Nutrition study guide chp 9

What are vitamins cont'd

-soluble vitamins are not stored in large amounts and are generally excreted when their intake is excessive. Even through they are not stored, excesses can still be harmful. Figure 9.2 shows the two paths for fat- and water-soluble vitamins in terms of their digestion and absorption. Not all vitamins in food are 100 percent bioavailable (available to be used in the body). Bioavailability is influenced by: the amount of vitamin in the food; if the food is coked, raw, or refined; efficiency of digestion and absorption; existing nutritional status of the individual; and if the vitamin is natural or synthetic. The greater need for a vitamin, the greater the amount that will be absorbed. Fat-soluble vitamins tend to be less bioavailable than water-soluble vitamins. Vitamins in plant-foods are generally less bioavailable than those in animal foods. Numerous conditions can change vitamin activity. For example, several environmental factors hinder vitamin activity and vitamin stability: air, heat, pH, oxygen levels, time since harvest, and storage conditions. Fat-soluble vitamins tend to be more stable than water-soluble vitamins. Vitamin toxicity (hypervitaminosis) is rare but can occur, particularly when megadoses of vitamin supplements are ingested. To prevent excessive intake, the DRIs include a tolerable upper intake level for most vitamins. Provitamins are really vitamin precursors and can be converted into an active form once they are absorbed. Preformed vitamins are already active and do not need to undergo conversion to be useful to the body.

What are vitamins cont'd

A compound is classified as a vitamin when (1) it cannot be built in ample amounts by the body and (2) physical symptoms have been linked to a chronic deficiency. Symptoms of deficiency should resolve when the vitamin has been fully restored in the body, as long as permanent damage has not resulted. Using these criteria, 13 compounds are classified as vitamins (four fat-soluble vitamins and the remaining nine compounds as water-soluble). Figure 9.1 categorizes vitamins as either water- or fat-soluble. Vitamins are organic (they contain carbon) and are single units so no digestion is required prior to absorption. Table 9.1 shows the many roles of vitamins in promoting health. All vitamins are absorbed in the small intestine, although the method of absorption differs in fat-soluble and water-soluble vitamins. Fat-soluble vitamins are primarily absorbed in the duodenum. Fat-soluble vitamins are packaged with fatty acids and bile, shuttled through cells in the intestinal wall, are packaged into chylomicrons with fat and other lipids, travel through the lymph system, and then enter the bloodstream. Absorption of these vitamins can be compromised if the body is lacking adequate fatty acids or bile. Fat-soluble vitamins are stored in the body; if too much of a fat-soluble vitamin is ingested, it can lead to toxicity, potentially causing the body harm.

Daily needs

Adults require 15 mgs of vitamin E each day. Most Americans are probably deficient in their vitamin E intake.

Absorption and transport

All forms of preformed vitamin A are absorbed by active transport in the small intestine. The rate of absorption of preformed vitamin A is 70 to 90 percent. Beta-carotene is absorbed via passive diffusion at a rate up to 22 percent. Dietary fat enhances the absorption of vitamin A. Beta-carotene absorption is improved when foods are cooked, but reduced with high fiber intakes. Vitamin A is stored in the liver and is difficult to excrete, leading to a possibility of vitamin toxicity (excretion through bile helps to prevent toxicity).

Vitamin E

Alpha-tocopherol is the most active form of vitamin E in the body. The type of vitamin E used in supplements is only half as active as the natural form. Figure 9.13 shows the structure of alpha-tocopherol.

Food sources

Animal foods (liver in particular) and some specific plant foods are the recommended source for vitamin A. Specific recommendations include milk, cereals, cheese, eggs, liver, carrots, spinach, and sweet potatoes. Inclusion of vegetable oil in a daily diet can improve absorption of carotenoids.

What are Antioxidants?

