Chapter 18: Nutrition and Body Composition for Health

By studying this chapter, you should be able to do the following:

1. Describe what is meant by the terms Recommended Dietary Allowance (RDA) and Adequate Intake (IA), and how they relate to the Daily Value (DV) used in food labeling.

2. List the six classes of nutrients.

3. Identify the fat- and water-soluble vitamins.

4. Describe why there is concern about the levels of intake of calcium, iron, and sodium in terms of health and disease.

5. Identify the two major classes of carbohydrates, and describe what the glycemic index is.

6. Identify the recommended changes in fat intake to improve health status.

7. List the nutrients that should be limited according to the 2015 Dietary Guidelines for Americans.

8. Identify the five food groups in the U.S. Style Healthy Eating Plan.

9. Describe the limitation of height/weight tables and the Body Mass Index in determining overweight and obesity.

10. Describe the two-component model of body composition and the assumptions made about the density values for the fat-free mass and the fat mass.

11. Explain the principle underlying the measurement of whole-body density with underwater weighing and why one must correct for residual volume.

12. Explain the principle by which bioelectrical impedance analysis (BIA) can be used to estimate the fat-free mass.

13. Explain how a sum of skinfolds can be used to estimate a percentage of body fatness value.

14. List the recommended percentage of body fatness values for health and fitness for males and females, and explain the concern for both high and low values.

15. Describe the roles of genetics and environment in the development of obesity.

16. Describe the pattern of change in body weight and caloric intake over the adult years.

17. Discuss the changes in body composition when weight is lost by diet alone versus diet plus exercise.

18. Describe the relationship of the fat-free mass to the BMR.

19. Define and give an example of thermogenesis.

20. Describe physical activity recommendations for preventing overweight and obesity, and for the prevention of weight regain in those who were previously obese.

• Standards of Nutrition 412

• Classes of Nutrients 413

  • Water 414

  • Vitamins 414

  • Minerals 414

  • Carbohydrates 418

  • Fats 419

  • Protein 419

• Dietary Guidelines for Americans 420

  • Healthy Eating Plans 421

  • Evaluating the Diet 422

• Body Composition 422

  • Methods of Assessing Overweight and Obesity 423

  • Methods of Measuring Body Composition 423

  • Two-Component Model of Body Composition 424

  • Body Fatness for Health and Fitness 427

• Obesity and Weight Control 428

  • Obesity 429

  • Causes of Obesity 429

• Diet, Physical Activity, and Weight Control 430

  • Energy Balance 430

  • Diet and Weight Control 431

  • Energy Expenditure and Weight Control 432

Adequate Intakes (AIs)

anorexia nervosa

basal metabolic rate (BMR)

bulimia nervosa

cholesterol

Daily Value (DV)

deficiency

Dietary Guidelines for Americans

Dietary Reference Intakes (DRIs)

elements

Estimated Energy Requirement (EER)

ferritin

food records

HDL cholesterol

high-density lipoproteins (HDLs)

LDL cholesterol

lipoprotein

low-density lipoproteins (LDLs)

major minerals

nutrient density

osteoporosis

provitamin

Recommended Dietary Allowance (RDA)

resting metabolic rate (RMR)

thermogenesis

Tolerable Upper Intake Level (UL)

toxicity

trace elements

transferrin

twenty-four-hour recall

underwater weighing

whole-body density

Chap. 14 described factors that limit health and fitness. These included hypertension, obesity, and elevated serum cholesterol. These three risk factors are linked to an excessive consumption of salt, total calories, and dietary fat, respectively. Clearly, knowledge of nutrition is essential to our understanding of health-related fitness. Whereas Chaps. 3 and 4 described the metabolism of carbohydrates, fats, and proteins, this chapter focuses on the type of diet that should provide them. The first part of the chapter presents nutrition standards and what they mean, a summary of the six classes of nutrients, dietary guidelines, and eating patterns (diets) to help meet those guidelines. The second part of the chapter discusses methods for measuring body composition, and the third part examines the role of exercise and diet in weight control. Nutrition related to athletic performance is covered in Chap. 23.

