Chapter 12: Water and the Major Minerals (Understanding Nutrition)

Suppose that a soup recipe calls for 2 tsp of salt. How many grams of salt is that?

12 g

If a cup of milk provides 300 mg of calcium, and you are trying to consume 1000 mg of calcium per day, how many cups of milk do you need to drink (assuming this is your only source of calcium)?

3.3 c

A sedentary 40-year-old male basketball player expends 2200 kcal per day. Calculate the water recommendation for the sedentary 40-year-old male.

8.8 c

Some mineral waters provide as much as 500 milligrams of calcium per liter, offering a convenient way to meet both calcium and water needs. Similarly, calcium-fortified orange juice and other fruit and vegetable juices allow a person to obtain both calcium and vitamins easily. Other examples of calcium-fortified foods include high-calcium milk (milk with extra calcium added) and calcium-fortified cereals. Fortified juices and foods help consumers increase calcium intakes, but depending on the calcium sources, the bioavailability may be significantly less than quantities listed on food labels. Highlight 12 discusses calcium supplements.

A generalization that has been gaining strength throughout this book is supported by the information given here about calcium. A balanced diet that supplies a variety of foods is the best plan to ensure adequacy for all essential nutrients. All food groups should be included, and none should be overemphasized. In our culture, calcium intake is usually inadequate wherever milk is lacking in the diet. By contrast, iron is usually lacking whenever milk is overemphasized

some electrolytes are concentrated primarily outside the cells (notably, sodium, chloride, and calcium), whereas others are concentrated predominantly inside the cells (notably, potassium, magnesium, phosphate, and sulfate). Cell membranes are selectively permeable, meaning that they allow the passage of some molecules, but not others.

All of these electrolytes are found both inside and outside the cells, but each can be found mostly on one side or the other of the cell membrane.: Ca- calcium, Cl- chloride, K-potassium, Mg-magnesium, Na-sodium, P-phosphorous, S-sulfate

Calcium Recommendations Calcium is unlike most other nutrients in that hormones maintain its blood concentration regardless of dietary intake. As Figure 12-14 shows, when calcium intake is high, the bones benefit; when intake is low, the bones suffer. Calcium recommendations are therefore based on the amount needed to retain the most calcium in bones. By retaining the most calcium possible, the bones can develop to their fullest potential in size and density—their peak bone mass—within genetic limits. Calcium recommendations have been set high enough to accommodate a 30 percent absorption rate. Because obtaining enough calcium during growth helps ensure that the skeleton will be strong and dense, the recommendation for adolescents to the age of 18 years is 1300 milligrams daily. Between the ages of 19 and 50, recommendations are lowered to 1000 milligrams a day; for women older than 50 and all adults older than 70, recommendations are raised again to 1200 milligrams a day to minimize the bone loss that tends to occur later in life.

Almost half of the people in the United States have calcium intakes below current recommendations. Those meeting recommendations for calcium are likely to be using calcium supplements. High intakes of calcium from supplements may have adverse effects such as kidney stone formation. For this reason, a UL has been established. A high-protein diet increases urinary calcium losses, but does not seem to impair bone health. In fact, protein may even improve calcium absorption and bone strength. The DRI Committee considered these nutrient interactions in establishing the RDA for calcium and did not adjust dietary recommendations based on this information.

Many people, for a variety of reasons, cannot or do not drink milk. Some cultures do not use milk in their cuisines; some vegetarians exclude milk as well as meat; and some people are allergic to milk protein or are lactose intolerant. Others simply do not enjoy the taste of milk. It is possible for people who do not drink milk to obtain adequate calcium, but only if they carefully select other calcium-rich foods. Some brands of tofu, corn tortillas, some nuts (such as almonds), and some seeds (such as sesame seeds) can supply calcium for the person who doesn't use milk products. A slice of most breads contains only about 5 to 10 percent of the calcium found in milk, but it can be a major source for people who eat many slices because the calcium is well absorbed. Oysters are also a rich source of calcium, as are small fish eaten with their bones, such as canned sardines.

Among the vegetables, mustard and turnip greens, bok choy, kale, parsley, watercress, and broccoli are good sources of available calcium. So are some seaweeds such as the nori popular in Japanese cooking. Some dark green, leafy vegetables—notably spinach and Swiss chard—appear to be calcium-rich but actually provide little, if any, calcium because they contain binders that limit absorption. It would take 8 cups of spinach—containing six times as much calcium as 1 cup of milk—to deliver the equivalent in absorbable calcium. With the exception of foods, such as spinach, that contain calcium binders, the calcium content of foods is usually more important than bioavailability. Consequently, recognizing that people eat a variety of foods containing calcium, the DRI Committee did not adjust for calcium bioavailability when setting recommendations. Figure 12-16 ranks selected foods according to their calcium bioavailability.

Proteins Attract Water Chapter 6 described how proteins attract water and help regulate fluid movement. It explains that when proteins leak out of the blood vessels into the spaces between the cells, fluids follow and cause the swelling of edema. In addition, transport proteins in the cell membranes regulate the passage of positive ions and other substances from one side of the membrane to the other. Negative ions follow positive ions, and water flows toward the more concentrated solution.

An example of a protein that regulates the flow of fluids and ions in and out of cells is the sodium-potassium pump. The pump actively exchanges sodium for potassium across the cell membrane, using ATP as an energy source. Figure 6-10 illustrates this action.

Regulation of Fluid Balance Fluids maintain the blood volume, which in turn influences blood pressure. The kidneys are central to the regulation of blood volume and blood pressure. All day, every day, the kidneys reabsorb needed substances and water and excrete wastes with some water in the urine (see Figure 12-5). The kidneys meticulously adjust the volume and the concentration of the urine to accommodate changes in the body, including variations in the day's food and beverage intakes. Instructions on whether to retain or release substances or water come from antidiuretic hormone, renin, angiotensin, and aldosterone.

Antidiuretic Hormone (ADH) Whenever blood volume or blood pressure falls too low, or whenever the extracellular fluid becomes too concentrated, the hypothalamus signals the pituitary gland to release antidiuretic hormone (ADH). ADH is a water-conserving hormone that stimulates the kidneys to reabsorb water. Consequently, the more water you need, the less your kidneys excrete. These events also trigger thirst. Drinking water and retaining fluids raise the blood volume and dilute the concentrated fluids, thus helping to restore homeostasis. (Recall from Highlight 7 that alcohol depresses ADH activity, thus promoting fluid losses and dehydration.)

The two types of bone handle calcium in different ways. Supplied with blood vessels and metabolically active, trabecular bone is sensitive to hormones that govern day-to-day deposits and withdrawals of calcium. It readily gives up minerals whenever blood calcium needs replenishing. Losses of trabecular bone start becoming significant for men and women in their 30s, although losses can occur whenever calcium withdrawals exceed deposits. Cortical bone also gives up calcium, but slowly and at a steady pace. Cortical bone losses typically begin at about age 40 and continue slowly but surely thereafter. bone density: a measure of bone strength. When minerals fill the bone matrix (making it dense), they give it strength.

As bone loss continues, bone density declines, and osteoporosis becomes apparent (see Figure H12-1). Bones become so fragile that even the body's own weight can overburden the spine—vertebrae may suddenly disintegrate and crush down, painfully pinching major nerves. Or the vertebrae may compress into wedge shapes, forming what is often called a "dowager's hump," the posture many older people assume as they "grow shorter." Figure H12-2 shows the effect of compressed spinal bone on a woman's height and posture. Because both the cortical shell and the trabecular interior weaken, breaks most often occur in the hip, as mentioned in the opening paragraph.

Sodium and Hypertension Most sodium is consumed as salt (sodium chloride), and a high salt intake correlates strongly with high blood pressure and heart disease. A salt-restricted diet lowers blood pressure and improves heart disease risk in healthy people as well as in those with hypertension. For this reason, the Dietary Guidelines for Americans advise limiting daily salt intake to about 1 teaspoon (the equivalent of about 2.3 grams or 2300 milligrams of sodium). For adults with prehypertension and hypertension, a reduction to 1500 milligrams of sodium per day is of even greater benefit. Most people will benefit from a sodium intake within this range (1500 to 2300 milligrams). Limited evidence suggests lower sodium intakes may be associated with health problems for some groups of people.

Because sodium is so prevalent in the food supply, consumers need to use multiple strategies to lower their intake. Given the current US food supply and typical eating habits, creating a nutritionally balanced diet that meets sodium recommendations can be a challenge. One eating pattern, known as the DASH (Dietary Approaches to Stop Hypertension) Eating Plan, is especially effective in lowering blood pressure. Like other USDA Food Patterns, the DASH Eating Plan reflects the Dietary Guidelines and allows people to stay within their energy allowance, meet nutrient needs, and lower disease risk. The DASH approach emphasizes potassium-rich fruits, vegetables, and low-fat milk products; includes whole grains, nuts, poultry, and fish; and calls for reduced intakes of sodium, red and processed meats, sweets, and sugar-containing beverages. Chapter 18 offers a complete discussion of hypertension and the dietary recommendations for its prevention and treatment.

Most of the body's calcium is in the bones, where it provides a rigid structure and a reservoir of calcium for the blood. Blood calcium participates in muscle contraction, blood clotting, and nerve impulses, and it is closely regulated by a system of hormones and vitamin D. Calcium is found predominantly in milk and milk products. Even when calcium intake is inadequate, blood calcium remains normal, but at the expense of bone loss, which can lead to osteoporosis. The accompanying table provides a summary of calcium.

Calcium RDA Adults: 1000 mg/day (adults, 19-50 yr) 1000 mg/day (men, 51-70 yr) 1200 mg/day (men, ≥71 yr) 1200 mg/day (women, ≥51 yr) UL Adults: 2500 mg/day (adults, 19-50 yr) 2000 mg/day (adults, ≥51 yr) Chief Functions in the Body Mineralization of bones and teeth; also involved in muscle contraction and relaxation, nerve functioning, blood clotting, blood pressure Deficiency Symptoms Stunted growth in children; bone loss (osteoporosis) in adults Toxicity Symptoms Constipation; increased risk of urinary stone formation and kidney dysfunction; interference with absorption of other minerals Significant Sources Milk and milk products, small fish (with bones), calcium-set tofu (bean curd), greens (bok choy, broccoli, chard, kale), legumes

calcium Calcium is the most abundant mineral in the body. It receives much emphasis in this chapter and in the highlight that follows because an adequate intake helps grow a healthy skeleton in early life and minimize bone loss in later life.

Calcium: the most abundant mineral in the body; found primarily in the body's bones and teeth.

Chloride is the major anion outside cells, and it associates closely with sodium. In addition to its role in fluid balance, chloride is part of the stomach's hydrochloric acid. The accompanying table provides a summary of chloride.