Antioxidants counter the oxidation that takes place in cells. The group of antioxidant compounds includes vitamin C, vitamin E, selenium, flavonoids, and carotenoids. Free radicals (unstable molecules with an unpaired electron) damage cells by altering cell structure, body proteins, and DNA. They are the by-products of metabolic reactions, a result of exposure to environmental chemicals or from the damaging effects of ultraviolet rays (from the sun). Free radicals search for an electron to incorporate for stability, stealing from another molecule that then becomes a free radical itself; they can also release an electron onto another molecule, creating another free radical. Antioxidants help neutralize free radicals to halt further cell destruction. Oxidative stress can occur when free radicals accumulate at a rate faster than the body can neutralize them. Damaging results of oxidative stress can contribute to chronic diseases and conditions, including heart disease, cancer, aging, diabetes, arthritis, Parkinson's disease, and Alzheimer's disease.

Daily needs

Based on the best estimates, adult women need 90 mcgs and men need 120 mcgs per day of dietary vitamin K.

Functions of vitamin A

Each form of retinol has a specific role in the body. The most well-known function of vitamin A is the role it plays in vision; see Figure 9.7 for a detailed diagram. Vitamin A also plays roles in protein synthesis, cell division and differentiation, growth, reproduction, bone health, and as an antioxidant. Figure 9.8 shows the role of vitamin A in producing healthy epithelial cells. Retinoid-containing medications are used to help treat acne; for example, topical medication can be used to encourage turnover of skin cells and inhibit acne formation.

What are Antioxidants

Figure 9.3 illustrates the action of free radicals and how antioxidants neutralize or overcome the damaged atoms. Free radicals play a part in eye damage, contributing to age-related macular degeneration (AMD) and cataracts. AMD damages the portion of the eye needed for central vision. A cataract is diagnosed when the lens of the eye becomes cloudy, resulting in blurred vision. Figure 9.4 shows three photographs of normal and impaired vision in relationship to cataracts and macular degeneration. Some current research from the National Eye Institute reveals favorable activity from vitamin C, E, and beta carotene regarding vision protection and macular degeneration. Phytochemicals, naturally occurring plant compounds that give plant food their vibrant colors, act as antioxidants in the body. Carotenoids and flavenoids are considered antioxidants. Table 9.2 lists the various colors or plant pigments, their phytochemicals, and actual foods that contain that nutrient. Studies are underway to determine the worth of antioxidant supplements in the diet. Currently, supplements are not recommended; instead, phytochemical- and antioxidant-rich foods should be part of a balanced diet. Antioxidants can potentially play a role in cancer prevention (see pages 346-349).

Food sources

Green vegetables, salads, cabbage, vegetable oils, and margarine are good sources of vitamin K.

Daily needs

Sunshine provides some of our vitamin D, but not everyone can depend on sun exposure to meet all vitamin D needs. Figure 9.12 shows several cities across the nation that are considered unreliable regarding vitamin D provision, especially in the winter season. Additionally, individuals with dark skin pigments and those who faithfully apply sunscreen may need even more sun exposure to receive adequate vitamin D. Daily intake recommendations are based on intake from foods rather than intake from the sun. The recommendation for adults is 5 to 15 mcgs (200 to 600 IU) per day of vitamin D. Research suggests it is safe to recommend 400 IU per day for children. Some individuals may need 800 to 1,000 IU per day to compensate for the lack of sunlight exposure. Food sources Fortified milk, breakfast cereals, yogurt, and fatty fish are the best food sources of vitamin D. Too much or too little The upper level is 50 mcgs (or 2,000 IU). Hypervitaminosis D can occur from excess intake (generally through supplements), causing overabsorption of calcium from intestines and calcium loss from bones. Hypercalcemia leads to damaging calcium deposits in soft tissues. High amounts of vitamin D can also affect the nervous system and cause severe depression. Excess intake from the sun or from foods is rarely a worry, unless intake from fish oil is excessive. Insufficient vitamin D is also a worry. Rickets is on the rise as a consequence of a lack of sunshine and low dietary levels of vitamin D intake by children. Bones are inadequately mineralized, causing them to weaken; the result is often bowed legs. Rickets became less of a concern in the United States due to the fortification of milk with vitamin D, but rates are rising again as a result of changes in children's diet and lifestyle. Osteomalacia is another vitamin D deficiency condition and it is considered the adult equivalent of rickets. Osteoporosis is also linked to vitamin D deficits because of deceased calcium absorption.