Food provides the carbohydrates, fats, proteins, minerals, vitamins, and water needed for life. The quantity of each nutrient needed for proper function and health is defined in one of the Dietary Reference Intakes (DRIs), an umbrella term encompassing specific standards for dietary intake. The DRIs include the following (61, 62, 63):

• Recommended Dietary Allowance (RDA). The average daily dietary nutrient intake level sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals in a particular group.

• Adequate Intakes (AIs). The AI describes levels that are assumed to be adequate when an RDA cannot be determined.

• Tolerable Upper Intake Level (UL). The highest average daily nutrient intake level likely to pose no risk of adverse health effects.

• Acceptable Macronutrient Distribution Ranges (AMDRs). These are defined as ranges of intakes for a particular energy source that is associated with a reduced risk of chronic disease, while providing adequate intake of essential nutrients. The ranges are 20% to 35% for fat, 45% to 65% carbohydrate, and the balance, 10% to 35%, is protein.

• Estimated Energy Requirement (EER). The average dietary energy intake that is predicted to maintain energy balance in a healthy adult of a defined age, gender, weight, height, and level of physical activity, consistent with good health (see appendix B for the EERs for males and females across various age groups).

• Daily Value (DV). Standard used in nutritional labeling to indicate how much of various nutrients is contained in the food we eat based on a 2000-calorie diet. For example, if one serving of a product provides 11% of the DV for fat, it contains 11% of the total amount of fat recommended for one day. Figure 18.1 provides an example of a food label.

  Figure 18.1

  Food packages must list product name, name and address of the manufacturer, amount of product in the package, and ingredients. The Nutrition Facts panel is required on virtually all packaged food products. The % Daily Value listed on the label is the percent of the amount of a nutrient needed daily that is provided by a single serving of the product.

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There are six classes of nutrients: water, vitamins, minerals, carbohydrates, fats, and proteins. In the following sections, each nutrient is described briefly, and the primary food sources of each are identified.

Water

Water is absolutely essential for life. Under normal conditions without exercise, water loss equals about 2500 ml/day, with most lost in urine. However, as higher environmental temperatures and heavy exercise are added, water loss can increase dramatically to 6 to 7 liters per day (165). Under normal conditions the 2500 ml of water per day is replaced with beverages (1500 ml), “solid” food (750 ml), and the water derived from metabolic processes (250 ml) (161). Most people are surprised by the large volume of water contributed by “solid” food until they consider the following percentages of water in solid food: baked potato—75%, apple—75%, lettuce—96% (161). A general recommendation under ordinary circumstances is to consume 1 to 1.5 ml of water per kcal of energy expenditure (107). The AI for total water intake (food and beverages) for women and men 19 to 70 years of age is set at 2.7 L/day and 3.7 L/day, respectively (62). However, to avoid potential problems associated with dehydration, one should drink water before and during exercise; thirst is not an adequate stimulus to achieve water balance (see Chap. 23).

Water weight can fluctuate depending on the body stores of carbohydrate and protein. Water is involved in the linkage between glucose molecules in glycogen and amino acid molecules in protein. The ratio is about 2.7 grams of water per gram of carbohydrate, and if an individual stores 454 g (1 lb) of carbohydrate, body weight would increase by 3.7 lbs. Of course, when one diets and depletes this carbohydrate store, the reverse occurs. This results in an apparent (and rapid) weight loss of 3.7 lbs when only 1816 kcal (454 g of carbohydrate times 4 kcal/g) have been lost.

Vitamins

Vitamins were introduced in Chap. 3 as organic catalysts involved in metabolic reactions. They are needed in small amounts and are not “used up” in the metabolic reactions. However, they are degraded (metabolized) like any biological molecule and must be replaced on a regular basis to maintain body stores. Several vitamins are in a precursor, or provitamin, form in foods and are converted to the active form in the body. Beta-carotene, the most important of the provitamin A compounds, is a good example. A chronic lack of certain vitamins can lead to deficiency diseases, and an excess of others can lead to a toxicity condition (161). In our presentation, vitamins will be divided into the fat-soluble and water-soluble groups.