Chloride AI Adults: 2300 mg/day (19-50 yr) 2000 mg/day (51-70 yr) 1800 mg/day (>70 yr) UL Adults: 3600 mg/day Chief Functions in the Body Maintains normal fluid and electrolyte balance; part of hydrochloric acid found in the stomach, necessary for proper digestion Deficiency Symptoms Do not occur under normal circumstances Toxicity Symptoms Vomiting Significant Sources Table salt, soy sauce; moderate amounts in meats, milks, eggs; large amounts in processed foods

Chloride Recommendations and Intakes Chloride is abundant in foods (especially processed foods) as part of sodium chloride and other salts. Chloride recommendations are slightly higher than, but still equivalent to, those of sodium. In other words, ¾ teaspoon of salt will deliver some sodium, more chloride, and still meet the AI for both.

Chloride Deficiency and Toxicity Diets rarely lack chloride. Like sodium losses, chloride losses may occur in conditions such as heavy sweating, chronic diarrhea, and vomiting. The only known cause of elevated blood chloride concentrations is dehydration due to water deficiency. In both cases, consuming ordinary foods and beverages can restore chloride balance.

Chloride The element chlorine is a poisonous gas. When chlorine reacts with sodium or hydrogen, however, it forms the negative chloride ion (Cl-). Chloride, an essential nutrient, is required in the diet. Chloride: the major anion in the extracellular fluids of the body. Chloride is the ionic form of chlorine, (Cl-).

Chloride Roles in the Body Chloride (KLO-ride) is the major anion of the extracellular fluids (outside the cells), where it occurs mostly in association with sodium. Chloride moves passively across membranes through channels and so also associates with potassium inside cells. Like sodium and potassium, chloride maintains fluid and electrolyte balance. In the stomach, the chloride ion is part of hydrochloric acid, which maintains the strong acidity of gastric juice. One of the most serious consequences of vomiting is the loss of this acid from the stomach, which upsets the acid-base balance. Such imbalances are commonly seen in bulimia nervosa

Nutrient Interactions Chapter 10 described how the presence or absence of one vitamin can affect another's absorption, metabolism, and excretion. The same is true of the minerals. The interactions between sodium and calcium, for example, cause both to be excreted when sodium intakes are high. Phosphorus binds with magnesium in the GI tract, so magnesium absorption is limited when phosphorus intakes are high. These are just two examples of the interactions involving minerals featured in this chapter. Discussions in both this chapter and the next point out additional problems that arise from such interactions. Notice how often they reflect an excess of one mineral creating an inadequacy of another and how supplements—not foods—are most often to blame.

Compared with the trace minerals, major minerals are found, and needed, in larger quantities in the body. Unlike vitamins and the energy-yielding nutrients, minerals are inorganic elements that retain their chemical identities. Minerals usually receive special handling and regulation in the body, and they may bind with other substances or interact with other minerals, thus limiting their absorption.

Sodium People have held salt (sodium chloride) in high regard throughout recorded history. We describe someone we admire as "the salt of the earth" and people who are not productive as "not worth their salt." The word salary comes from the Latin word for salt, a valued commodity.

Cultures vary in their use of salt, but most people find its taste innately appealing. Salt brings its own tangy taste and enhances other flavors, most likely by suppressing the bitter flavors. You can taste this effect for yourself: tonic water with its bitter quinine tastes sweeter with a little salt added.

Gender and Hormones After age, gender is the next strongest predictor of osteoporosis. The sex hormones play a major role in regulating the rate of bone turnover. In general, men have greater bone density than women at maturity, and women have greater losses than men in later life. Consequently, men develop bone problems about 10 years later than women, and women account for two out of three cases of osteoporosis. Menopause imperils women's bones. Bone dwindles rapidly when the hormone estrogen diminishes and menstruation ceases. The lack of estrogen contributes to the release of cytokines that produce inflammation and accelerate bone loss. Women may lose up to 20 percent of their bone mass during the 6 to 8 years following menopause. Eventually, losses taper off so that women again lose bone at the same rate as men their age. Losses of bone minerals continue throughout the remainder of a woman's lifetime, but not at the free-fall pace of the menopause years (review Figure H12-3). Rapid bone losses also occur when young women's ovaries fail to produce enough estrogen, causing menstruation to cease. In some cases, diseased ovaries are to blame and must be removed; in others, the ovaries fail to produce sufficient estrogen because the women suffer from anorexia nervosa and have unreasonably restricted their body weight (see Highlight 8). The amenorrhea and low body weights explain much of the bone loss seen in these young women, even years after diagnosis and treatment.

Estrogen therapy may help some women prevent further bone loss and reduce the incidence of fractures. Because estrogen therapy may increase the risks for breast cancer, women must carefully weigh any potential benefits against the possible dangers. Several drug therapies have been developed to inhibit bone loss and enhance bone formation. A combination of drugs or of hormone replacement and a drug may be most beneficial. Some women who choose not to use estrogen therapy turn to soy as an alternative treatment. Interestingly, the phytochemicals commonly found in soy mimic the actions of estrogen in the body. Research results have been mixed and controversial, but overall seem to indicate a lack of benefit for soy and its phytochemicals in helping to prevent the rapid bone losses of the menopause years. As is true of all dietary supplements and herbal products, there may be risks associated with their use; in the case of soy, research suggests relative safety with modest benefit. Because the risks and benefits vary depending on each person's medical history, women should discuss soy options with their physicians. As in women, sex hormones appear to play a key role in men's bone loss as well. Other common causes of osteoporosis in men include excessive use of corticosteroids and alcohol.

Respiration in the Lungs The lungs control the concentration of carbonic acid by raising or lowering the respiration rate, depending on whether the pH needs to be increased or decreased. If too much carbonic acid builds up, the respiration rate speeds up; this hyperventilation increases the amount of carbon dioxide exhaled, thereby lowering the carbonic acid concentration and restoring homeostasis. Conversely, if bicarbonate builds up, the respiration rate slows; carbon dioxide is retained and forms more carbonic acid. Again, homeostasis is restored.

Excretion in the Kidneys The kidneys control the concentration of bicarbonate by either reabsorbing or excreting it, depending on whether the pH needs to be increased or decreased. Their work is complex, but the net effect is easy to sum up. The body's total acid burden remains nearly constant; the acidity of the urine fluctuates to accommodate that balance.

health effects of water Water supports good health. Physical and mental performances depend on it, as does the optimal functioning of the GI tract, kidneys, heart, and other body systems. The kind of water a person drinks may also make a difference to health. Water is usually either hard or soft. Hard water has high concentrations of calcium and magnesium; the principal mineral of soft water is sodium or potassium. (See Glossary 12-1 for other common terms used to describe water.) In practical terms, soft water makes more bubbles with less soap; hard water leaves a ring on the tub, a crust of rocklike crystals in the teakettle, and a gray residue in the laundry. Soft water may seem more desirable around the house, and some homeowners purchase water softeners that replace magnesium and calcium with sodium. In the body, however, soft water with sodium may aggravate hypertension and heart disease. In contrast, the minerals in hard water may benefit these conditions. Soft water also more easily dissolves certain contaminant minerals, such as cadmium and lead, from old plumbing pipes. As Chapter 13 explains, these contaminant minerals harm the body by displacing the nutrient minerals from their normal sites of action. Many people select bottled water, believing it to be safer than tap water and therefore worth its substantial cost. Chapter 19 offers a discussion of bottled water safety and regulations.

Hard water: water with a high calcium and magnesium content. soft water: water with a high sodium or potassium content. Water makes up about 60 percent of the adult body's weight. It assists with the transport of nutrients and waste products throughout the body, participates in chemical reactions, acts as a solvent, serves as a shock absorber, and regulates body temperature. To maintain water balance, intake from liquids, foods, and metabolism must equal losses from the kidneys, skin, lungs, and GI tract. Whenever the body experiences low blood volume, low blood pressure, or highly concentrated body fluids, the actions of ADH, renin, angiotensin, and aldosterone restore homeostasis. Electrolytes (charged minerals) in the fluids help distribute the fluids inside and outside the cells, thus ensuring the appropriate water balance and acid-base balance to support all life processes. Excessive losses of fluids and electrolytes upset these balances, and the kidneys play a key role in restoring homeostasis.

Fluid and Electrolyte Imbalance Normally, the body defends itself successfully against fluid and electrolyte imbalances. With minor imbalances, people can replace the fluids and minerals lost in sweat or in a temporary bout of diarrhea, for example, by drinking plain cool water and eating regular foods. Certain situations and some medications, however, may overwhelm the body's ability to compensate. Severe, prolonged vomiting and diarrhea as well as heavy sweating, burns, and traumatic wounds may incur such great fluid and electrolyte losses as to precipitate a medical emergency. Appropriate medical intervention depends on the circumstances surrounding the loss of fluids and their solutes. If fluid is lost by vomiting or diarrhea, sodium is lost indiscriminately. If the adrenal glands oversecrete aldosterone, as may occur when a tumor develops, the kidneys may excrete too much potassium. A person with uncontrolled diabetes may lose glucose (a solute not normally excreted) and large amounts of fluid with it. Each situation results in dehydration, but drinking water alone will not restore balance. Medical intervention is required to restore balance.

In some cases, restoring balance demands rapid replacement of fluids and electrolytes—for example, when diarrhea threatens the life of a malnourished child. Caregivers around the world have learned to use oral rehydration therapy (ORT)—a simple solution of sugar, salt, and water, taken by mouth—to treat dehydration caused by severe diarrhea. These lifesaving formulas do not require hospitalization and can be prepared from ingredients available locally. Caregivers need only learn to measure ingredients carefully and use sanitary water. Once rehydrated, a person can begin eating foods. oral rehydration therapy (ORT): the administration of a simple solution of sugar, salt, and water, taken by mouth, to treat dehydration caused by diarrhea.

Water constitutes about 60 percent of an adult's body weight and a higher percentage of a child's (see Figure 1-1). Because water makes up about 75 percent of the weight of lean tissue and less than 25 percent of the weight of fat, a person's body composition influences how much of the body's weight is water. The proportion of water is generally smaller in females, obese people, and the elderly because of their smaller proportion of lean tissue.

In the body, water is the fluid in which all life processes occur. The water in the body fluids: - Carries nutrients and waste products throughout the body - Maintains the structure of large molecules such as proteins and glycogen - Participates in metabolic reactions - Serves as the solvent for minerals, vitamins, amino acids, glucose, and many other small molecules so that they can participate in metabolic activities - Acts as a lubricant and cushion around joints and inside the eyes, the spinal cord, and, in pregnancy, the amniotic sac surrounding the fetus in the womb - Aids in the regulation of normal body temperature, as the evaporation of sweat from the skin removes excess heat from the body - Maintains blood volume These activities take place in fluids that are carefully distributed in various compartments throughout the body.

Like calcium and phosphorus, magnesium supports bone mineralization. Magnesium is also involved in numerous enzyme systems and in heart function. It is found abundantly in legumes and dark green, leafy vegetables and, in some areas, in water. The accompanying table offers a summary of magnesium.

