Energy Balance and Obesity Chapter 11 Nutrition

Analyzing Body Composition BMI and waist circumference provide useful estimates of body fat in most people, but both are indirect measures; they do not actually measure an individual's body composition in terms of fat, muscle, and bone, for example. A variety of additional methods that better determine body composition are available to researchers and health professionals. These include measuring skinfold thickness with calipers, underwater weighing, and air displacement. All methods estimate percent body fat but vary in terms of their sophistication, cost, and accuracy. Levine and his colleagues, for example, used a highly regarded research method called dual-energy x-ray absorptiometry (DEXA), which uses a small dose of radiation in the form of two x-ray beams with different energy levels to distinguish between fat, muscle, and bone. One method of estimating body composition that has become accessible to the general public is bioelectrical impedance, which is the method used by home bathroom scales or handheld devices that measure body fat content. It is interesting to note that although scientists and healthcare providers agree that excess body fat is risky, there is little scientific consensus and no established standards on exactly how much excess body fat would identify someone as obese or at increased health risk. (INFOGRAPHIC 11.13)

For the average person, BMI is still the most accessible indicator of health risk, because it only requires knowing your height and weight. BMI is also what health professionals (and insurance companies) typically use to help determine what intervention may be warranted for overweight or obese patients.

THE BIOLOGY OF HUNGER Given energy's central role in sustaining life, it is not surprising that obtaining and storing energy has a complex physiological regulation. Our bodies have built-in mechanisms that let us know when we are hungry and when we have eaten enough (if we pay attention). Through this complex physiological control system, which involves a constant dialogue between our brains and our gastrointestinal tract, we are able to obtain and maintain sufficient energy stores to power our activities. Two different systems regulate energy balance and food intake—a short-term system and a long-term system. The short-term system, mediated by hormones and stomach pressure, triggers hunger and satiety (the opposite of hunger), respectively, before and after individual meals. The long-term system, mediated by a different set of hormones, adjusts food intake and energy expenditure to maintain adequate fat stores (adipose cells and tissue). Hormones Participate in Energy Balance When we haven't eaten for a while, our stomach begins to grumble. That grumbling is a sign that a hormone called ghrelin is racing into action. Ghrelin, nicknamed the "hunger hormone," is a 28-amino acid peptide hormone that is produced primarily in the stomach. It is the only hormone that has been found to increase hunger. Circulating ghrelin levels in the blood surge just before meals and decrease after eating. Ghrelin stimulates hunger by activating specific neurons in the brain. Ghrelin secretion decreases only when nutrients from the meal are absorbed into the blood. The secretion of ghrelin is most effectively inhibited by carbohydrates and then by proteins; dietary fat is least effective at decreasing ghrelin secretion. Satiation is the process that leads to the termination of a meal and refers to the sense of fullness that we feel while eating. Satiety is the effect the meal has on our interest in food after a meal; it operates in the interval between meals and affects when we feel hungry again. The primary factors affecting these two processes are gastric distention—how much our stomach has expanded to take in food—and the release of hormones produced by specialized cells in the gastrointestinal tract. Nerves in the stomach sense its expansion and relay signals to the brain to communicate satiation, a sense of fullness. At the same time, several gut peptide hormones are produced by specialized cells in the small intestine in response to the detection of nutrients in the gut. Importantly, calories in beverages appear to bypass mechanisms of satiation. A number of studies have demonstrated that soft drink calories are less satiating than calories from solid foods. Over the long term, energy balance is affected by a hormone called leptin. Leptin is produced primarily by adipose tissue. The circulating concentration of leptin in our blood is closely associated with total body fat. When fat stores increase, more leptin is produced. The leptin level increase in the blood acts on the brain to suppress hunger and increases energy expenditure to avoid excess weight gain. (INFOGRAPHIC 11.5)

If leptin suppresses hunger and overweight or obese people have more leptin-producing fat cells, then why do people ever become obese? It turns out that obese individuals have higher levels of circulating leptin than lean individuals, but the obese individuals seem to be resistant to the hunger-suppressing effects of leptin. Thus, they don't experience the same diminished hunger or increased expenditure of energy as their lean counterparts. Some evidence suggests that a high-fructose diet (primarily through excess consumption of sweetened beverages) may contribute to leptin resistance in humans and contribute to increased appetite and food intake.