Fat-Soluble Vitamins

The fat-soluble vitamins include A, D, E, and K. These vitamins can be stored in large quantities in the body; thus, a deficiency state takes longer to develop than for water-soluble vitamins. However, because of their solubility, so much can be stored that a toxicity condition can occur. Table 18.1 summarizes the information on these vitamins, including the RDA/AI standards, dietary sources, and functions.

Water-Soluble Vitamins

The water-soluble vitamins include vitamin C and the B vitamins: thiamin (B₁), riboflavin (B₂), niacin, pyridoxine (B₆), folic acid, B₁₂, pantothenic acid, and biotin. Most are involved in energy metabolism. You have already seen the role of niacin, as NAD, and riboflavin, as FAD, in the transfer of energy in the Krebs cycle and electron transport chain. Table 18.1 summarizes the information on these vitamins, including the RDA/AI standards, dietary sources, and functions. See Appendix C for recommended intakes of vitamins for males and females across various age groups.

Minerals

Minerals are the chemical elements, other than carbon, hydrogen, oxygen, and nitrogen, associated with the structure and function of the body. We have already seen the importance of calcium in bone structure and in the initiation of muscle contraction, iron in O₂ transport by hemoglobin, and phosphorus in ATP. Minerals are important inorganic nutrients and are divided into two classes: (a) major minerals and (b) trace elements. The major minerals include calcium, phosphorus, magnesium, sulfur, sodium, potassium, and chloride, with whole-body quantities ranging from 35 g for magnesium to 1200 g for calcium in a 70-kg man (161). The trace elements include iron, iodine, fluoride, zinc, selenium, copper, cobalt, chromium, manganese, molybdenum, arsenic, nickel, and vanadium. There are only 4 g of iron and 0.0009 g of vanadium in a 70-kg man. Like vitamins, some minerals taken in excess (e.g., iron and zinc) can be toxic. The following sections focus attention on calcium, iron, and sodium.

Calcium

Calcium (Ca⁺⁺) and phosphorus combine with organic molecules to form the teeth and bones. The bones are a “store” of calcium that helps to maintain the plasma Ca⁺⁺ concentration when dietary intake is inadequate (see parathyroid hormone in Chap. 5). Bone is constantly turning over its calcium and phosphorus, so diet must replace what is lost. If the diet is deficient in calcium for a long time, loss of bone, or osteoporosis, can occur. Three major factors are implicated: dietary calcium intake, inadequate estrogen, and lack of physical activity (see Chap. 17 for more on this) (3).

There is concern that the increase in osteoporosis in our society is related to an inadequate calcium intake. The adult RDA for calcium is 1000 mg/day, and although many men come close to meeting the standard, few women do (161). Given that the RDA is higher for 9–18-year-olds (1300 mg/day) and for females 51 to 70 years of age (1200 mg/day), the concern is greater for those age groups. Although menopause is the usual cause of a reduced secretion of estrogen, young, extremely active female athletes are experiencing this problem associated with amenorrhea (4). The decrease in estrogen secretion is associated with the acceleration of osteoporosis. Because exercise has been shown to slow the rate of bone loss, we now have physical activity recommendations to build bone mass in early childhood and to hold on to it as we age (2, 3, 153) (see Ask the Expert 17.1 in Chap. 17).

Iron

The majority of iron is found in hemoglobin in the red blood cells, which is involved with oxygen transport to cells (see Chap. 10). Other iron-containing molecules include myoglobin in muscle and the cytochromes in mitochondria, accounting for about 25% of the body’s iron. A large portion of the remaining iron is bound to ferritin in the liver, with the serum ferritin concentration being a sensitive measure of iron status (161, 165).

To remain in iron balance, the RDA is set at 8 mg/day for an adult male and 18 mg/day for an adult female; the higher amount is needed to replace that which is lost in the menses. In spite of the higher need for iron, American women take in only 13.2 mg/day, whereas men take in 17.5 mg/day (150). This is due to higher caloric intakes in males than females. The diet provides iron in two forms, heme (ferrous) and nonheme (ferric). Heme iron, found primarily in meats, fish, and organ meats, is absorbed better than nonheme iron, which is found in vegetables. However, the absorption of nonheme iron can be increased by the presence of meat, fish, and vitamin C (55, 161, 165).