Functions of vitamin K

The best-known function of vitamin K is the regulation of blood clotting. Figure 9.16 displays the role of vitamin K in blood clotting. Anticoagulation medications and severe liver disease can result in low blood-levels of vitamin K, increasing the risk of hemorrhage. 2. Vitamin K also promotes strong bones. 3. Most dietary vitamin K is absorbed in the jejunum; both forms of vitamin K are incorporated into chylomicrons and transported to the liver for storage in small amounts.

What Are Vitamins

The body requires vitamins (tasteless, organic compounds) in small amounts to maintain normal metabolic functions. Vitamins help regulate metabolism, convert energy into ATP, promote growth and reproduction, and help avoid serious conditions related to deficiency. Vitamins remained mostly undiscovered until about a century ago. While it was recognized that certain foods were necessary for health, historically it was not known why the foods were needed (such as citrus to help ward off scurvy). After specific vitamins were identified to help prevent diseases such as beriberi, scurvy, and rickets, it was recognized that some vitamins could be added to grains and milk to promote public health. Focus on vitamin research has shifted from curing disease to disease prevention. Vitamins were originally named alphabetically as they were identified, but this has evolved as B vitamins were found to have more than one physiological function. Some vitamins are now better known by their chemical names, such as thiamin and riboflavin.

What are the fat- soluble vitamins

The fat-soluble vitamins, A, D, E, and K are insoluble in water and require bile and the formation of micelles for absorption. After absorption in the small intestine, the vitamins are packaged into chylomicrons for transport throughout the body. Chylomicron remnants (containing fat-soluble vitamins) are taken up by the liver for storage. When the vitamins are needed, a protein transports each vitamin through the blood. Fat-soluble vitamins do not need to be consumed daily, but should be part of a varied diet. Table 9.4 provides a detailed comparison of the vitamins, including food sources, functions, deficiency symptoms, toxicity concerns, and adult RDA value.

Too little or too much

The upper level is 3,000 mgs per day. a. Those with hypervitaminosis A have chronically much more than the daily recommended dose, leading to toxic buildup of vitamin A in the liver. Hypervitaminosis A can lead to deterioration and scarring of the liver and possibly death. Taking vitamin A supplements can lead to difficulties. Too much vitamin A in a single dose can cause nausea, vomiting, headache, dizziness, and blurred vision. Higher intake during pregnancy can cause facial and skull defects in the child, and potential CNS damage. Some research suggests that high intake levels can contribute to osteoporosis. Upper levels apply only to preformed vitamin A from foods, fortified foods, and supplements; provitamin A carotenoids in foods are not toxic and don't pose health risks. Carotenodermia (orange-tinged skin) is considered much less serious than hypervitaminosis, and can be cured by cutting back on carotenoid-rich food consumption. High intake of carotenoid supplements in smokers can potentially lead to higher risk of lung cancer. A deficiency in vitamin A can lead to problems, particularly related to vision. If a diet is deficient in vitamin A, night blindness (the inability to see in the dark) can result. A prolonged vitamin A deficiency can lead to complete blindness. A severe eye disease called xerophthalmia is primarily an international condition, but it does impact thousands of children around the globe. Vitamin A deficiency is the leading cause of preventable blindness in children. Vitamin A deficiency can also cause keratinization of the epithelial tissues, resulting in a hard, dry epithelial cell that is unable to secrete a protective layer of mucus. i. Cells are unable to function properly, resulting in an environment susceptible to infection.

Too much too little

The upper limit is 1,000 mgs per day. Oversupplementation may cause harm, but there is no known concern from consuming foods rich in vitamin E. Excess amounts of vitamin E can increase risk of hemorrhage, and may be dangerous for those at risk of heart disease. It is unlikely but possible to be low in vitamin E, particularly for those who can't properly absorb fat. Nerve problems, muscle weakness, and poor red blood cell membrane production could be the result of a chronic deficiency of vitamin E.

Food sources

Vegetable oils, avocado, nuts, seeds, green vegetables, and fortified cereals are good sources of vitamin E.