Magnesium RDA Men (19-30 yr): 400 mg/day Women (19-30 yr): 310 mg/day UL Adults: 350 mg nonfood magnesium/day Chief Functions in the Body Bone mineralization, building of protein, enzyme action, normal muscle contraction, nerve impulse transmission, maintenance of teeth, and functioning of immune system Deficiency Symptoms Weakness; confusion; if extreme, convulsions, bizarre muscle movements (especially of eye and face muscles), hallucinations, and difficulty in swallowing; in children, growth failure Toxicity Symptoms From nonfood sources only; diarrhea, alkalosis, dehydration Significant Sources Nuts, legumes, whole grains, dark green vegetables, seafood, chocolate, cocoa

Magnesium Roles in the Body In addition to maintaining bone health, magnesium acts in all the cells of the soft tissues, where it forms part of the protein-making machinery and is necessary for energy metabolism. It participates in hundreds of enzyme systems. A major role of magnesium is as a catalyst in the reaction that adds the last phosphate to the high-energy compound ATP, making it essential to the body's use of glucose; the synthesis of protein, fat, and nucleic acids; and the cells' membrane transport systems. Together with calcium, magnesium is involved in muscle contraction and blood clotting: calcium promotes the processes, whereas magnesium inhibits them. This dynamic interaction between the two minerals helps regulate blood pressure and lung function. Like many other nutrients, magnesium supports the normal functioning of the immune system.

Magnesium Intakes The brown bars in Figure 12-19 indicate that legumes, nuts, and seeds make significant magnesium contributions. Magnesium is part of the chlorophyll molecule, so dark green, leafy vegetables are also good sources. In areas with hard water, the water contributes both calcium and magnesium to daily intakes. Mineral waters noted earlier for their calcium content may also be magnesium-rich and can be important sources of this mineral for those who drink them. Bioavailability of magnesium from mineral water is about 50 percent, but it improves when the water is consumed with a meal. foods: broccoli, tomato juice, pinto beans, peanut butter, sunflower seeds, tofu, halibut, cashews, artichocke

Magnesium Deficiency Average magnesium intakes typically fall below recommendations, which may exacerbate inflammation and contribute to chronic diseases such as heart disease, stroke, hypertension, diabetes, and cancer. A severe magnesium deficiency causes a tetany similar to the calcium tetany described earlier. Magnesium deficiencies also impair central nervous system activity and may be responsible for the hallucinations experienced during alcohol withdrawal. Magnesium Toxicity Magnesium toxicity is rare, but it can be fatal. The UL for magnesium applies only to nonfood sources such as supplements or magnesium salts.

Magnesium and Hypertension Magnesium is critical to heart function and seems to protect against hypertension and support heart health. Interestingly, people living in areas of the country with hard water, which contains high concentrations of calcium and magnesium, tend to have low rates of heart disease. With magnesium deficiency, the walls of the arteries and capillaries tend to constrict—a possible explanation for the hypertensive effect.

Trace minerals refers to the essential minerals that are needed in amounts less than 100 mg per day.

Major minerals include the essential minerals that are needed in amounts greater than 100 mg per day. They are also present in larger amounts in the body compared to trace minerals. Bioavailability is a term relating to how much of an ingested nutrient is actually absorbed from the gut and used by the body. Binders refer to chemical compounds present in foods that combine with nutrients, especially minerals, to form complexes the body cannot absorb. Binders present in plant foods that affect the bioavailability of minerals include phytates and oxalates.

Calcium Absorption Because many factors affect calcium absorption, the most effective way to ensure adequacy is to increase calcium intake. On average, adults absorb about 30 percent of the calcium they ingest. The stomach's acidity helps keep calcium soluble, and vitamin D helps make the calcium-binding protein needed for absorption. This relationship explains why calcium-rich milk is a good choice for vitamin D fortification. Whenever calcium is needed, the body increases its calcium absorption. The result is obvious in the case of a newborn infant, whose calcium absorption is 55 to 60 percent. Similarly, a pregnant woman doubles her absorption of calcium. Growing children and teens absorb up to 50 percent of the calcium they consume. Then, when bone growth slows or stops, absorption falls to the adult level of about 30 percent. In addition, absorption becomes more efficient during times of inadequate intakes.

Many of the conditions that enhance calcium absorption limit its absorption when they are absent. For example, sufficient vitamin D supports absorption, and a deficiency impairs it. In addition, fiber in general, and the binders phytate and oxalate in particular, interfere with calcium absorption, but their effects are relatively minor in typical US diets. Vegetables with oxalates and whole grains with phytates are nutritious foods, of course, but they are not useful calcium sources. calcium-binding protein: a protein in the intestinal cells, made with the help of vitamin D, that facilitates calcium absorption.

Age and Bone Calcium Two major stages of life are critical in the development of osteoporosis. The first is the bone-acquiring stage of childhood and adolescence. The second is the bone-losing decades of late adulthood, especially in women after menopause. The bones gain strength and density all through the growing years and into young adulthood. As people age, the cells that build bone gradually become less active, but those that dismantle bone continue working. The result is that bone loss exceeds bone formation. Some bone loss is inevitable, but losses can be curtailed by maximizing bone mass.

Maximizing Bone Mass To maximize bone mass, the diet must deliver an adequate supply of calcium during the first three decades of life. Children and teens who consume milk products and get enough calcium have denser bones than those with inadequate intakes. With little or no calcium from the diet, the body must depend on bone to supply calcium to the blood—bone mass diminishes, and bones lose their density, integrity, and strength. When people reach the bone-losing years of middle age, those who formed dense bones during their youth have the advantage. They simply have more bone starting out and can lose more before suffering ill effects. Peak bone mass is achieved by age 30. Women gradually lose bone mass until menopause, when losses accelerate dramatically and then gradually taper off.

Calcium Food Sources Figure 12-15 and Photo 12-8 show that calcium is found most abundantly in one food group—milk and milk products. The person who doesn't like to drink milk may prefer to eat cheese or yogurt. Alternatively, milk and milk products can be concealed in foods. Powdered fat-free milk can be added to casseroles, soups, and other recipes during preparation. This simple step is an excellent way to obtain not only extra calcium, but more protein, vitamins, and minerals as well.

Milk and milk products are well known for their calcium, but calcium-set tofu, bok choy, kale, calcium-fortified orange juice, and broccoli are also rich in calcium. broccoli, milk, yogurt, cheddar cheese, tofu, sardines, bok choy

Fresh herbs add flavor to a recipe without adding salt.

Most people eat more salt (and therefore sodium) than they need. Some people can lower their blood pressure by avoiding highly salted foods and removing the saltshaker from the table. Foods eaten without salt may seem less tasty at first, but with repetition, people can learn to enjoy the natural flavors of many unsalted foods.

Dietary Calcium For older adults, an adequate calcium intake alone cannot protect against bone fractures. Bone strength later in life depends primarily on how well the bones were built during childhood and adolescence. Adequate calcium nutrition during the growing years is essential to achieving optimal peak bone mass. Simply put, growing children who do not get enough calcium do not develop strong bones. For this reason, the DRI Committee recommends 1300 milligrams of calcium per day for everyone 9 through 18 years of age. Unfortunately, few girls meet the recommendations for calcium during these bone-forming years. (Boys generally obtain intakes close to those recommended because they eat more food.) Consequently, most girls start their adult years with less-than-optimal bone density. As adults, women rarely meet their recommended intakes of 1000 to 1200 milligrams from food. Some authorities suggest 1500 milligrams of calcium for postmenopausal women who are not receiving estrogen.

Other Nutrients Much research has focused on calcium, but other nutrients support bone health too. Adequate protein protects bones and reduces the likelihood of hip fractures. As mentioned earlier, vitamin D is needed to maintain calcium metabolism and optimal bone health. Vitamin K regulates bone and cartilage mineralization and decreases bone turnover. Vitamin C may slow bone losses. The minerals magnesium and potassium also help maintain bone mineral density. Vitamin A is needed in the bone-remodeling process, but too much vitamin A may be associated with osteoporosis. Carotenoids may inhibit bone loss. Omega-3 fatty acids may help preserve bone integrity. Additional research points to the bone benefits not of a specific nutrient, but of a diet rich in fruits, vegetables, and whole grains. In contrast, diets containing too much salt are associated with bone losses. Similarly, diets containing too many sodas or commercially baked snack and fried foods are associated with low bone mineral density. Clearly, a well-balanced diet that depends on all the food groups to supply a full array of nutrients is central to bone health.

Phosphorus accompanies calcium both in the crystals of bone and in many foods such as milk. Phosphorus is also important in energy metabolism as part of ATP, in lipid structures as part of phospholipids, and in genetic materials as part of DNA and RNA. The accompanying table provides a summary of phosphorus.

Phosphorus RDA Adults: 700 mg/day UL Adults (19-70 yr): 4000 mg/day Chief Functions in the Body Mineralization of bones and teeth; part of every cell; important in genetic material, part of phospholipids, used in energy transfer and in buffer systems that maintain acid-base balance Deficiency Symptoms Muscular weakness, bone pain Toxicity Symptoms Calcification of nonskeletal tissues, particularly the kidneys Significant Sources Foods derived from animals (meat, fish, poultry, eggs, milk)

Phosphorus Roles in the Body Phosphorus is found not only in bones and teeth, but also in all body cells as part of a major buffer system. Phosphorus is also part of DNA and RNA and is therefore necessary for all growth. Phosphorus assists in energy metabolism. The high-energy compound ATP uses three phosphate groups to do its work. Many enzymes and the B vitamins become active only when a phosphate group is attached. Phospholipids provide stability to the lipoprotein vehicles that help transport lipids in the blood. Phospholipids are also the major structural components of cell membranes, where they control the transport of nutrients into and out of the cells. Some proteins, such as the casein in milk, contain phosphorus as part of their structures (phosphoproteins). foods: broccoli, milk, yogurt, liver, almonds, cheddar cheese, cottage cheese, sunflower seeds, tofu

Phosphorus Recommendations and Intakes Because phosphorus is commonly found in almost all foods, dietary deficiencies are unlikely. As Figure 12-18 shows, foods rich in proteins—such as meat, poultry, fish, milk, and cheese—are the best sources of phosphorus. Many processed foods and soft drinks contain phosphate-based additives, and phosphorus intakes in the United States have increased as consumption of processed foods and beverages has increased. Phosphate toxicity is rare and usually reflects a significant problem such as kidney failure. Still, phosphorus intakes can be excessive when processed foods take center stage, disrupting kidney function and bone metabolism; high intakes are also associated with increased mortality. A UL of 4000 milligrams has been established.

Phosphorus Phosphorus is the second most abundant mineral in the body. About 85 percent of it is found combined with calcium in the hydroxyapatite crystals of bones and teeth.

