Students are generally introduced to nutrition in grade school, where the Food Guide Pyramid is discussed. They are taught the need for a balanced diet in the context of growth and maintaining a healthy body. However, little consideration may be given to physical activity and energy expenditure. Additionally, students may be unaware of the need to provide energy in excess of their activity requirements to maintain the growth they are experiencing. Consequently, this curriculum supplement was created to introduce students to the key concept of energy balance and provide a context within which they can better understand nutrition concepts they learn at other times. A key concept is that energy balance is a long-term, rather than a short-term, goal. Two important consequences of energy imbalance for adolescents are obesity and undernutrition.
Obesity is a chronic metabolic disease resulting from an imbalance between energy intake and energy output. It is caused by the interaction of multiple genetic and environmental factors. Among these are excessive caloric and food intake, insufficient physical activity, genetic predisposition, family history of obesity, individual metabolism, and behavioral factors. The defining feature is excess body fat.11 Obesity increases the risk of developing many conditions, including heart disease and stroke, high blood pressure, type 2 (non-insulin-dependent) diabetes, gallstones, sleep apnea, back pain, osteoarthritis of the weight-bearing joints, and some forms of cancer (for example, breast and colon). Fortunately, many of these conditions improve with successful treatment for obesity.53
Undernutrition is a complex issue. Individuals may suffer from an insufficient intake of both calories and specific nutrients. In some cases, energy intake may be sufficient, but the diet may be lacking in nutrient content. The diagnosis of undernutrition used to depend on deficits in weight compared with reference standards. More recent recommendations classify children as undernourished depending on their height-for-age and weight-for-height measurements. Interestingly, low height-for-age (that is, stunting) is much more prevalent worldwide than is low weight-for-height (that is, wasting).41
Nutrition in general is a concern for adolescents, who are entering a stressful, confusing, and sometimes frightening time of social, emotional, and physical development.25,26,28 Many adolescents do not understand the changes that are occurring during this time, including the weight gain associated with growth and sexual development. There is great pressure to fit in and gain acceptance. Teenagers may worry excessively about what others think of them, especially their physical appearance. Unfortunately, emphasis in the media generally is on being thin, and the “ideal” body image is that of today’s models and TV and movie stars.2 Additionally, adolescents may be confronted with problems in their family environment. Some teenagers may develop eating disorders as a result of complex psychological, environmental, and/or genetic factors. An important goal of this curriculum supplement is to help adolescents and teenagers recognize that healthy bodies come in a variety of shapes and sizes.
Nutrition in general is a concern for adolescents, who are entering a stressful, confusing, and sometimes frightening time of social, emotional, and physical development.
Eating disorders have both mental and physical components that can have serious medical consequences.37 However, not all individuals with eating disorders suffer serious medical consequences. Indeed, some individuals may not suffer from any apparent medical problems. These disorders may develop as a means of gaining control, of focusing on something pleasant, of blocking out painful feelings or experiences, or of providing punishment through self-abuse. Three eating disorders are among the key health issues affecting adolescents and young adults:20
Eating disorders may begin at very young ages. Children as young as 8 years of age have voiced complaints about their body size and shape and expressed a fear of being fat. Furthermore, young people in all ethnic and cultural groups may develop eating disorders.
Early detection of an eating disorder is important and contributes to the likelihood of successful treatment and recovery. Treatment is complex and requires input from psychological, medical, and nutritional experts, as well as strong support from family and friends.
Healthy diet and regular physical activity help children and adults feel better, learn and work more effectively, and avoid developing a variety of risk factors for disease. Unfortunately, most adolescents in the United States are not moderately active—that is, they do not get 30 or more minutes of physical activity on five or more days of the week.10 The proportion of high school students who participated daily in physical education classes declined from 42 percent in 1991 to 25 percent in 1995. Furthermore, 48 percent of high school students are not enrolled in any type of physical education class.8 In general, older children are less active than younger children, and girls are less active than boys.22
Additionally, recent surveys by the U.S. Department of Agriculture (USDA) point out some shortcomings in children’s diets. For instance, over half of children and adolescents aged 2 to 17 consume more total fat, saturated fat, and sodium than is recommended in dietary guidelines for Americans.26,33 Additionally, they do not eat enough fruits, vegetables, or dairy products.13,26,33 Adolescent girls ingest considerably less iron and calcium than recommended by the Food and Nutrition Board of the National Research Council.26,33,50
The proportion of high school students who participated daily in physical education classes declined from 42 percent in 1991 to 25 percent in 1995.