Vitamin A

Vitamin A is a group of fat-soluble retinoids including retinol, retinal, and retinoic acid. As illustrated in Figure 9.6, there are several vitamin A compounds. Retinoids are preformed vitamin A and, as such, are in a form that can be readily used by the body; preformed vitamin A is primarily found in animal foods. Retinol is the most usable to the body. Plant foods usually contain provitamin A compounds, precursors to retinol in the body. Beta-carobene, beta-cryptoxanthin, and alpha carotene are carotenoids, pigments that give some foods a deep-orange color.

Daily needs

Vitamin A is measured in two ways: micrograms of retinol activity equivalents (RAE) and in international units (IU). The retinol equivalent is a relatively new way of expressing vitamin A activity; it takes into consideration the conversion of precursor forms of vitamin A into functional vitamin A. i. 1 µg RAE = 3.3 IU Adult women need 700 mcgs of RAE and men need 900 mcgs of RAE each day. The Institute of Medicine suggest an intake of 3 to 6 mgs of beta-carotene per day through foods, although a daily recommendation hasn't been established.

Vitamin D

Vitamin D (calciferol) is derived from the reaction between ultraviolet rays and a form of cholesterol found in the skin. Up to 100 percent of vitamin D needed by the body can be synthesized by exposure to sunlight, making it a conditionally essential nutrient. A deficiency of the nutrient can cause symptoms that are cured once adequate intake has been restored. Vitamin D is found in the form of cholecalciferol (produced in the skin and found in animal foods) and ergocalciferol (found in plants and dietary supplements). Figure 9.9 shows the chemical structures of vitamin D. Vitamin D enters the body in an inactive form, is converted into precalciferol, which is then changed to cholecalciferol and absorbed through the skin into the blood, then taken up by the liver. Figure 9.10 shows the elaborate use, activation, and metabolism of vitamin D. Vitamin D is absorbed like a fat or lipid compound and circulates in the lymph system before it is stored in the liver. Vitamin D's metabolism is influenced by blood calcium levels. Figure 9.11 illustrates the relationship between vitamin D and parathyroid hormone and calcium.

Function of Vitamin D

Vitamin D has multiple established functions and new functions seem to emerge all the time. Vitamin D regulates calcium and phosphorus and plays a role in bone growth, disease prevention, immune system regulation, and blood pressure management. Calciferol stimulates the absorption of calcium and phosphorus in the intestinal tract and reduces the amount of these minerals excreted in urine. A lack of vitamin D may allow cancer cells to flourish. People with low levels of vitamin D in the blood may have a higher risk of diabetes mellitus. It's possible that people with hypertension may be able to alleviate symptoms by increased sun exposure, leading to increased intake of vitamin D.

Functions of vitamin E

Vitamin E is a potent antioxidant and a blood anticoagulant. Figure 9.14 illustrates the role that vitamin E plays in cell membrane protection as an antioxidant. Bile salts and micelles aid absorption of vitamin E. Once absorbed in the small intestine, it is transported through lymph fluid as part of a chylomicron. Most vitamin E is stored in adipose tissue.

Vitamin K

Vitamin K is found naturally in two forms, phylloquinone and menaquinone, and supplied in vitamin supplements in a third form (menadione). Figure 9.15 shows the structure of two naturally occurring forms of vitamin K.

Too much too little

Vitamin K is not a vitamin that has excess intake concerns, so there is no upper intake level at this time. 2. Vitamin K deficiency is also rare in healthy people. a. People who take prescription medicine to thin their blood or who have illnesses affecting fat absorption should monitor the intake of vitamin K in their diet.

What's the Best Source of Vitamins?

Whole foods are rich in phytochemicals, antioxidants, and fiber. The Dietary Guidelines for Americans recommended intake of fruits, vegetables, whole grains, and dairy products has increased to help encourage the opportunity to meet daily vitamin needs. Figure 9.5 divides the MyPyramid into five food groups and shows the primary vitamin contributions from those foods. Table 9.3 illustrates how the DRIs can be met with healthy food choices. If consuming a balanced diet, it's generally unnecessary to consume vitamins from synthetic sources. See pages 352-354 for an in-depth discussion of fortified foods and supplements.