Phosphorus: a major mineral found mostly in the body's bones and teeth.

Genetics The role of genetics in osteoporosis is strong, although still unclear. Most likely, genes influence both the peak bone mass achieved during growth and the bone loss incurred during the later years. The extent to which a given genetic potential is realized, however, depends on many outside factors. Diet and physical activity, for example, can maximize peak bone density during growth, whereas alcohol and tobacco abuse can accelerate bone losses later in life. Importantly, these factors are within a person's control. Smoking and Alcohol Add bone damage to the list of ill consequences associated with smoking. The bones of smokers are less dense than those of nonsmokers—even after controlling for differences in age, body weight, and physical activity habits. Fortunately, the damaging effects can be reversed with smoking cessation. Blood indicators of beneficial bone activity are apparent 6 weeks after a person stops smoking. In time, bone density is similar for former smokers and nonsmokers. People who abuse alcohol often suffer from osteoporosis and experience more bone breaks than others. Several factors appear to be involved. Alcohol enhances fluid excretion, leading to excessive calcium losses in the urine; upsets the hormonal balance required for healthy bones; slows bone formation, leading to lower bone density; stimulates bone breakdown; and increases the risk of falling.

Physical Activity and Body Weight Physical activity may be the single most important factor supporting bone growth during adolescence. Active adolescents have stronger bones in adulthood. Muscle strength and bone strength go together. When muscles work, they pull on the bones, stimulating them to grow denser. The hormones that promote new muscle growth also support bone growth. As a result, active bones are denser and stronger than sedentary bones. Both the muscle contraction and the gravitational pull of the body's weight create a load that benefits bone metabolism. As Photo H12-2 shows, to keep bones healthy, a person should engage in weight training or weight-bearing endurance activities (such as tennis and jogging or sprint cycling) regularly. Regular physical activity combined with an adequate calcium intake helps maximize bone density in children and adolescents. Adults can also maximize and maintain bone density with a regular program of weight training. Even past menopause, when most women are losing bone, weight training improves bone density. strength training helps build strong bones Heavier body weights and weight gains place a similar stress on the bones and promote their density. In contrast, weight losses reduce bone density and increase the risk of fractures—in part because energy restriction diminishes calcium absorption and compromises calcium balance. As mentioned in Highlight 8, the relative energy deficiency that results from a combination of restricted energy intake and extreme daily exercise reliably predicts bone loss.

Potassium, like sodium and chloride, is an electrolyte that plays an important role in maintaining fluid balance. Potassium is the primary cation inside cells; fresh foods, notably fruits and vegetables, are its best sources. The accompanying table provides a summary of potassium.

Potassium AI Adults: 4700 mg/day Chief Functions in the Body Maintains normal fluid and electrolyte balance; facilitates many reactions; supports cell integrity; assists in nerve impulse transmission and muscle contractions Deficiency Symptoms Irregular heatbeat, muscular weakness, glucose intolerance Toxicity Symptoms Muscular weakness; vomiting; if given into a vein, can stop the heart Significant Sources All whole foods: meats, milks, fruits, vegetables, grains, legumes

Potassium Like sodium, potassium is a positively charged ion. In contrast to sodium, potassium is the body's principal intracellular cation, inside the body cells. Potassium Roles in the Body Potassium plays a major role in maintaining fluid and electrolyte balance and cell integrity. During nerve transmissions and muscle contractions, potassium and sodium briefly trade places across the cell membrane. The cell then quickly pumps them back into place. Controlling potassium distribution is a high priority for the body because it affects many aspects of homeostasis, including a steady heartbeat. potassium: the principal cation within the body's cells; critical to the maintenance of fluid balance, nerve impulse transmissions, and muscle contractions.

Potassium Recommendations and Intakes Potassium is abundant in all living cells. Because cells remain intact unless foods are processed, the richest sources of potassium are fresh foods—as Figure 12-12 shows. In contrast, most processed foods such as canned vegetables, ready-to-eat cereals, and luncheon meats contain less potassium—and more sodium (recall Figure 12-11). To meet the AI for potassium, most people need to increase their intake of fruits and vegetables Fresh foods, especially fruits and vegetables, provide potassium in abundance. (broccoli, carrots, tomato juice, strawberries, squash, artichoke, potato, banana, orange, watermelon, milk, yogurt etc.)

Potassium and Hypertension Diets low in potassium, especially when combined with high sodium intakes, raise blood pressure and increase the risk of death from heart disease. In contrast, high potassium intakes reduce the risks of hypertension, heart disease, stroke, and related deaths. Unfortunately, most US adults consume too much sodium and too little potassium. Recall that the DASH eating pattern used to lower blood pressure emphasizes potassium-rich foods such as fruits and vegetables. Potassium Deficiency Potassium deficiency is characterized by an increase in blood pressure, kidney stones, and bone turnover. As deficiency progresses, symptoms include irregular heartbeat, muscle weakness, and glucose intolerance.

Potassium Toxicity Potassium toxicity does not result from overeating foods high in potassium; therefore a UL has not been set. It can result from overconsumption of potassium salts or supplements (including some "energy fitness shakes") and from certain diseases or treatments. Given more potassium than the body needs, the kidneys accelerate excretion. If potassium is injected directly into a vein, however, it can stop the heart.

What Processing Does to the Sodium and Potassium Contents of Foods People who eat foods high in salt often happen to be eating fewer potassium-containing foods at the same time. Notice how potassium is lost and sodium is gained as foods become more processed, causing the potassium-to-sodium ratio to fall dramatically. Even when potassium isn't lost, the addition of sodium still lowers the potassium-to-sodium ratio. Selecting fresh, unprocessed foods lowers blood pressure in two ways, then—by lowering sodium intakes and by raising potassium intakes.

Sodium Deficiency Sodium deficiency does not develop from an inadequate diet. The body only needs a little and typical diets provide more than enough. Blood sodium may drop with vomiting, diarrhea, or heavy sweating, and in these cases, both sodium and water must be replenished. Under normal conditions of sweating due to physical activity, salt losses can easily be replaced later in the day with ordinary foods. Salt tablets are not recommended because too much salt, especially if taken with too little water, can induce dehydration. During intense activities, such as ultra-endurance events, athletes can lose so much sodium and drink so much water that they develop hyponatremia—the dangerous condition of having too little sodium in the blood. Symptoms of hyponatremia include headache, confusion, stupor, seizures, and coma. Importantly, hyponatremia is caused by excessive sodium losses, not from inadequate sodium intake. (Chapter 14 offers details about hyponatremia and guidelines for ultra-endurance athletes.)

Sodium Roles in the Body Sodium is the principal cation of the extracellular fluid and the primary regulator of its volume. Sodium also helps maintain acid-base balance and is essential to nerve impulse transmission and muscle contraction. Sodium is readily absorbed by the intestinal tract and travels freely in the blood until it reaches the kidneys, which filter all the sodium out of the blood. Then, with great precision, the kidneys return to the blood the exact amount of sodium the body needs. Normally, the amount excreted is approximately equal to the amount ingested on a given day. When blood sodium rises, as when a person eats salted foods, thirst signals the person to drink until the appropriate sodium-to-water concentration is restored. Then the kidneys excrete both the excess water and the excess sodium together. Both too much and too little sodium in the diet can be harmful. The key to good health, then, is finding the balance that meets the relatively small need for this essential nutrient but does not exceed the amount that leads to hypertension and heart disease.

Sodium: the principal cation in the extracellular fluids of the body; critical to the maintenance of fluid balance, nerve impulse transmissions, and muscle contractions. Sodium Recommendations Diets rarely lack sodium, and even when intakes are low, the body adapts by reducing sodium losses in urine and sweat, thus making deficiencies unlikely. Sodium recommendations are set low enough to protect against high blood pressure, but high enough to allow an adequate intake of other nutrients with a typical diet. Because high sodium intakes correlate with high blood pressure, the Upper Level (UL) for adults is set at 2300 milligrams per day, as is the Daily Value used on food labels (see Photo 12-5). An estimated 90 percent of the US population exceeds the recommended intake for sodium. Food labels indicate the sodium content of foods, making it easier for consumers to select products with less sodium.

Minimizing Bone Loss Not only does dietary calcium build strong bones in youth, but it remains important in protecting against losses in the later years. Unfortunately, calcium intakes of older adults are typically low, and calcium absorption declines after menopause. The kidneys do not activate vitamin D as well as they did earlier (recall that vitamin D enhances calcium absorption). Also, sunlight is needed to form vitamin D, and many older people spend little or no time outdoors in the sunshine. For these reasons, and because intakes of vitamin D are typically low anyway, blood levels of vitamin D decline.

Some of the hormones that influence bone and calcium metabolism—parathyroid hormone, calcitonin, estrogen, and testosterone—also change with age and accelerate bone loss. Together, these age-related factors contribute to bone loss: inefficient bone remodeling, reduced calcium intakes, impaired calcium absorption, poor vitamin D status, and hormonal changes that favor bone mineral withdrawal.

Which of the following is not a function of water in the body?

Source of energy

Sulfate Sulfate is the oxidized form of the mineral sulfur, as it exists in foods and water. The body's need for sulfate is easily met by a variety of foods and beverages. In addition, the body receives sulfate from the amino acids methionine and cysteine, which are found in dietary proteins. These sulfur-containing amino acids help determine the contour of protein molecules. The sulfur-containing side chains in cysteine molecules can link to each other via disulfide bridges, which stabilize the protein structure. (See the drawing of insulin with its disulfide bridges in Figure 6-4) Skin, hair, and nails contain some of the body's more rigid proteins, which have a high sulfur content. Because the body's sulfate needs are easily met with normal protein intakes, there is no recommended intake for sulfate. Deficiencies do not occur when diets contain protein. Only when people lack protein to the point of severe deficiency will they lack the sulfur-containing amino acids.

Sulfate: a salt produced from the oxidation of sulfur. sulfur: a mineral present in the body as part of some proteins.

Inorganic Elements Unlike the organic vitamins, which are easily destroyed, minerals are inorganic elements that always retain their chemical identity. Once minerals enter the body, they remain there until excreted; they cannot be changed into anything else. Iron, for example, may temporarily combine with other charged elements in salts, but it is always iron. Neither can minerals be destroyed by heat, air, acid, or mixing. Consequently, little care is needed to preserve minerals during food preparation. In fact, the ash that remains when a food is burned contains all the minerals that were in the food originally. Minerals can be lost from food only when they leach into cooking water that is then poured down the drain.

The Body's Handling of Minerals The minerals also differ from the vitamins in the amounts the body can absorb and in the extent to which they must be specially handled. Some minerals, such as potassium, are easily absorbed into the blood, transported freely, and readily excreted by the kidneys, much like the water-soluble vitamins. Other minerals, such as calcium, are more like fat-soluble vitamins in that they must have carriers to be absorbed and transported. And, like some of the fat-soluble vitamins, minerals consumed in excess can be toxic.