Social and technological trends of the 20th century contributed to the current poor state of diet and physical activity among Americans, most notably innovations in transportation, communication, computer technologies, food processing, and food marketing. Fewer daily opportunities exist today to burn calories: leisure and workplace activities are increasingly sedentary, motorized travel for all but the shortest distances has become almost universal, and school systems continue to cut back on physical education programs. At the same time, there are more opportunities to eat each day now than 20 years ago because the marketing and distribution of high-calorie fast foods and snacks has increased, the number of restaurants has increased, and social interactions increasingly involve food and drink.
This combination of poor diet and lack of physical activity is associated with an “epidemic of obesity” in the United States.31,36 Obesity among adults increased in the population from about 15 percent in 1980 to about 30 percent in 2000.18 Increases were recorded in all states and demographic groups. Overweight is increasing as well among children and adolescents. The percentage of overweight adolescents (12 to 19 years of age) in the United States has tripled during the past 20 years, and about 15 percent of this age group is considered overweight now.9,10,12,40,51
The health burden of overweight and physical inactivity is substantial.38 Poor diet and physical inactivity together account for 300,000 deaths each year in the United States.30 Only tobacco consumption exceeds the combination of poor diet and lack of exercise as an actual cause of preventable deaths each year (20 percent versus 14 percent of all deaths).30 In young people, obesity is associated with an increased incidence of elevated blood cholesterol,16 elevated blood pressure,24 and type 2 diabetes.42 Further-more, obesity during childhood or adolescence is associated with increased morbidity and mortality during adulthood in both males and females. Significantly, the risk of persistence of adolescent obesity into adulthood is three times greater for adolescent girls than for boys.14
Poor diet and physical inactivity together account for 300,000 deaths each year in the United States.
Because adolescence is a period of intense physiological, psychological, and psychosocial development, young people’s bodies may change more rapidly than do their attitudes toward them. Adolescence can be an awkward period of adjustment under the best of circumstances. However, overweight or underweight adolescents in particular may have feelings of low self-esteem, self-consciousness, social isolation, failure, and depression.3 Any classroom discussion of nutrition and its effects on the body must be sensitive to these issues. Nutrition curricula that are informative, nonjudgmental, and inclusive of all students may be better accepted by and more effective with adolescents.4,29,39
Some preconceptions about energy balance and healthy nutrition are presented below. Detailed information about the topics introduced here follows in subsequent sections.
Students should appreciate that being overweight in childhood or adolescence is likely to persist into adulthood unless they take steps toward weight control. About 80 percent of obese adolescents become obese adults, and there is evidence that they become heavier than those who become obese as adults.55 Significantly, the risk of persistence of adolescent obesity into adulthood is three times greater for adolescent girls than for boys.14
The teenager is a rapidly changing organism. Students should appreciate that the events of puberty and the simultaneous growth spurt during adolescence place special demands on their nutritional needs. Adolescents require adequate calories and nutrients in order to sustain the physical growth and maturation, cognitive development, and psychosocial development that characterize this time of life. Students should appreciate that growth during adolescence involves an increase in body tissues that results in increases in both height and weight. Hunger is an important signal, and students should learn the value of responding to this signal by choosing nutritious foods, such as fruits, vegetables, whole-grain products, dairy products, and meats, to satisfy hunger. Additionally, students benefit by knowing the effects of unsafe weight-loss methods and the characteristics of safe weight-loss programs. Overweight adolescents should be aware that it is important to stop gaining weight (or to slow the rate of weight gain), and to be under the supervision of a healthcare provider to ensure proper growth and maturation.