LIFESTYLE AND ENERGY BALANCE One hundred fifty years ago, 90% of the world's population lived in agricultural regions. Much like our distant evolutionary ancestors, they walked to work, performed manual labor, and walked home at the end of the day. They manually prepared their food and washed their clothes. Physical work—and thus caloric expenditure—was required to get the job of living done.

Modern conveniences and technologies encourage inactivity. Today, in developed countries, most people live in cities and work behind a computer. A modern person sits during his or her drive to work; sits all day at work; sits to drive home; and sits in the evening watching television, surfing the Internet, or playing video games. "In a mere 150 years," Levine estimates, "Homo sapiens has become addicted to the chair." Notably, in adopting this sedentary lifestyle, Levine estimates that humans have decreased their NEAT by approximately 1500 kcal per day. At the same time, we have continued to eat similar amounts of—if not more—food. As a result, we are experiencing a population-wide positive energy balance, which Levine believes has led to our current obesity epidemic. (INFOGRAPHIC 11.9) Of course, not everyone who lives in our modern society becomes obese. Why is that? Levine would say that is because some people naturally have higher NEAT than others. These high-NEAT individuals are the toe-tappers and fidgeters among us who just can't seem to sit still. In recent studies, Levine installed an under-the-table apparatus that promotes leg movement in office workers to mimic fidgeting and found the device increased energy expenditure by approximately 20%. Fortunately, our NEAT quotient is made up of more than our propensity to fidget, and some of it is within our conscious control. "The brain is just like a muscle," explains Levine. "If you exercise a muscle, it gets bigger and stronger, and if you exercise and activate your brain, your brain becomes NEATER."

KEY IDEAS

Obesity is at epidemic proportions in the United States, with approximately 70% of adults classified as overweight or obese. Obesity increases the risk of multiple chronic diseases and premature death. Hormonal changes associated with excess body fat cause a low-grade chronic inflammation throughout the body that has adverse health effects. Energy, the capacity to do work, is required to perform all functions necessary to sustain life and is obtained through the breakdown of carbohydrates, fats, proteins, and alcohol in food. A calorie, a unit of measure, is defined as the energy required to raise 1 g of water 1°C. The energy in food is commonly measured in units of kilocalories (1000 calories). A kilocalorie (kcal) is the energy required to raise 1 kg of water 1°C. Energy balance is a reflection of energy intake versus expenditure. Although many factors contribute to the development of obesity, fundamentally it results from chronic positive energy balance. Negative energy balance is necessary for weight loss. Energy balance and food intake are regulated through a short-term system and a long-term system. The short-term system is mediated by hormones and stomach pressure and is responsible for triggering hunger and satiety during individual meals. The long-term system, mediated by a different set of hormones, adjusts food intake and energy expenditure to maintain adequate fat stores. Satiation is the sense of fullness we feel while eating and leads to the termination of a meal. Satiety is the effect the meal has on our interest in food and hunger levels after and between meals. The hormone ghrelin stimulates hunger by activating specific neurons in the brain. Ghrelin levels in the blood increase just before meals and decrease after eating. The hormone leptin is produced primarily by adipose (fat) tissue and has a role in long-term energy balance. Its circulating concentration is closely associated with total body fat. Hunger is the biological impulse that drives us to seek out food and consume it to meet our energy needs. Appetite is a desire for or liking of food for reasons other than, or in addition to, hunger. Total energy expenditure (TEE) is composed of basal metabolism, the thermic effect of food, and activity energy expenditure. Basal metabolism is the energy expenditure required to maintain the essential functions that sustain life. It accounts for about 60% of TEE in a typical sedentary individual, with most variation from person to person accounted for by differences in fat-free mass (FFM). Thermic effect of food (TEF) is the energy needed to digest, absorb, and metabolize nutrients in our food. TEF is generally equivalent to 10% of the energy content of the food ingested and does not vary greatly between people. Activity energy expenditure (AEE) is the amount of energy individuals expend in physical activity per day and is the most variable component of TEE. Body mass index (BMI) is an indirect measure of body fat calculated from a person's weight and height. "Underweight," "normal," "overweight," and "obese" are all labels for ranges of body weight on the BMI scale. The greater the BMI, the higher the risk of premature mortality and the risk of obesity-associated diseases. A BMI in the underweight range is also associated with increased premature mortality. Waist circumference indicates body fat distribution and the presence of excess visceral fat, which has been shown to be an independent health risk. Body composition can be measured in a variety of ways, including skinfold thickness with calipers, underwater weighing, air displacement, bioelectrical impedance, and dual-energy x-ray absorptiometry. Weight loss, including dietary modifications and increased physical activity, is recommended for anyone with a BMI of 30 or higher or those who are overweight and have two or more risk factors or have a large waist circumference. For those with extreme obesity (a BMI of at least 40 or a BMI of at least 35 with additional risk factors), weight loss (bariatric) surgery may be recommended. Modest weight loss and maintenance (5-10% of body weight) can improve health as well as reduce the risk of chronic diseases and premature death. Healthy weight loss plans maximize nutrient density while reducing calorie intake, increase physical activity, incorporate behavioral strategies to enhance compliance, address individual health concerns, and consider maintenance of a lower body weight.