Acid-Base Balance The body uses its ions not only to help maintain fluid and electrolyte balance, but also to regulate the acidity (pH) of its fluids. The pH scale introduced in Chapter 3 is repeated here in Figure 12-7 with the normal and abnormal pH ranges of the blood added. As you can see, the body must maintain the pH within a narrow range to avoid life-threatening consequences. Slight deviations in either direction can denature proteins, rendering them useless—for example, enzymes couldn't catalyze reactions and hemoglobin couldn't carry oxygen.

The acidity of the body's fluids is determined by the concentration of hydrogen ions (H+). A high concentration of hydrogen ions is acidic. Normal energy metabolism generates hydrogen ions, as well as many other acids, that must be neutralized. Three systems defend the body against fluctuations in pH—buffers in the blood, respiration in the lungs, and excretion in the kidneys.

A Perspective on Calcium Supplements Bone health depends, in part, on calcium. People who do not consume milk products or other calcium-rich foods in amounts that provide even half the recommendation should consider consulting a registered dietitian nutritionist who can assess the diet and suggest food choices to correct any inadequacies. Calcium from foods may support bone health better than calcium from supplements. For those who are unable to consume enough calcium-rich foods, however, taking calcium supplements—especially in combination with vitamin D—may help enhance bone density and protect against bone loss and fractures. Because some research suggests that calcium from supplements may increase the risk of heart attacks and strokes, women should consult their physicians when making this decision. Selecting a calcium supplement requires a little investigative work to sort through the many options. Before examining calcium supplements, recognize that multivitamin-mineral pills contain little or no calcium. The label may list a few milligrams of calcium, but remember that the recommended intake is a gram (1000 milligrams) or more for adults. Calcium supplements are typically sold as compounds of calcium carbonate (common in antacids and fortified chocolate candies), citrate, gluconate, lactate, malate, or phosphate. These supplements often include magnesium, vitamin D, or both. In addition, some calcium supplements are made from bone meal, oyster shell, or dolomite (limestone). Many calcium supplements, especially those derived from these natural products, contain lead—which impairs health in numerous ways, as Chapter 13 points out. Fortunately, calcium interferes with the absorption and action of lead in the body. bone meal: crushed or ground bone preparations intended to supply calcium to the diet. Calcium from bone is not well absorbed and is often contaminated with toxic minerals such as arsenic, mercury, lead, and cadmium. oyster shell: a product made from the powdered shells of oysters that is sold as a calcium supplement, but it is not well absorbed by the digestive system. dolomite: a compound of minerals (calcium magnesium carbonate) found in limestone and marble. Dolomite is powdered and is sold as a calcium-magnesium supplement. However, it may be contaminated with toxic minerals, is not well absorbed, and interferes with absorption of other essential minerals.

The first question to ask is how much calcium the supplement provides. Most calcium supplements provide between 250 and 1000 milligrams of calcium. To be safe, total calcium intake from both foods and supplements should not exceed the UL. Read the label to find out how much a dose supplies. Unless the label states otherwise, supplements of calcium carbonate are 40 percent calcium; those of calcium citrate are 21 percent; lactate, 13 percent; and gluconate, 9 percent. Select a low-dose supplement and take it several times a day rather than taking a large-dose supplement all at once. Taking calcium supplements in doses of 500 milligrams or less improves absorption. Small doses also help ease the GI distress (constipation, intestinal bloating, and excessive gas) that sometimes accompanies calcium supplement use. The next question to ask is how well the body absorbs and uses the calcium from various supplements. Most healthy people absorb calcium equally well from milk and any of these supplements: calcium carbonate, citrate, or phosphate. More important than supplement solubility is tablet disintegration. When manufacturers compress large quantities of calcium into small pills, the stomach acid has difficulty penetrating the pill. To test a supplement's ability to dissolve, drop it into a 6-ounce cup of vinegar, and stir occasionally. A high-quality formulation will dissolve within a half-hour. Finally, people who choose supplements must take them regularly. Furthermore, consideration should be given to the best time to take the supplements. To circumvent adverse nutrient interactions, take calcium supplements between, not with, meals. (Importantly, do not take calcium supplements with iron supplements or iron-rich meals; calcium inhibits iron absorption.) To enhance calcium absorption, take supplements with meals. If such contradictory advice drives you crazy, reconsider the benefits of food sources of calcium. Most experts agree that foods are the best source of most nutrients.

Calcium Roles in the Body Only 1 percent of the body's calcium is in the body fluids. The remaining 99 percent of the body's calcium is in the bones (and teeth), where it plays two roles. First, it is an integral part of bone structure, providing a rigid frame that holds the body upright and serves as attachment points for muscles, making motion possible. Second, it serves as a calcium bank, offering a readily available source of calcium to the body fluids should a drop in blood calcium occur. As bones begin to form, calcium salts form crystals, called hydroxyapatite, on a matrix of the protein collagen. During mineralization, as the crystals become denser, they give strength and rigidity to the maturing bones. As a result, the long leg bones of children can support their weight by the time they have learned to walk. Many people have the idea that once a bone is built, it is inert like a rock. Actually, the bones are gaining and losing minerals continuously in an ongoing process of remodeling. Growing children gain more bone than they lose, and healthy adults maintain a reasonable balance. When withdrawals substantially exceed deposits, problems such as osteoporosis develop (as described in Highlight 12). hydroxyapatite: crystals made of calcium and phosphorus. mineralization: the process in which calcium, phosphorus, and other minerals crystallize on the collagen matrix of a growing bone, hardening the bone. calmodulin: a calcium-binding protein that regulates such cell activities as muscle contractions.

The formation of teeth follows a pattern similar to that of bones. The turnover of minerals in teeth is not as rapid as in bone, however; fluoride hardens and stabilizes the crystals of teeth, opposing the withdrawal of minerals from them. Although only 1 percent of the body's calcium circulates in the extracellular and intracellular fluids, its presence there is vital to life. Calcium in the extracellular fluids helps maintain normal blood pressure and participates in blood clotting. The calcium in intracellular fluids binds to proteins within the cells and activates them. For example, when the protein calmodulin binds with calcium, it activates the enzymes involved in breaking down glycogen, which releases energy for muscle contractions. Many such proteins participate in the regulation of muscle contractions, the transmission of nerve impulses, the secretion of hormones, and the activation of some enzyme reactions.

Water Intake Thirst and satiety influence water intake in response to changes sensed by the mouth, hypothalamus, and nerves. When water intake is inadequate, the blood becomes concentrated (having lost water but not the solutes within it), the mouth becomes dry, and the hypothalamus initiates drinking behavior. When water intake is excessive, the stomach expands, and stretch receptors send signals to stop drinking. Similar signals are sent from receptors in the heart as blood volume increases. When too much water is lost from the body and not replaced, dehydration develops. A first sign of dehydration is thirst, the signal that the body has lost some fluid. If a person is unable to obtain water or, as in many elderly people, fails to perceive the thirst message, the symptoms of dehydration may progress rapidly from thirst to weakness, exhaustion, and delirium—and end in death if not corrected (see Table 12-3). Notice that an early sign of dehydration is fatigue; keep that in mind when considering caffeinated beverages for an afternoon "pick-me-up" and choose water instead. Dehydration develops with either inadequate water intake or excessive water losses. Figure 12-9 illustrates how the color of urine may indicate possible dehydration. (Chapter 14 revisits dehydration and the fluid needs of athletes.)

Thirst: a conscious desire to drink. dehydration: the condition in which body water output exceeds water input. Symptoms include thirst, dry skin and mucous membranes, rapid heartbeat, low blood pressure, and weakness. signs of dehydration (body weight lost % and symptoms) 1-2: thirst, fatigue, weakness, vague discomfort, loss of appetite 3-4: impaired physical performance, dry mouth, reduction in urine, flushed skin, impatience, apathy 5-6: Difficulty concentrating, headache, irritability, sleepiness, impaired temperature regulation, increased respiratory rate 7-10: Dizziness, spastic muscles, loss of balance, delirium, exhaustion, collapse NOTE: The onset and severity of symptoms at various percentages of body weight lost depend on the activity, fitness level, degree of acclimation, temperature, and humidity. If not corrected, dehydration can lead to death.

Osteoporosis and Calcium Osteoporosis (OS-tee-oh-pore-OH-sis) becomes apparent during the later years, but it develops much earlier—and without warning. Few people are aware that their bones are being robbed of their strength. The problem often first becomes evident when someone's hip suddenly gives way. People say, "She fell and broke her hip," but in fact the hip may have been so fragile that it broke before she fell. Even bumping into a table may be enough to shatter a porous bone into fragments so numerous and scattered that they cannot be reassembled. Removing them and replacing them with an artificial joint requires major surgery. An estimated 300,000 people in the United States are hospitalized each year because of hip fractures related to osteoporosis. About one in five die of complications within a year; one in three will never walk or live independently again. Their quality of life slips downward.

This highlight examines low bone density and osteoporosis, one of the most prevalent diseases of aging, affecting more than 53 million people in the United States—most of them women older than 50. It reviews the many factors that contribute to the 2 million fractures in the bones of the hips, vertebrae, wrists, arms, and ankles each year. And it presents strategies to reduce the risks, paying special attention to the role of dietary calcium.

The calcium content of various vegetables is presented below. Which of these vegetables would be the best choice to help you meet your calcium DRI?

Turnip greens, 125 mg in ½ cup cooked

Cut Salt (and Sodium) Intake Salt (sodium chloride) is about 40% sodium and 60% chloride. 1 g salt contributes about 400 mg sodium and 600 mg chloride 6 g salt = 1 tsp 1 tsp salt contributes about 2300 mg sodium and 3700 mg chloride Most people eat more salt (and therefore sodium) than they need. Some people can lower their blood pressure by avoiding highly salted foods and removing the salt shaker from the table. Foods eaten without salt may seem less tasty at first, but with repetition, people can learn to enjoy the natural flavors of many unsalted foods. Strategies to cut salt intake include: Read labels with an eye open for sodium. (See Glossary 2-1, for terms used to describe the sodium contents of foods on labels.) Select products with less sodium and buy low-salt or salt-free products when available. Select fresh or frozen vegetables. If buying canned vegetables, drain and rinse in water to remove some of the sodium or select those labeled low-sodium or no-salt-added. Select fresh or frozen meat, fish, and poultry instead of processed items or select products labeled low-sodium or no-salt-added. Cook foods from scratch at home more often; limit sauces, mixes, and instant products. Cook with little or no added salt. Prepare foods with sodium-free herbs and spices such as basil, bay leaves, curry, garlic, ginger, mint, oregano, pepper, rosemary, and thyme; lemon juice; vinegar; or wine (see Photo 12-6). Add little or no salt at the table; taste foods before adding salt.