Students should appreciate that being overweight in childhood or adolescence is likely to persist into adulthood unless they take steps toward weight control.
Many fad diets may offer the promise of rapid and permanent weight loss. Although these diets may allow an initial weight loss, many may limit the intake of essential nutrients to unhealthy levels. Also, many of these diets are not appealing for long-term use. However, the same precaution applies here as above: adolescents should be aware that weight loss, if undertaken, should be on the advice of a physician and done under a physician’s care. Depending on the adolescent’s age and stage of growth, it may be better not to attempt weight loss because this may hamper normal development.
The time of food consumption is not a factor in weight gain. Instead, consideration should be given to energy balance, that is, balancing energy (calories) consumed with energy expended. Any energy consumed in excess of energy expended is stored as fat.
There are no foods that can “burn fat,” although some foods with caffeine or certain spices can increase metabolism for a short time. Eating these foods does not constitute an effective strategy for weight loss. The most effective means of maintaining a healthy body weight is balancing calories consumed with calories used for activities.
The energy balance equation includes terms that refer to energy intake and energy output. For some individuals, the equation must also include terms for the energy required for growth and for energy that is stored.
Energy In (Ein): Energy is available from the foods we eat; this energy input is represented by Energy In, or Ein. Although foods contain a number of nutrients, energy is provided by proteins, carbohydrates, and fats. Vitamins and minerals in foods, although essential for normal metabolic functions, do not contribute calories to our diets. Each gram of protein or carbohydrate we consume contributes 4 calories of energy. In contrast, fat provides 9 calories per gram. Interestingly, alcohol has 7 calories per gram. Alcohol-containing products have calories and few nutrients; their consumption may upset both energy balance and nutritional status.
A food calorie is the equivalent of 1,000 calories, or 1 kilocalorie. The food calorie is sometimes represented by “Calorie,” with a capital C. In keeping with the usual format in nutrition studies, this curriculum supplement uses “calorie” to mean the food calorie. A food calorie (1 kilocalorie) is defined as the amount of energy required to raise the temperature of a liter of water 1ºC at sea level.
The Food Guide Pyramid was developed by the U.S. Department of Agriculture and is supported by the U.S. Department of Health and Human Services.7 The Food Guide Pyramid, which is based on the foods Americans typically eat and the nutrients in these foods, is designed to help people choose what and how much to eat from each food group to get needed nutrients, without excessive intake of calories, saturated fat, cholesterol, total fat, and sugar. The Food Guide Pyramid shows a range of servings for each major food group. This range is based on the caloric needs of the individual. Almost everyone should have at least the lowest number of servings in the ranges. However, older children and teenagers (ages 9 to 18 years) and adults over the age of 50 need three servings from the milk group daily.
The Food Guide Pyramid is easily adaptable to ethnic and cultural preferences by including specific types of fruits, vegetables, and grain products. It is also adaptable to vegetarian diets by allowing for meat substitutes, such as beans, soy-based meat substitutes, and eggs (although some vegetarians (vegans) do not eat eggs).
A balanced diet provides 45 to 65 percent of total daily calories as carbohydrate, most of which should be from complex carbohydrates, such as starches; 10 to 35 percent of daily calories from protein; and no more than 30 percent of calories from fat.52 The needs of athletes and other more physically active people may differ in both energy and nutrient intakes, depending on the intensity and duration of their physical activities. People with special needs due to illness or medications should consult a physician and a registered dietitian to create an appropriate plan to meet nutritional demands.
Energy Out (Eout): Total energy expenditure is represented by Energy Out, or Eout. Eout has three major components, which, added together, provide an accurate measure of an individual’s daily caloric requirement: the basal metabolic rate (BMR), the energy used for physical activity, and the thermic effect of food.