Weight Loss Maintenance The likelihood of regaining weight lost through any diet or program is well known and discouraging. However, the National Weight Control Registry (NWCR) provides hope and strategies through the findings from data collected from more than 10,000 people who have successfully maintained long-term weight loss. According to the NWCR, most "successful losers" share common strategies that include maintaining a lower-fat and reduced-calorie eating plan, eating breakfast, weighing themselves at least once a week, watching fewer than 10 hours of television per week, and exercising on average about one hour per day. When we first caught up with Levine to discuss NEAT, we found him in transit, walking to his office in downtown Phoenix, Arizona. Levine walks a lot. He routinely conducts meetings, interviews, and many other tasks on the go. "Contrary to popular belief, I do sit from time to time," says Levine, but he makes a habit of doing most things standing if he can. Asked just how bad sitting is for you, Levine rattles off a list of 16 associated health risks including obesity, diabetes, hypertension, high cholesterol, cardiovascular disease, depression, swollen ankles, joint problems, back pain, depression, and cancer. Even one's creativity, he suggests, may be dulled from sitting too much. That's why Levine has called for a "moratorium on the chair." He believes it is time to fundamentally redesign our environments so that higher NEAT is the norm. Toward that end, he is working with the mayor of Phoenix on initiatives to encourage more walking among commuters, and Levine works with businesses and school systems to make workplaces and classrooms more active—or, as Levine puts it, "NEATER." "What's really cool about NEAT is that everyone can do it," says Levine. "You can have somebody who's 150 pounds going for a 'walk and talk' meeting with someone who's 350 pounds. You can promote NEAT in all people without having to change their clothes." Although it's easy to get discouraged about our modern obesity epidemic—as discouraged, perhaps, as a person who tries desperately and fails to lose weight—Levine stresses that obesity has crept up on us slowly. "The obesity epidemic has occurred over about four generations," he notes. If little steps got us into this mess, he notes, then little steps may get us out—provided they are done consistently. The degree of positive energy balance that has produced our obesity epidemic has been termed our "energy gap." Addressing the energy gap means identifying the change in energy expenditure relative to energy intake necessary to restore energy balance. It turns out that the amount is less than you might think. By some estimates, a lifestyle change that reduced energy intake or increased energy expenditure by 100 kcal per day would completely abolish the energy gap, and hence weight gain, for most of the population. Walking just one extra mile each day would increase one's energy expenditure by about 50-100 kcal per day, depending on body weight. You can estimate the energy you expend in walking by using this relationship: For every mile you walk (between 2 and 4 miles per hour), the energy you expend (in kilocalories) is essentially equal to your body weight in kilograms. For example, if you weigh 150 pounds (68 kg), you will expend approximately 68 kcal for every mile you walk. Similarly, taking two or three fewer bites of food at each meal will reduce energy intake by 100 kcal per day. The problem, of course, is that our current environment strongly discourages both of these things. (INFOGRAPHIC 11.16)

Taking a walk with a friend is a pleasant way to increase energy expenditure. James Levine once referred to NEAT as the "crouching tiger, hidden dragon" of societal weight gain. What he meant was that the behaviors that lead to obesity may be both sneakier and more deadly than we ever imagined. "Whoever thinks about the amount of time they spend sitting?" Levine asks. "No one does."

A NEAT CAUSE OF WEIGHT GAIN We all know people who seem to be able to eat nearly anything they wish and never gain weight. Likewise, there are those who "merely look at food" and put on pounds. It turns out there is scientific support for these subjective observations. Studies that have looked at weight gain in response to overfeeding have shown that people vary greatly in how much body fat they accumulate. The biological mechanism that allows some individuals to resist weight gain more than others, however, has not been identified. In 1999, Levine set out to solve the mystery. He and a team of researchers at the Mayo Clinic in Rochester, Minnesota, recruited 16 nonobese adults (12 men and 4 women from 25 to 36 years of age). These individuals underwent measures of body composition and energy expenditure before and after eight weeks of supervised overfeeding by 1000 kcal per day. The researchers found that fat gain varied 10-fold among individuals in the study, ranging from a gain of only 0.36 kg to a gain of 4.23 kg (1 kg = 2.2 lb). Why was the weight gain so disparate? One obvious possibility is that each person expended a different amount of energy.