Use these foods sparingly: Foods prepared in brine, such as pickles, olives, and sauerkraut Salty or smoked meats, such as bologna, corned or chipped beef, bacon, frankfurters, ham, lunchmeats, salt pork, sausage, and smoked tongue Salty or smoked fish, such as anchovies, caviar, salted and dried cod, herring, sardines, and smoked salmon Snack items such as potato chips, pretzels, salted popcorn, salted nuts, and crackers Condiments such as bouillon cubes; seasoned salts; MSG; soy, teriyaki, Worcestershire, and barbeque sauces; prepared horseradish, ketchup, and mustard Cheeses, especially processed types Canned and instant soups Packaged instant or flavored rice, pasta, and cereal mixes

Physicians can determine bone loss and diagnose osteoporosis by measuring bone density using dual-energy X-ray absorptiometry (DEXA) scanning (see Photo H12-1). They also consider risk factors for osteoporosis, including age, personal and family history of fractures, and physical inactivity. Table H12-1 summarizes the major risk factors for osteoporosis. The more risk factors that apply to a person, the greater the chances of bone loss. Notice that several risk factors that are influential in the development of osteoporosis—such as age, gender, and genetics—cannot be changed. Other risk factors—such as diet, physical activity, body weight, smoking, and alcohol use—are personal behaviors that can be changed. By eating a calcium-rich, well-balanced diet; being physically active; abstaining from smoking; and drinking alcohol in moderation (if at all), people can defend themselves against osteoporosis. These decisions are particularly important for those with other risk factors that cannot be changed. Whether a person develops osteoporosis seems to depend on the interactions of several factors, including nutrition. The strongest predictor of bone density is age.

Using a DEXA (dual-energy X-ray absorpiometry) test to measure bone mineral density identifies osteoporosis, determines risks for fractures, and tracks responses to treatment. risk factors for osteoporosis Nonmodifiable Female gender Older age (>50 yr) Small frame Caucasian, Asian, or Hispanic/Latino Family history of osteoporosis or fractures Personal history of fractures Estrogen deficiency in women (amenorrhea or menopause, especially early or surgically induced); testosterone deficiency in men Female gender Older age (>50 yr) Small frame Caucasian, Asian, or Hispanic/Latino Family history of osteoporosis or fractures Personal history of fractures Estrogen deficiency in women (amenorrhea or menopause, especially early or surgically induced); testosterone deficiency in men Modifiable Sedentary lifestyle Diet inadequate in calcium and vitamin D Diet excessive in protein, sodium, caffeine Cigarette smoking Alcohol abuse Low body weight Certain medications, such as glucocorticoids, aluminumcontaining antacids, and antiseizure drugs

Electrolytes Attract Water Whenever electrolytes move across the membrane, water follows because electrolytes attract water. Each water molecule has a net charge of zero, but the oxygen side of the molecule has a slightly negative charge, and the hydrogen molecules have a slightly positive charge. Figure 12-3 shows the result in an electrolyte solution: both positive and negative ions attract clusters of water molecules around them. This attraction allows salts to dissolve in water and enables the body to move fluids into appropriate compartments.

Water Dissolves Salts and Follows Electrolytse The structural arrangement of the two hydrogen atoms and one oxygen atom enables water to dissolve salts. Water's role as a solvent is one of its most valuable characteristics.

Water intoxication, on the other hand, is rare but can occur with excessive water intake and kidney disorders that reduce urine production. The symptoms may include confusion, convulsions, and even death in extreme cases. Excessive water ingestion (10 to 20 liters) within a few hours dilutes the sodium concentration of the blood and contributes to a dangerous condition known as hyponatremia. For this reason, guidelines suggest limiting fluid intake during times of heavy sweating to between 1 and 1.5 liters per hour. (Chapter 14 revisits the possibility of hyponatremia in endurance athletes.)

Water intoxication: the rare condition in which body water contents are too high in all body fluid compartments. hyponatremia: a decreased concentration of sodium in the blood.

Dehydration is a condition in which body water output exceeds water input. Dehydration may progress rapidly from thirst to weakness, exhaustion, and delirium, and even death if dehydration is not corrected. The onset and severity of symptoms at various percentages of body weight lost depend upon the activity, fitness level, degree of acclimation, temperature, and humidity.

When the percent of body weight lost is 1 to 2 percent, the symptoms include thirst, fatigue, weakness, loss of appetite. When it is 3 to 4 percent, the symptoms include dry mouth, flushed skin, apathy, impaired physical performance, reduced urine volume, impatience. When it is 5 to 6 percent, the symptoms include headache, sleepiness, increased respiratory rate, difficulty concentrating, irritability, impaired temperature regulation. When it is 7 to 10 percent, the symptoms include dizziness, spastic muscles, loss of balance, delirium, exhaustion, collapse.

Water Recommendations Because water needs vary depending on diet, activity, environmental temperature, and humidity, a general water requirement is difficult to establish. Recommendations are sometimes expressed in proportion to the amount of energy expended under average environmental conditions; for adults, for example, 1.0 to 1.5 milliliters per kcalorie expended (roughly one-half cup per 100 kcalories). The recommended water intake for a person who expends 2000 kcalories a day, then, is 2 to 3 liters of water (about 8 to 12 cups). This recommendation is in line with the Adequate Intake (AI) for total water set by the DRI Committee. Total water includes not only drinking water, but water in other beverages and in foods as well. On average, most adults in the United States consume about the AI for total water; older adults tend to consume less. Because a wide range of water intakes will prevent dehydration and its harmful consequences, the AI is based on average intakes. People who are physically active or who live in hot environments may need more. Physically active people must remember to replace their body fluids.

Which beverages are best? Any beverage can readily meet the body's fluid needs, but those with few or no kcalories do so without contributing to weight gain. Given that obesity is a major health problem and that beverages currently represent more than 20 percent of the total energy intake in the United States, water is the best choice for most people. Other choices include tea, coffee, nonfat and low-fat milk and soymilk, artificially sweetened beverages, fruit and vegetable juices, sports drinks, and lastly, sweetened nutrient-poor beverages. Because caffeine acts as a diuretic, people who drink caffeinated beverages may lose slightly more fluid than when drinking water, but the losses are relatively insignificant. The DRI Committee considered such findings in their recommendations for water intake and concluded that caffeinated beverages contribute to the daily total water intake similar to that contributed by noncaffeinated beverages. In other words, it doesn't seem to matter whether people rely on caffeine-containing beverages or other beverages to meet their fluid needs. As Highlight 7 explained, alcohol acts as a diuretic and can impair a person's health. To limit the risks associated with alcoholic beverages, consumers should keep intake moderate and drink plenty of water as well.

sodium: Maintains normal fluid and electrolyte balance; assists in nerve impulse transmission and muscle contraction chloride: Maintains normal fluid and electrolyte balance; part of hydrochloric acid found in the stomach, necessary for proper digestion potassium: Maintains normal fluid and electrolyte balance; part of hydrochloric acid found in the stomach, necessary for proper digestion calcium: Mineralization of bones and teeth; also involved in muscle contraction and relaxation, nerve functioning, blood clotting, and blood pressure Phosphorus Mineralization of bones and teeth; part of every cell; important in genetic material, part of phospholipids, used in energy transfer and in buffer systems that maintain acid-base balance Magnesium Bone mineralization, building of protein, enzyme action, normal muscle contraction, nerve impulse transmission, maintenance of teeth, and functioning of immune system Sulfate As part of proteins, stabilizes their shape by forming disulfide bridges; part of the vitamins biotin and thiamin and the hormone insulin

With all of the tasks these minerals perform, they are of great importance to life. Consuming enough of each of them every day is easy, given a variety of foods from each of the food groups. Whole-grain breads supply magnesium; fruits, vegetables, and legumes provide magnesium and potassium too; milk products offer calcium and phosphorus; meats, poultry, and seafood offer phosphorus and sulfate as well; all foods provide sodium and chloride, with excesses being more problematic than inadequacies. The message is quite simple and has been repeated throughout this text: for an adequate intake of all the nutrients, including the major minerals, choose a variety of foods from each of the five food groups. And drink plenty of water.

angiotensin II

a hormone involved in blood pressure regulation

antidiuretic hormone (ADH)

a hormone produced by the pituitary gland in response to dehydration (or a high sodium concentration in the blood) that stimulates the kidneys to reabsorb more water and therefore to excrete less.

aldosterone

a hormone secreted by the adrenal glands that regulates blood pressure by increasing the reabsorption of sodium by the kidneys

renin

an enzyme from the kidneys that hydrolyzes the protein angiotensinogen to angiotensin I, which results in the kidneys reabsorbing sodium

Angiotensin In addition to its role in sodium retention, renin hydrolyzes angiotensinogen (a protein from the liver) to angiotensin I. Angiotensin I is inactive until another enzyme converts it to its active form—angiotensin II. Angiotensin II is a powerful vasoconstrictor that narrows the diameters of blood vessels, thereby raising the blood pressure. Aldosterone In addition to acting as a vasoconstrictor, angiotensin II stimulates the release of the hormone aldosterone from the adrenal glands. Aldosterone signals the kidneys to excrete potassium and to retain more sodium, and therefore water, because when sodium moves, water follows. Again, the effect is that when more water is needed, less is excreted. All of these actions are presented in Figure 12-6 and help explain why high-sodium diets aggravate conditions such as hypertension and edema. Too much sodium causes water retention and an accompanying rise in blood pressure, swelling in the interstitial spaces, or both. Chapter 18 discusses hypertension in detail.

angiotensinogen: a precursor protein that is hydrolyzed to angiotensin I by renin. angiotensin I: an inactive precursor that is converted by an enzyme to yield active angiotensin II. angiotensin II: a hormone involved in blood pressure regulation. vasoconstrictor: a substance that constricts or narrows the blood vessels. aldosterone: a hormone secreted by the adrenal glands that regulates blood pressure by increasing the reabsorption of sodium by the kidneys. Aldosterone also regulates chloride and potassium concentrations. adrenal glands: glands adjacent to, and just above, each kidney. The renin-angiotensin-aldosterone system helps regulate blood volume and therefore blood pressure.

Variable Bioavailability The bioavailability of minerals varies. Some foods contain binders that combine chemically with minerals, preventing their absorption and carrying them out of the body with other wastes. Examples of binders include phytates, which are found primarily in legumes, seeds, nuts, and grains, and oxalates, which are present in rhubarb, beet greens, sweet potatoes, and spinach, among other vegetables. These foods contain more minerals than the body actually receives for use.

binders: chemical compounds in foods that combine with nutrients (especially minerals) to form complexes the body cannot absorb. Examples include phytates and oxalates.