The BMR represents the energy used to carry out the basic metabolic needs of the body. Energy must be provided for maintaining a heartbeat, breathing, regulating body temperature, and carrying out other activities that we take for granted. Most of our daily energy expenditure, about 60 to 70 percent, is represented by our BMR. The BMR can be estimated as follows: for adult males, multiply the body weight (in pounds) by 10, and add double the body weight to this value (example: for a 160-pound male, BMR = 1,600 + (2 × 160) = 1,920 calories/day); for adult females, multiply the body weight by 10, and add the body weight to this value (example: for a 110-pound female, BMR = 1,100 + 110 = 1,210 calories/day).
BMR calculations, as in the examples above, are average estimations. A person’s actual BMR changes over time. This depends on a number of factors, including several that distinguish groups of people:
Age—Younger people have higher-than-average BMRs. As children grow, their body composition (percent body fat and muscle mass) changes. As they continue to age, BMR decreases as the percent muscle mass decreases.
Growth—Children and pregnant women have higher-than-average BMRs.
Height—Tall, thin people have higher-than-average BMRs.
Body Composition—People with higher-than-average or increased muscle mass have higher-than-average BMRs.
Other factors cause variation within individuals:
Fever—Fever increases your BMR.
Stress—Physical stress, such as recovering from an illness, increases your BMR; mental or emotional stress may lead to lethargy or depression and decrease your BMR.
Inside/Outside Temperature—Both heat and cold raise your BMR.
Fasting—Fasting lowers your BMR.
Physical activity amounts to about 20 to 30 percent of the body’s total energy output. Energy expended during physical activity varies with the level and duration of the activity. It is also affected by the age, gender, height, and weight of the individual performing the activity. Examples of the calories used by different individuals for walking and running are presented in the table below. The values in the table include the calories for BMR.
The “thermic effect of food” refers to the energy required to digest food. This term indicates what is usually obvious: we must expend some energy to make materials available in the body that will be used for the production of much larger amounts of energy. The thermic effect of food can be estimated as approximately 10 percent of total calories consumed. Because it makes a relatively small contribution to energy expended, the thermic effect of food is not included as part of Eout in the lessons in this curriculum supplement.
As an alternative to adding the three terms discussed above, daily caloric requirements (Eout) can also be estimated as follows: For less-active individuals: weight (in pounds) × 14 = estimated calories/day. For moderately active individuals (3 to 4 aerobic sessions/week): weight (in pounds) × 17 = estimated calories/day. For active individuals (5 to 7 aerobic sessions/ week): weight (in pounds) × 20 = estimated calories/day.
|Individual||Calories Used Per Hour*|
|Walking for pleasure (2.5–3.0 mph)||Walking for exercise (3.5–4.0 mph)||Running (5.0 mph)|
|12-year-old girl, 4’11”, 92 pounds||187||203||426|
|12-year-old boy, 4’11”, 89 pounds||188||204||430|
|30-year-old woman, 5’4”, 133 pounds||204||221||465|
|30-year-old man, 5’10”, 155 pounds||251||273||574|
|*Compiled by BSCS staff using various health education resources.|
The energy balance equation. The equation for energy balance is Ein = Eout. This means that caloric intake equals caloric output. It is the desirable condition for adults who are at a healthy weight. One way to understand the concept of energy balance is to use a two-pan-balance analogy. On one pan of the balance are weights representing Ein (foods, which contain carbohydrate, protein, fat, and alcohol). On the other pan are weights representing energy expenditures (Eout) as metabolic activities and physical activities (and the thermic effect of food). If adults consume more calories than are used for metabolic and physical activities, then Ein > Eout, and the extra energy is stored as body fat. They are in a state of positive energy balance. The pan scale would tip to the Ein side. If adults lose weight (as, for example, with dieting), they are in a state of negative energy balance. In this case, Ein < Eout, and the pan scale would tip to the Eout side.