Understanding Energy Expenditure A person's total energy expenditure (TEE) is the combination of three main components: (1) basal metabolism, (2) the thermic effect of food, and (3) activity energy expenditure. Basal metabolism is the energy expenditure required to maintain the essential functions that sustain life. This energy is required for the chemical reactions in our cells; the maintenance of muscle tone; and the work done by our heart, lungs, brain, liver, and kidneys—with much of this work depending on the active transport of electrolytes and other nutrients in our cells. In research studies, basal metabolism is measured while the person is lying completely still but awake at a comfortable temperature following an overnight fast and without any physical activity for the preceding eight hours. For most individuals, basal metabolism is the largest component of total daily energy expenditure, accounting for about 60% of TEE in a typical sedentary individual. Thermic effect of food (TEF) is the energy needed to digest, absorb, and metabolize nutrients in our food. TEF is generally equivalent to 10% of the energy content of the food ingested and does not vary greatly between people. Activity energy expenditure (AEE) is the amount of energy individuals expend in physical activity per day, both planned and spontaneous. It includes all of the energy expended in the contraction of skeletal muscles to move our body and to maintain posture (sitting or standing versus lying down). AEE is the most variable component of TEE. (INFOGRAPHIC 11.6)

abdominal obesity

excessive fat distributed around the stomach and abdomen; the most often used measure of abdominal obesity is waist size; abdominal obesity in women is a waist size of 35 inches or higher, and in men, it is a waist size of 40 inches or higher.

visceral fat

fat in the abdominal area that surrounds the body's internal organs; has been shown to be an independent health risk.

WEIGHT LOSS RECOMMENDATIONS Weight loss is recommended for anyone with a BMI of 30 or higher, for those who are overweight and have two or more risk factors, and for individuals with a large waist circumference. Examples of risk factors associated with high BMI include cardiovascular disease or a family history of it, smoking, hypertension, age (men: 45 years or older; women: 55 years or older or postmenopausal), diabetes, and physical inactivity. Surgical Approaches to Weight Loss Dietary modifications and increased physical activity are generally recommended to help people achieve and maintain a healthier body weight. But for very obese individuals, additional treatment options or interventions may be warranted. There are several FDA-approved antiobesity drugs that can be prescribed as an adjunct to diet, exercise, and behavior therapy for these individuals. Medications to treat obesity function to aid in weight regulation by promoting satiety and satiation or by reducing absorption of calories from dietary fat. For those with extreme obesity (BMI of 40 or above or a BMI of 35 or above with additional risk factors), weight loss surgery may be recommended. At present, in the case of extreme obesity, weight loss surgery (also known as bariatric surgery) is the most effective treatment to yield significant weight loss and reduction of weight-related disorders. Although not without significant risk, these procedures dramatically reduce stomach capacity, limit food intake, and increase satiety. Losses of 50% of excess body weight after bariatric surgery are not unusual. The most commonly performed procedures are gastric bypass and "sleeve" gastrectomy, both of which significantly reduce stomach capacity as well produce changes in gut hormones that suppress hunger and increase satiety. (INFOGRAPHIC 11.14)

A modest reduction in body weight can result in significant improvements in health for people who are overweight or obese. Losing 5-10% of body weight (and maintaining that lower weight) can reduce the risk of chronic diseases (cardiovascular disease and diabetes, for example) and premature all-cause mortality by about 50%.

Obesity is a complex disease that is influenced by multiple factors; behavior, environment, and genetics are the primary contributors. Genetics is often viewed by many among the public as the primary factor determining an individual's susceptibility to obesity. However, the rapid rise in the prevalence of obesity in the last few decades is largely attributed to changing environmental factors, because human genes have essentially remained the same over this time. For this reason, it is said that "Genetics loads the gun, and environment pulls the trigger," because the environments in which we live and work can strongly affect our behaviors and consequently our weight. The built environment—our surroundings that are designed by humans—can strongly affect how likely we are to engage in physical activity, which in turn determines whether we maintain, increase, or decrease our body weight. As an example, a recent study found that residents of walkable neighborhoods (compact neighborhoods with a mix of residences and businesses, sidewalks, and parks), weighed 6-10 pounds less than residents of sprawling neighborhoods (low-density neighborhoods characterized by cul-de-sacs and winding streets with few businesses and shops nearby) that were less walkable. Also, the food environment (the social and physical factors that influence the foods we eat) can make it challenging for us to make healthy food choices. Consequently, the dramatic rise in obesity is believed to result from environmental changes that promote low levels of physical activity and the consumption of energy-dense foods that are particularly likely to promote weight gain in genetically susceptible individuals.