Blood calcium above normal results in calcium rigor: the muscles contract and cannot relax. Similarly, blood calcium below normal causes calcium tetany (TET-ah-nee)—also characterized by uncontrolled muscle contraction. These conditions do not reflect a dietary excess or lack of calcium; they are caused by a lack of vitamin D or by abnormal secretion of the regulatory hormones. A chronic dietary deficiency of calcium, or a chronic deficiency due to poor absorption over the years, depletes the bones. Again: the bones, not the blood, are robbed by a calcium deficiency.

calcium rigor: hardness or stiffness of the muscles caused by high blood calcium concentrations. calcium tetany: intermittent spasm of the extremities due to nervous and muscular excitability caused by low blood calcium concentrations.

Regulation by the Buffers Bicarbonate (a base) and carbonic acid (an acid) in the body fluids, as well as some proteins, protect the body against changes in acidity by acting as buffers—substances that can neutralize acids or bases. Carbon dioxide, which is formed all the time during energy metabolism, dissolves in water to form carbonic acid in the blood. Carbonic acid, in turn, dissociates to form hydrogen ions and bicarbonate ions. The appropriate balance between carbonic acid and bicarbonate is essential to maintaining optimal blood pH. Figure 12-8 presents the chemical reactions of this buffer system, which is primarily under the control of the lungs and kidneys.

carbonic acid: a compound with the formula ‍H2CO3 that results from the combination of carbon dioxide (CO2) and water (H20); of particular importance in maintaining the body's acid-base balance. Bicarbonate-Carbonic Acid Buffer System The reversible reactions of the bicarbonate-carbonic acid buffer system help to regulate the body's pH and maintain homeostasis. Recall from Chapter 7 that carbon dioxide and water are formed during energy metabolism.

Bone Development and Disintegration Bone has two compartments: the outer, hard shell of cortical bone and the inner, lacy matrix of trabecular bone. Both types of bone can lose minerals, but in different ways and at different rates. The first photograph in Figure H12-1 shows a human leg bone sliced lengthwise, exposing the lacy, calcium-containing crystals of trabecular bone. These crystals give up calcium to the blood when the diet runs short, and they take up calcium again when the supply is plentiful (review Figure 12-14 ). For people who have eaten calcium-rich foods throughout the bone-forming years of their youth, these deposits make bones dense and provide a rich reservoir of calcium. A close look at the first photograph in Figure H12-1 reveals that protecting the trabecular bone is a dense, ivorylike exterior shell—the cortical bone. Cortical bone composes the shafts of the long bones, and a thin cortical shell caps the ends of the bones too. Both compartments confer strength on bone: cortical bone provides the sturdy outer wall, and trabecular bone provides support along the lines of stress.

cortical bone: the very dense bone tissue that forms the outer shell surrounding trabecular bone and comprises the shaft of a long bone. trabecular bone: bone: the lacy inner structure of calcium crystals that supports the bone's structure and provides a calcium storage bank.

parathyroid hormone

hormone that regulates blood calcium by raising it when levels fall too low

Every cell contains fluid of the exact composition that is best for that cell. Fluid inside cells is called intracellular fluid, whereas fluid outside cells is called extracellular fluid. The extracellular fluid that surrounds each cell is called interstitial fluid, whereas the extracellular fluid in the blood vessels is called intravascular fluid. Figure 12-1 illustrates a cell and its associated fluids. The compositions of intercellular and extracellular fluids differ from one another. They continuously lose and replace their components, yet the composition in each compartment remains remarkably constant under normal conditions. Maintaining a balance of about two-thirds of the body fluids inside the cells and one-third outside is vital to the life of the cells. If too much water were to enter the cells, they might rupture; if too much water were to leave, they would collapse.

intracellular fluid: fluid inside the cells, usually high in potassium and phosphate. Intracellular fluid accounts for approximately two-thirds of the body's water. extracellular fluid: fluid outside the cells. Extracellular fluid includes two main components—the interstitial fluid between cells and the intravascular fluid inside blood vessels. Extracellular fluid accounts for approximately one-third of the body's water. interstitial fluid: fluid between the cells (intercellular), usually high in sodium and chloride. Interstitial fluid is a large component of extracellular fluid. intravascular fluid: fluid within blood vessels.

calcium

involved in mineralization of bones and teeth, muscle contraction and relaxation, and in promoting blood clotting

phosphorous

involved in mineralization of bones and teeth, part of phospholipids, used in energy transfer, and in buffer systems that maintain acid-base balance

magnesium

involved in mineralization of bones, forms part of protein-making machinery, and is necessary for energy metabolism

Magnesium Only about 1 ounce of magnesium is present in the body of a 132-pound person (review Figure 12-10). More than half of the body's magnesium is in the bones. Much of the rest is in the muscles and soft tissues, with only 1 percent in the extracellular fluid. As with calcium, bone magnesium may serve as a reservoir to ensure normal blood concentrations.

magnesium: a cation within the body's cells, active in many enzyme systems.

functions of water in the body

maintains blood volume maintains the structure of large molecules such as proteins and glycogen carries nutrients and waste products throughout the body participates in metabolic reactions

chloride

major anion in extracellular fluid; maintains normal fluid and electrolyte balance

sodium

major cation in the extracellular fluid; maintains normal fluid and electrolyte balance

potassium

major cation in the intracellular fluid; maintains normal fluid and electrolyte balance and cell integrity

The Minerals—An Overview Figure 12-10 shows the amounts of the major minerals found in the body and, for comparison, some of the trace minerals. The distinction between the major and trace minerals does not mean that one group is more important than the other—all minerals are vital. The major minerals are so named because they are present, and needed, in larger amounts in the body. They are shown at the top of the figure and are discussed in this chapter. The trace minerals, shown at the bottom of the figure, are discussed in Chapter 13. A few generalizations pertain to all of the minerals and distinguish them from the vitamins. Especially notable is their chemical nature.

major minerals: essential mineral nutrients the human body requires in relatively large amounts (greater than 100 milligrams per day); sometimes called macrominerals. trace minerals: essential mineral nutrients the human body requires in relatively small amounts (less than 100 milligrams per day); sometimes called microminerals. Minerals in a 60-kilogram (132-pound) Human Body Not only are the major minerals needed by the body in larger amounts, but they are also present in the body in larger amounts than the trace minerals. MAJOR: calcium: 1150 phosphorous: 600 potassium: 210 sulfur: 150 sodium: 90 chloride: 90 magnesium: 30 TRACE: iron: 2.4 zinc: 2.0 copper: 0.09 manganese: 0.02 iodine: 0.02 selenium: 0.02

artesian water: water drawn from a well that taps a confined aquifer in which the water is under pressure. carbonated water: water that contains carbon dioxide gas, either naturally occurring or added, that causes bubbles to form in it; also called bubbling or sparkling water. The FDA defines seltzer, soda, and tonic waters as soft drinks; they are not regulated as water. distilled water: water that has been vaporized and recondensed, leaving it free of dissolved minerals. filtered water: water treated by filtration, usually through activated carbon filters that reduce the lead in tap water, or by reverse osmosis units that force pressurized water across a membrane, removing lead, arsenic, and some microorganisms from tap water. mineral water: water from a spring or well that naturally contains at least 250 parts per million (ppm) of minerals. Minerals give water a distinctive flavor. Many mineral waters are high in sodium. well water: water drawn from groundwater by tapping into an aquifer.

natural water: water obtained from a spring or well that is certified to be safe and sanitary. The mineral content may not be changed, but the water may be treated in other ways such as with ozone or by filtration. public water: water from a municipal or county water system that has been treated and disinfected. purified water: water that has been treated by distillation or other physical or chemical processes that remove dissolved solids. Because purified water contains no minerals or contaminants, it is useful for medical and research purposes. spring water: water originating from an underground spring or well. It may be bubbly (carbonated), or "flat" or "still," meaning not carbonated. Brand names such as "Spring Pure" do not necessarily mean that the water comes from a spring.

Water Losses At the very least, the body must excrete enough water to carry away the waste products generated by a day's metabolic activities. This obligatory water excretion (ah-BLIG-ah-TORE-ee) is a minimum of about 500 milliliters (about 2 cups) of water each day. Above this amount, excretion adjusts to balance intake. If a person drinks more water, the kidneys excrete more urine, and the urine becomes more dilute. In addition to urine, water is lost from the lungs as vapor and from the skin as sweat; some is also lost in feces. The amount of fluid lost from each source varies, depending on conditions in the environment (such as heat or humidity) and the body's physical condition (such as exercise or fever). On average, daily losses total about 2500 milliliters. Table 12-2 shows how daily water losses and intakes balance; maintaining this balance requires healthy kidneys and an adequate intake of fluids. An adequate intake of fluids, in turn, helps maintain healthy kidneys and prevent kidney stone formation.

obligatory water excretion: the minimum amount of water the body has to excrete each day to dispose of its wastes—about 500 milliliters (about 2 cups or 1 pint). water sources and amount (mL) beverages-550 to 1500 foods- 700 to 100 metabolism- 200 to 300 total- 1450 to 2800 water losses and amount (mL) kidneys (urine)-500 to 1400 skin (sweat)- 450 to 900 lungs (breath)- 350 GI tract (feces)- 150 total- 1450 to 2800

Calcium Deficiency A low calcium intake during the growing years limits the bones' ability to reach their peak bone mass. Most people achieve a peak bone mass by their late 20s, and dense bones best protect against age-related bone loss and fractures (see Figure 12-17). All adults lose bone as they grow older, beginning between the ages of 30 and 40. When bone losses reach the point of causing fractures under common, everyday stresses, the condition is known as osteoporosis (OS-tee-oh-pore-OH-sis). Osteoporosis and osteopenia (OS-tee-oh-PEE-nee-ah) affect an estimated 54 million people in the United States, mostly older women.

osteoporosis: a disease in which the bones become porous and fragile due to a loss of minerals; also called adult bone loss. osteopenia: a bone condition in which bone loss is significant, but not as severe as in osteoporosis. The active growth phase occurs from birth to approximately age 20. The phase of peak bone mass development occurs between the ages of 12 and 30. The final phase, when bone resorption exceeds formation, begins between the ages of 30 and 40 and continues through the remainder of life.

Calcium Balance Calcium homeostasis involves a system of hormones and vitamin D. Whenever blood calcium falls too low or rises too high, three body systems respond: the intestines, bones, and kidneys. Figure 12-13 illustrates how vitamin D and two hormones—parathyroid hormone and calcitonin—return blood calcium to normal.

parathyroid hormone: a hormone from the parathyroid glands that regulates blood calcium by raising it when levels fall too low; also known as parathormone calcitonin: a hormone secreted by the thyroid gland that regulates blood calcium by lowering it when levels rise too high.