Healthy children and adolescents (until they stop growing) are in a state of positive energy balance. The extra calories are used primarily to increase the amount of important body tissues such as bone, muscle, blood, and body organs. Some of the extra calories may also be stored as body fat, which can be used at a later time as a source of energy. Thus, food components (protein, fat, and carbohydrate) taken into the body have the following fates: they can be used to fuel metabolic activities and physical activities, they can be incorporated into growing body tissues, and they can be stored as fat. If food intake contributes to all three of these fates, then Ein = Eout + Egrowth + Estored, and the body is in positive energy balance. If Ein < Eout + Egrowth for children or adolescents the body will be in negative energy balance and will not be able to grow properly.
Extra Ein is an important consideration during periods of growth. The amount of energy (calories) required for growth during most of a child’s life accounts for about 1 to 2 percent of the youngster’s daily energy intake. However, during infancy and adolescence, growth does have a significant impact on energy requirements. The appetites of healthy youngsters, at any age, are usually reliable guides to the amount of food they should eat. Presented with a well-balanced diet, healthy children will eat all they need. Importantly, attempts to force low-calorie diets on children and adolescents may interfere with normal growth processes. Furthermore, such diets alone have not been very successful in achieving long-term weight control (discussed below). An important consideration is that growing children and adolescents are not in energy balance until they stop growing. Rather, they are in positive energy balance, taking in more energy than is expended in physical activities and in maintaining the BMR. That extra energy is used for growth.
An important consideration is that growing children and adolescents are not in energy balance until they stop growing.
In summary, there are two important concepts of energy balance for adolescents. First, to allow for normal body growth, more food energy must be consumed than can be accounted for solely on the basis of energy required for metabolic and physical activities. Second, insufficient energy intake may affect cellular metabolic activities, body weight, growth, tissue formation, and health.
Understanding the relationship between energy requirements and desirable body weights should take into account not only the total weight, but also the composition of the weight. This is important because muscle mass and body fat make different demands on daily energy requirements15 and can have different long-term health consequences.23 Considerable variation among individuals in resting metabolic rates is due in part to variation in body composition or, more specifically, to the ratio of muscle to fat tissue in the body.43 Muscle tissue is more effective than fat at burning calories, expending more than three times as much energy under resting conditions.15 Therefore, the ratio of muscle to fat tissue is an important determinant of the total daily energy requirement.
Significant changes in the ratio of muscle tissue to fat tissue occur during adolescence. In females, body fat increases from a mean of 17 percent of body weight to 25 percent of body weight during adolescence. Males, in contrast, experience a decline in body fat, from a mean of 18 percent of body weight to 11 percent of body weight during this period. In addition, the pattern of body-fat distribution changes during adolescence: in both genders, body fat redistributes from peripheral sites to central sites (gluteal region in females, abdomen in males).19
Consequently, it is important to emphasize to students that great variation exists in body shapes and sizes among healthy individuals.
These normal changes during adolescence are due primarily to genetic and physiologic factors. Nevertheless, energy balance still plays an important role in influencing the direction and magnitude of these changes. Because of changes in body composition with growth, weight is a less reliable measure of body composition for children and adolescents than for adults. Consequently, it is important to emphasize to students that great variation exists in body shapes and sizes among healthy individuals. Conditions of overweight and obesity should be diagnosed only by qualified health professionals and should not be based on appearance.
Body composition is a much better predictor of one’s level of health and risk of disease than is weight. Muscle mass or a measure of body fat is used to assess body composition. Professionals estimate body-fat content using tools and techniques such as circumference measurements (of abdomen, hips); a height and hip-girth chart; ultrasound measurements; electrical-impedance measurements; the caliper-measurement method; and the water-weighing-measurement method.
BMI expresses the relationship between an individual’s weight in kilograms (or pounds divided by 2.2) and height in meters (or inches divided by 39.4). For adults, the formula is BMI = weight/ height2. For children over two years old and adolescents, the formula is BMI = (weight in pounds/height in inches2) × 703.35 BMI is a helpful indicator of obesity and underweight and has two primary uses. It can be used to screen and monitor a population for risks to health or for nutritional disorders. Alternatively, BMI, along with other necessary information, can be used to assess risks to health for an individual.