Although almost everyone agrees that obesity seriously compromises our health and longevity, scientists are divided about just what is causing our waistlines to expand at an ever-alarming pace. Is it increased food intake—linked perhaps to larger portion sizes in restaurants and in supermarkets? Or is it decreased energy expenditure—a result of our increasingly sedentary lifestyle? Arguments have been made on both sides of the coin, but no strong consensus has emerged. Now with the help of some clever experiments using sophisticated undergarments, Levine thinks he has found insights to aid in answering this question.

Body Mass Index The most common tool for assessing body fat is the body mass index (BMI). BMI is calculated from a person's weight and height and provides an indirect estimate of body fat. "Underweight," "normal," "overweight," and "obese" are all labels for ranges of weight on the BMI scale. The greater the BMI, the higher the risk of obesity-associated diseases such as coronary heart disease, hypertension, stroke, and type 2 diabetes. (INFOGRAPHIC 11.11) INFOGRAPHIC 11.11 Determine Your Body Mass Index To find your BMI, use the table or one of the equations provided above. Calculate your BMI using one of the methods above. Although easy to use, BMI has limitations. For example, BMI does not distinguish between excess fat and muscle and bone mass. Thus, it may overestimate body fat in muscular individuals, such as highly trained athletes who have increased muscle mass.

Although easy to use, BMI has limitations. For example, BMI does not distinguish between excess fat and muscle and bone mass. Thus, it may overestimate body fat in muscular individuals, such as highly trained athletes who have increased muscle mass.

Although a lower BMR does not appear to contribute significantly to the initial cause of obesity, it may contribute to the difficulty that most individuals encounter in preventing weight regain following weight loss. Recent studies involving former participants in the "Biggest Loser" competition found that BMR is about 20% (500 kcal) lower than expected (for a particular body weight and composition) six years following weight loss. Furthermore, it was found that contestants who were successful in maintaining weight loss were engaging in very high levels of physical activity, amounting to about 80 minutes of additional moderate daily activity above pre-weight loss levels. These results indicate that continually high levels of physical activity may be necessary for maintenance of weight loss. In Levine's study, he found only very minor increases in BMR and TEF in his study participants, which was not enough to account for the 10-fold variance in fat gain among them. By contrast, levels of physical activity varied markedly between study participants. Interestingly, intentional exercise was not the crucial difference in activity level. Levine designed his study in such a way that exercise was kept at a constant and minimum level across all study participants. Therefore, the differences in physical activity were principally non-exercise related. Levine has a name for this type of activity: He calls it NEAT, short for nonexercise activity thermogenesis. NEAT includes all activities of daily living, such as performing household chores, doing yard work, shopping, carrying out occupational activities, walking the dog, and playing a musical instrument. It also includes the energy expended to maintain posture and spontaneous movements such as fidgeting, pacing, and even chewing gum. In other words, NEAT encompasses all of the activities we do as part of daily living but separate from planned, intentional exercise such as going for a run, taking an aerobics class, or hopping on an exercise bike. In Levine's study, NEAT varied greatly among individuals (fewer than 100 kcal to more than 700 kcal per day). More important, changes in NEAT were inversely correlated with changes in weight gain. In other words, NEAT proved to be the principal mediator of resistance to fat gain in these individuals. (INFOGRAPHIC 11.8)