Water Sources The obvious dietary source of water is water itself, which provides about one-third of the total water intake in the United States. In addition, other beverages and nearly all foods also contain water. Most fruits and vegetables contain up to 90 percent water, and many meats and cheeses contain at least 50 percent. See Table 12-4 for selected foods. Also, metabolic water is generated as an end product during condensation reactions and the oxidation of energy-yielding nutrients. Recall from Chapter 7 that when the energy-yielding nutrients break down, their carbon and hydrogen molecules combine with oxygen to yield carbon dioxide and water . As Table 12-2 shows, the water derived daily from these three sources—beverages, foods, and metabolism—averages about 2500 milliliters (roughly 2.5 quarts, or 10.5 cups).

percentage of water in selected foods 90-99: fat-free milk, strawberries, watermelon, lettuce, cabbage, celery, spinach, broccoli 80-89: fruit juice, yogurt, apples, grapes, oranges, carrots 70-79: shrimp, bananas, corn, sweet potatoes, avocados, cottage cheese, ricotta cheese 60-69: pasta, legumes, salmon, ice cream, chicken breast 50-59: ground beef, hot dogs, feta cheese 40-49: pizza 30-39: cheddar cheese, bagels, bread 20-29: pepperoni sausage, cake, biscuits 10-19: butter, margarine, raisins 1-9: crackers, cereals, pretzels, taco shells, peanut butter, nuts 0: oils, sugars

Most of the sodium consumed in the diet comes from:

processed foods

Renin Cells in the kidneys respond to low blood pressure by releasing an enzyme called renin (REN-in). Through a complex series of events, renin causes the kidneys to reabsorb sodium. Sodium reabsorption, in turn, is always accompanied by water retention, which helps to raise blood volume and blood pressure.

renin: an enzyme from the kidneys that hydrolyzes the protein angiotensinogen to angiotensin I, which results in the kidneys reabsorbing sodium.

Electrolytes When a mineral salt such as sodium chloride (NaCl) dissolves in water, it separates (dissociates) into ions—positively and negatively charged particles (Na+ and Cl+). The positive ions are cations; the negative ones are anions. (To remember the difference between cations and anions, think of the "t" in cations as a "plus" sign and the "n" in anions as a "negative.") Unlike pure water, which conducts electricity poorly, ions dissolved in water carry electrical current. For this reason, salts that dissociate into ions are called electrolytes, and fluids that contain them are electrolyte solutions. In all electrolyte solutions, anion and cation concentrations are balanced (the number of negative and positive charges are equal). If a fluid contains 1000 negative charges, it must also contain 1000 positive charges. If an anion enters the fluid, a cation must accompany it or another anion must leave so that electrical neutrality will be maintained. For example, whenever sodium (Na+) ions leave a cell, potassium (K+) ions enter. In fact, it's a good bet that whenever Na+ and K+ ions are moving, they are going in opposite directions. Table 12-1 shows that, indeed, the positive and negative charges inside and outside cells are perfectly balanced, even though the numbers of each kind of ion differ over a wide range. Inside the cells, the positive charges total 202 and the negative charges balance these perfectly. Outside the cells, the amounts and proportions of the ions differ from those inside, but again the positive and negative charges balance. Scientists count these charges in milliequivalents per liter (mEq/L).

salt: a compound composed of a positive ion other than H+ and a negative ion other than OH-. An example is sodium chloride (Na+Cl-) dissociates: physically separates. ions: atoms or molecules that have gained or lost electrons and therefore have electrical charges. Examples include the positively charged sodium ion (Na+) and the negatively charged chloride ion (Cl-). cations: positively charged ions. anions: negatively charged ions. electrolytes: salts that dissolve in water and dissociate into charged particles called ions. electrolyte solutions: solutions that can conduct electricity. milliequivalents per liter (mEq/L): the concentration of electrolytes in a volume of solution. Milliequivalents reveal characteristics about the solution that are not evident when the concentration is expressed in terms of weight.

Sodium is the main cation outside cells and one of the primary electrolytes responsible for maintaining fluid balance. Dietary deficiency is unlikely, and excesses raise blood pressure. For this reason, health professionals advise a diet moderate in salt and sodium. The accompanying table provides a summary of sodium.

sodium AI - adults: 1500 mg/day (19-50yr) 1300 mg/day (51-70 yr) 1200 mg/day (>70 yr) UL - adults: 2300 mg/day Chief functions in body - maintains normal fluid and electrolyte balance; assists in nerve impulse transmission and muscle contraction deficiency symptoms - not from inadequate intakes - hyponatremia from excessive losses toxicity symptoms - edema, acute hypertension significant sources - table salt, soy sauce; moderate amounts in meats, milks, breads, and vegetables; large amounts in processed foods

Sodium and Bone Loss (Osteoporosis) A high salt intake is also associated with increased calcium excretion, but its influence on bone loss is less clear. In addition, potassium may prevent the calcium excretion caused by a high-salt diet. For these reasons, dietary advice to prevent bone loss parallels that suggested for hypertension—a DASH eating pattern that is low in sodium and abundant in potassium-rich fruits and vegetables and calcium-rich low-fat milk.

sodium in foods In general, processed foods have the most sodium, whereas unprocessed foods such as fresh fruits and vegetables have the least. In fact, most of the sodium in people's diets comes from salt added to foods during food processing and preparation; only a small proportion comes from salt added during cooking and at the table or from the natural content in foods. Among foods with the highest sodium density (milligrams of sodium per kcalorie) are mixed foods such as sandwiches, pizza, tacos, and soups. Because sodium intake tends to increase as kcalories increase, making food choices based on low sodium density is a practical and effective way to meet sodium recommendations. To help consumers limit their intake, public health organizations and policymakers worldwide have called for manufacturers and restaurants to reduce sodium in the food supply. Quite simply, removing salt from processed foods is a more effective option than educating consumers. Unfortunately, several studies report that these voluntary reductions in sodium by the food industry are slow and not particularly meaningful. Because processed foods may contain sodium without chloride, as in additives such as sodium bicarbonate or sodium saccharin, they do not always taste salty. Most people are surprised to learn that 1 ounce of some cereals contains more sodium than 1 ounce of salted peanuts—and that ½ cup of instant chocolate pudding contains still more. The peanuts taste saltier because the salt is all on the surface, where the tongue's taste receptors immediately pick it up. Figure 12-11 shows that processed foods not only contain more sodium than their less-processed counterparts but also have less potassium. Low potassium may be as significant as high sodium when it comes to blood pressure regulation, so processed foods have (at least) two strikes against them.

Solutes Attract Water The concentration of a solution reflects the amount of a solute relative to its fluid. Consider saltwater, for example. One cup of water with one teaspoon salt dissolved in it has the same concentration as one-half cup water with one-half teaspoon salt dissolved in it; the proportions of salt to water are the same in the two solutions and they would taste the same with respect to their saltiness. Adding more water would dilute the solution, making it less concentrated; it wouldn't taste as salty. Adding more salt would make the solution more concentrated; it would taste saltier. Solutes attract water. The movement of water across a membrane toward the more concentrated solutes is called osmosis. The amount of pressure needed to prevent the movement of water across a membrane is called the osmotic pressure. Figure 12-4 illustrates osmosis, and Photos 12-2 and 12-3 provide familiar examples.

solute: the substances that are dissolved in a solution. The number of molecules in a given volume of fluid is the solute concentration. osmosis: the movement of water across a membrane toward the side where the solutes are more concentrated. osmotic pressure: the amount of pressure needed to prevent the movement of water across a membrane. When immersed in water, raisins become plump because water moves toward the higher concentration of sugar inside the raisins. When sprinkled with salt, eggplant and other vegetables "sweat" because water moves toward the higher concentration of salt outside the vegetable.

sulfur

stabilize protein structure

Some Closing Thoughts Unfortunately, many of the strongest risk factors for osteoporosis are beyond a person's control: age, gender, and genetics. But several strategies are effective for prevention. First, ensure an optimal peak bone mass during childhood and adolescence by eating a balanced diet rich in calcium and vitamin D and by engaging in regular physical activity. Then, maintain that bone mass in early adulthood by continuing those healthy diet and activity habits, abstaining from cigarette smoking and using alcohol moderately, if at all. Finally, minimize bone loss in later life by maintaining an adequate nutrition and exercise regimen, and, especially for older women, consult a physician about bone density tests, calcium supplements, or drug therapies that may be effective both in preventing bone loss and in restoring lost bone. The reward is the best possible chance of preserving bone health throughout life.

summary: Water is the most essential nutrient to life, needed in greater quantities each day than any other nutrient. The body maintains its fluid balance with the help of minerals, inorganic elements also required by the body. Minerals play important roles in good health beyond their roles in maintaining water balance. Focal Points 12.1 Water and the Body Fluids Water makes up about 60 percent of the adult body weight; water balance is maintained through intake from liquids, foods and metabolism versus what is lost from the kidneys, skin, lungs, and GI tract. 12.2 The Minerals—An Overview Minerals are inorganic elements required for proper body function; they are divided into the major and trace mineral groups. 12.3 The Major Minerals Major minerals influence fluid balance, play important roles in nerve and muscle function, and participate in many reactions involving glucose

mineralization

the process in which calcium, phosphorus, and other minerals crystallize on the collagen matrix of a growing bone, hardening the bone.

The Color of Urine in Relation to Hydration The color of urine gives a hint to a person's hydration status, but cannot be used for diagnosis. In addition to dehydration, the color of urine may indicate the presence of food dyes; blood; bladder, liver, or kidney disease; and medications.

transparent: possible over-hydration pale straw: normal, well hydrated transparent yellow: normal dark yellow: normal, possible mild dehydration deep amber or honey: normal, possible moderate dehydration orange: possible severe dehydration

Water Balance and Recommended Intakes Because imbalances can be devastating, the body actively maintains an appropriate water balance between intake and output. Consequently, the entire system of cells and their fluids remains in a delicate, but controlled, state of homeostasis.

water balance: the balance between water intake and output (losses).

Water is an essential nutrient, more important to life than any of the others (see Photo 12-1). The body needs more water each day than any other nutrient. Furthermore, you can survive only a few days without water, whereas a deficiency of the other nutrients may take weeks, months, or even years to develop.

water is the most indispensable nutrient The body maintains an appropriate balance and distribution of fluids with the help of another class of nutrients—the minerals. In addition to introducing the minerals that help regulate body fluids, this chapter describes many of the other important functions minerals perform in the body. Chapter 19 revisits water as a beverage and addresses consumer concerns about its safety; Chapter 20 looks at water usage in agriculture and food production and examines the impending water shortages we face.

The calcium in bones provides a nearly inexhaustible bank of calcium for the blood. The blood borrows and returns calcium as needed so that even with an inadequate diet, blood calcium remains normal—even as bone calcium diminishes (see Figure 12-14). Blood calcium changes only in response to abnormal regulatory control, not to diet. A person can have an inadequate calcium intake for years and have no noticeable symptoms. Only later in life does it become apparent that bone integrity has been compromised.

with an adequate intake of calcium-rich food, blood calcium remains normal... and bones deposit calcium. The result is strong, dense bones with a dietary deficient, blood calcium still remains normal/// because bones give up calcium to the blood. The result is weak, osteoporotic bones