What does a BMI value mean? Scientists and health officials have arrived at the following classifications for adults, based on the effect that body weight has on disease and death:
|BMI less than 18.5||underweight|
|BMI between 18.5 and 24.9||healthy range|
|BMI between 25 and 29.9||overweight|
|BMI equal to or over 30.0||obese|
Justification for these ranges become apparent in the following figure:
More information about overweight in adults can be found at http://www.nhlbi.nih.gov/ guidelines/obesity/ob_home.htm.
Average BMI values rise throughout adolescence, and the changes are similar for both genders.14 Typical ranges for BMI during adolescence are 16 to 22 at age 12, rising to 18 to 25 at age 17.18 Students need to appreciate these age-related trends in order to interpret correctly the BMI as an assessment tool for their own weight status. Students should also understand that the similar range of BMIs for females and males during adolescence conceals significant gender differences in body composition. Adolescent females tend to have a higher ratio of fat to fat-free tissue than do males of comparable BMI. Students should understand that these gender-related differences in body composition are normal and that small gains in fat tissue and weight in girls are a normal part of physiological preparation for reproduction.
Portion size: Nutritionists caution that effective weight management places equal importance on both the kind and the amount of food consumed. However, in a recent survey by the American Institute for Cancer Research, 78 percent of those responding believed that the kind of food they eat is more important for managing weight than how much they eat.1 In part, this is attributed to advertising and emphasis on “low-fat” and “fat-free” foods. Trends in the food industry have also contributed to increased consumption. Competition has resulted in serving larger portion sizes. Fast-food restaurants have “super-sized” and “value” meals, while other restaurants have replaced the former industry standard 10-inch plate with a 12-inch plate. Interestingly, information from the U.S. Department of Agriculture shows that while the percentage of fat in the American diet dropped from 40 percent to 33 percent over the past 20 years, total daily caloric intake increased about 8 percent, from 1,854 calories to 2,002 calories.
In many ways, these trends are not surprising. Our lack of concern about portion size may result from our general lack of understanding about what actually constitutes “a serving” of food. Even though today’s food labels offer more complete nutrition information than they did in the past, the information is not always easy to translate into practical terms. For instance, to calculate our energy intake in calories, we must multiply the calories per serving given on the label by the number of servings we eat. But what is a serving? “Serving size” may not reflect what a person actually eats. Indeed, food consumption varies widely across the population, especially as a function of age. How many of us consume 1 ounce of dry cereal at breakfast? What is 1 ounce of dry cereal? How much is 3 ounces of meat? What is a medium apple as opposed to a small or a large apple? And a food-label serving size is not necessarily the same as a Food Guide Pyramid serving size. However, a label serving and a Food Guide Pyramid serving are each a standardized amount that reflects a certain nutritional content. No matter how confusing serving size might be or how inconvenient it might be to determine the number of servings consumed, it should be apparent that no understanding of energy intake can be achieved without attention to the actual amounts of foods consumed. Consider the examples in the Estimating Amounts of Common Foods table, which provides a practical and simple means of estimating how much of various foods we consume.
Hunger and appetite: The input side of the energy balance equation, Ein, is controlled in large part through the opposing sensations of hunger and satiety. Hunger is a physiological state modulated by internal factors. It is often associated with the question, Is there anything to eat? Key to understanding the physiological control of body weight is understanding how eating behavior (food intake) and metabolism are coordinated. This area is complex, and a detailed understanding of the mechanisms is lacking. Early models of the control of food intake held that hunger was a simple response to stomach contractions or to temporary low concentrations of glucose, amino acids, or fatty acids circulating in the blood. In the past decade, however, this simplistic view has been refined into a more comprehensive model involving the nervous and endocrine systems.