NFOGRAPHIC 11.8 NEAT and Its Impact on the Risk of Obesity Total daily energy expenditure can be increased significantly when we minimize the time spent sitting quietly and find ways to keep moving, which decreases our risk of weight gain. Next, Levine wanted to know if NEAT plays a role in obesity. Do lean and obese people differ in their levels of NEAT, for example? To get at this question, Levine and his colleagues recruited 20 healthy volunteers who were self-proclaimed "couch potatoes." Ten participants (5 women and 5 men) were lean, and 10 participants (also 5 women and 5 men) were mildly obese. The volunteers agreed to have all of their movements measured for 10 days. They were instructed to continue their normal daily activities and not to adopt new exercise regimens. To measure NEAT, Levine and colleagues devised a novel way to track activity levels in study participant volunteers. They built a special kind of underwear outfitted with electronic sensors that detect movement. The undergarments were built to allow people to wear them essentially all the time—even while going to the bathroom and having sex. "We literally have snapshots as to how real people live their lives every half second of every day for days and days and days on end," says Levine. From the electronic sensors measurements, Levine's team then calculated how many NEAT calories a person expends per day. Over the 10-day period, Levine's team collected 25 million data points on NEAT for each individual. The bottom line? Obese people sit on average 2.25 hours longer than their leaner counterparts. Moreover, this tendency seems to be innate: Lean individuals sit the same amount of time even after they are forced to gain weight, and obese individuals sit the same amount of time even after they are forced to lose weight. What does all this have to do with obesity? Levine thinks that it's not only that people are eating more than they did a few decades ago but also that we have been "seduced" (his word) by our environment into expending less energy by sitting more.

ENERGY IN, ENERGY OUT To understand our obesity crisis, says Levine, you have to realize that it didn't happen overnight. "Obesity doesn't occur over minutes and hours," says Levine. "Obesity occurs over years, decades, and generations." What that means is that even small changes in the way we eat or the way we move can seriously add up. Even something as simple as an extra spoonful or two at each meal or the amount of time we spend sitting or standing—if multiplied consistently across time—can profoundly affect how much energy we store or expend. STAY TUNED To learn more about the cell's energy supply, see Chapter 12 Nutrition and Fitness. Energy, defined as the capacity to do work, is required to perform all of the various functions that are necessary to sustain life, from breathing to moving to digesting food to maintaining a constant body temperature. Humans and other animals obtain energy through the breakdown of carbohydrates, fats, proteins, and alcohol in food and beverages. The energy contained in the chemical bonds of these molecules is released by the chemical reactions of metabolism and captured in a form that can be used to do the body's work. One way scientists measure energy is in units called calories. A calorie is defined as the energy required to raise 1 gram (g) of water 1° Celsius (C). The energy in food is measured in units of kilocalories (kcal = 1000 calories). A kilocalorie is the energy required to raise 1 kilogram (kg) of water 1°C. All food labels in the United States report the energy in foods in kilocalories, although these are usually just referred to as "calories." Obesity results from a chronic imbalance of energy intake and expenditure. According to the laws of thermodynamics, energy is neither created nor destroyed but merely changes form. This principle, known as the conservation of energy, means that when we consume more energy than we expend, the excess has to go somewhere. Most often, it seems to end up in the fat cells in our hips, thighs, and bellies.

There are two types of fat, or adipose tissue: white adipose tissue (WAT) and brown adipose tissue (BAT). WAT stores excess energy in the form of fatty acids in triglycerides and releases those fatty acids during times when energy intake is low. In contrast, BAT burns fatty acids and glucose to produce heat by a process called nonshivering thermogenesis (heat production). Infants are born with significant amounts of BAT that is localized primarily between the shoulder blades in concentrated and identifiable depots, and the heat produced there is critical for the maintenance of body temperature in the infant. It was previously thought that adults had little if any BAT, but recent studies have found that some adults have significant amounts. But unlike BAT in infants, in adults, it does not exist in specific depots, but brown fat cells, or adipocytes, are found scattered within depots of WAT. Another difference is that brown fat cells in adults appear to arise primarily from the conversion of WAT into brown fat cells, which is referred to as WAT "browning." Because these cells are similar but not identical to the brown adipocytes found in infants, they are often called "beige" adipocytes. Because the amount of beige fat cells present in adults has been shown to correlate with body leanness and the browning of WAT in adults can be induced with cold exposure, it is thought that the heat produced by these cells may make significant contributions to the maintenance of energy balance in some adults. To be in energy balance means that the amount of energy we take in ("energy in") equals the amount of energy we use ("energy out"). When this occurs, our body weight is stable. Any increase in our body weight indicates that "energy in" is greater than "energy out." Fundamentally, the only way to gain weight is through positive energy balance (to consume calories in excess of what is expended), and the only way to lose weight is through negative energy balance (to consume fewer calories than what is expended). Obesity always results from chronic positive energy balance. (INFOGRAPHIC 11.3) This discussion reveals something unique about energy nutrients compared with other nutrients: Body weight provides us with an easily monitored indicator of adequacy, excess, or insufficient energy nutrients.