Apple, orange, pear, peach
1 tennis ball
|Cooked beans or peas||
|1 cupcake wrapper|
|1 cupcake wrapper|
|Cooked or raw vegetables||1⁄2 cup||1⁄2 baseball|
|Cut or mixed fruit||1⁄2 cup||1⁄2 baseball|
|Dry cereal||1 cup||2 cupcake wrappers|
|Meat, fish, or poultry||2 to 3 ounces||1 deck of cards|
|Natural cheese||11⁄2 ounces||3 dominoes|
|Pasta||1⁄2 cup||1 cupcake wrapper|
|Peanut butter||2 tablespoons||1 golf or Ping Pong ball|
|Potato||1 medium||Size of standard computer mouse|
|Processed cheese||11⁄2 ounces||2 9-volt batteries|
|Raw, leafy vegetables||1 cup||4 outer romaine or iceberg lettuce leaves|
1 cupcake wrapper
Factors that may lead to decreased food intake include
Factors that may lead to increased food intake include
Although we do not yet understand all of the factors regulating food intake, it is clear that regulation is complex and involves the coordinated interaction of many signals.
Appetite is a learned condition influenced by both external and internal cues. It is often associated with the question, What do I want to eat? Appetite seems better related to energy need when we are regularly physically active. When physical activity is low, appetite may increase out of boredom or a need to do something. Eating also may be a means of achieving immediate satisfaction or gratification.
An important molecule that regulates energy balance through its effects on metabolism and appetite is leptin, a protein produced by fat cells.5,6,44 Leptin is able to reduce food intake, inhibit the synthesis and release of neuropeptide Y (a peptide that increases appetite), increase body temperature, increase metabolic rate, and reduce blood concentrations of glucose and insulin. The overall result is lower body weight and lower percentage of body fat. More information about leptin is presented in the next section (Genetics).
How does integration of all the signals relating to hunger and appetite occur? Experiments have shown that specific regions of the brain integrate various inputs to control food intake. It is thought that the most important region for control of hunger and satiety is the hypothalamus. This structure lies deep in the center of the brain, and it regulates biologically based motives such as hunger, thirst, and sex drive. The hypothalamus receives smell, taste, and visual inputs, and it senses changes in blood glucose concentration. One area of the hypothalamus contains the satiety center, which tells us when to stop eating. A second area contains the hunger center. Other areas of the brain are involved in the regulation of hunger and satiety as well. Additionally, the brain stem regulates the mechanical events of eating, such as saliva production, chewing, and swallowing.
Genetics. Food intake and body weight, like all physiological variables, are influenced by genes. The genetics of feeding behavior has been studied in rodents, in which single gene mutations that result in increased feeding and obesity have been identified. Functional analysis of the ways the mutated products of these genes contribute to overeating began only recently.
Progress has been made in understanding the genetic relationship between two defective gene products in the mouse. These are the OB protein (also called leptin; see above), which is the product of the recessive ob gene, and OB-R protein (leptin receptor), which is the product of the recessive db gene. The initial observation was that a protein secreted by fat tissue of normal lean mice was absent in obese mice (ob/ob). When leptin became available in recombinant form and was injected into obese (ob/ob) mice, they ate less and their metabolic heat production increased, followed by weight loss.6 ob/ob mice fail to produce leptin because of a mutation in the OB gene. Lacking leptin, which inhibits food intake, the mutants tend toward obesity.
Leptin may exert its inhibitory effect on food intake indirectly, by inhibiting production in the brain of the powerful eating stimulant neuropeptide Y and/or by increasing production of an appetite-suppressing brain peptide.45 In the db/db strain of obese mice, on the other hand, it is the receptor for leptin, OB-R, that is defective. In this case, a mutation in the receptor gene results in a receptor that has greatly reduced binding affinity for leptin in the brain. Such mutants fail to respond to injected leptin.45
Figure 2. A db/db mouse (right) next to a control white mouse.
Figure 3. Two control mice (left scale pan) and an ob/ob mouse (right scale pan).
The first single-gene defect associated with human obesity was described only recently and involved leptin deficiency in both a child and an adult.32 However, most obese humans have high circulating concentrations of leptin and normal leptin receptors, so any defect in the leptin signaling pathway likely resides beyond the receptor and leads to a state of leptin resistance. Also, children with Prader Willi syndrome, a genetic disorder caused by deletion of a small piece of chromosome 15 donated by the father, have a voracious appetite and insatiable hunger.47 They tend to become obese early in life. It is believed that genes for appetite regulation are contained on the missing piece of chromosome 15.
In general, the genetics of human food intake and obesity is not yet well understood. However, clear genetic effects do exist. Scientific studies have evaluated the contributions of genetic factors and the family environment to BMI. These studies have taken two different approaches. Some have compared adoptees with both their biological parents and their adoptive parents,46,49 while others have studied BMI in identical and fraternal twins.17,48 Scientists have concluded from these studies that BMI is under substantial genetic control. Additionally, one research study concluded that human obesity may be influenced by behaviors that themselves are regulated genetically.17 Nonetheless, some studies have indicated a large contribution of nonshared individual environmental influences on BMI.54
The relative importance of genetics and environment to BMI is not clearly defined. What is clear, however, is that despite the popular attribution of overweight to “glandular problems,” known endocrine disorders are actually a rare cause of obesity.
Attention to energy balance over time is required for promoting health and maintaining a stable body weight. For overweight people, steps must be taken to stop the weight gain and reduce weight to a healthy level, and then to maintain that healthy weight. Accomplishing these goals requires an understanding of energy balance—that is, of the general concepts of energy in and energy out. Individuals have direct control over both their food (calorie) intake and their physical activity level. By gaining knowledge of the caloric content of various foods and the caloric costs of various activities, individuals can evaluate their current Ein/Eout regimens and devise plans to achieve energy balance. People are generally surprised to learn just how small a contribution sedentary activities, such as watching TV or playing video games, make to daily calorie expenditures. On the other hand, any type of physical activity, from running or playing sports to walking or household work, increases the number of calories the body uses.
As emphasized by the National Institute of Diabetes and Digestive and Kidney Disorders, the key to successful weight control and improved overall health is making physical activity a part of our daily routine. Interestingly, behavioral research suggests that attempts to change the activity patterns of overweight students may be more effective when students are reinforced for choosing ways to limit their inactivity than when they are reinforced directly for activity.14
How much physical activity is necessary? The Dietary Guidelines for Americans recommends 30 minutes a day for adults and 60 minutes a day for children and adolescents.52 A new report from the National Academy of Sciences recommends a goal of one-hour-a-day total exercise for adults.34 The report indicates that energy expenditure is cumulative, including both low-intensity activities, such as stair climbing and house cleaning, and more vigorous exercise, such as swimming and cycling. Sixty minutes of moderate physical activity most days of the week is recommended for children, adolescents, and teenagers up to age 18. Substantial health benefits may still be gained by accumulating at least 30 minutes’ of moderate-to-intense physical activity a day, at least five times a week. However, care should be taken not to exercise more frequently and more intensely than is required for good health or to compete well.
Developing appropriate strategies for achieving and maintaining a healthy body size and weight can be challenging for some individuals, and it may require more than one approach to the problem. Education may be necessary for an understanding of energy balance and basic nutrition principles. Counseling with an appropriate professional may be essential, for instance, for helping individuals find suitable physical activities and motivating factors. Counseling with a registered dietician or other qualified professional may also be necessary for developing meals and daily food plans. Medical or psychological therapy may be necessary for dealing with issues of undereating or overeating. Additionally, physical activity classes and programs should be available and accessible.
For more information on this important topic, visit the following Web sites:
The following document is also recommended: U.S. Department of Health and Human Services. 2001. The Surgeon General’s call to action to prevent and decrease overweight and obesity. Rockville, Md.: U.S. Department of Health and Human Services, Public Health Service, Office of the Surgeon General. This publication is available online at http://www.surgeongeneral.gov/library or from the U.S. Government Printing Office by calling 866-512-1800.
The 2005 Dietary Guidelines for Americans, by the U.S. Department of Health and Human Services and the U.S. Department of Agriculture, was published as this supplement went to press. It is available at http://www.healthierus.gov/dietaryguidelines.