Anemia is a condition in which the hemoglobin concentration is low: less than 13 g/dl in men and less than 12 g/dl in women. This can be the result of blood loss (e.g., blood donation or bleeding) or a lack of vitamins or minerals in the diet. The most common cause of anemia in North America is a lack of dietary iron (161, 165). In fact, iron deficiency is the most common nutrient deficiency. In iron-deficiency anemia, the iron bound to transferrin in the plasma is reduced, and serum ferritin (an indicator of iron stores) is low (55). Although children aged one to five years, adolescents, young adult women, and the elderly are more apt to develop anemia, it also occurs in competitive athletes. This latter point is discussed in detail in Chap. 23.

Sodium

Sodium is directly involved in the maintenance of the resting membrane potential and the generation of action potential in nerves (see Chap. 7) and muscles (see Chap. 8). In addition, sodium is the primary electrolyte determining the extracellular fluid volume. If sodium stores fall, the extracellular volume, including plasma, decreases. This could cause major problems with the maintenance of the mean arterial blood pressure (see Chap. 9) and body temperature (see Chap. 12).

The problem in our society is not with sodium stores that are too small, but just the opposite. The estimated average intake of sodium for Americans ages two years and older is about 3400 mg/day. This is considerably more than the AI for adults (1500 mg/day) and exceeds the UL for adults (2300 mg/day) (152). There is no question about the link between sodium intake and blood pressure, with evidence showing that reducing salt intake is feasible and will result in a reduction in cardiovascular morbidity and mortality (70, 162). The Dietary Guidelines for Americans have consistently supported the need to reduce sodium intake, and they recommend eating patterns (see later) to accomplish that goal. Those involved in athletic competition or strenuous exercise or who work in the heat must be concerned about adequate sodium replacement. Generally, because these individuals consume more kcal of food (containing more sodium), this is usually not a problem. More on this is in Chap. 23.

The previous sections have focused attention on three minerals—calcium, iron, and sodium—because of their relationship to current medical and health-related problems. A summary of the minerals, including the RDA/AI values, functions, and food sources is presented in Table 18.2. See Appendix D for recommended intakes of minerals (elements) for males and females across various age groups.

Carbohydrates

Carbohydrates and fats are the primary sources of energy in the average American diet. The Acceptable Macronutrient Distribution Range (AMDR) for carbohydrate is 45% to 65% (63). Carbohydrates suffer a bad reputation from those trying to lose weight, especially when you consider that you would have to eat more than twice as much carbohydrate as fat to consume the same number of calories (4 kcal/g versus 9 kcal/g). Carbohydrates can be divided into two classes: those that can be digested and metabolized for energy (sugars and starches), and those that are indigestible (fiber). The sugars are found in jellies, jams, fruits, soft drinks, honey, syrups, and milk, whereas the starches are found in cereals, flour, potatoes, and other vegetables (161).

Sugars and Starches

Carbohydrate is a major energy source for all tissues and a crucial source for two: red blood cells and neurons. The red blood cells depend exclusively on anaerobic glycolysis for energy, and the nervous system functions well only on carbohydrate. These two tissues can consume 180 grams of glucose per day (31). Given this need, it is no surprise that the plasma glucose concentration is maintained within narrow limits by hormonal control mechanisms (see Chap. 5). During strenuous exercise, the muscle can use 180 grams of glucose in less than one hour. As a result of these needs, one might expect that carbohydrate would make up a large fraction of our energy intake. Currently, about 50% of energy intake is derived from carbohydrate (150), within the AMDR carbohydrate intakes (63). There is great interest not only in the amount of carbohydrate in the diet but also in the type of carbohydrate as it relates to the prevention and management of diabetes (see Clinical Applications 18.1). The Dietary Guidelines for Americans have consistently stressed the need to avoid too much sugar (152), and the recommended eating patterns that will be presented later address this need.

Dietary Fiber

Dietary fiber is divided into the following classes (63):

• Dietary fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants. Examples of dietary fiber include plant nonstarch polysaccharides such as cellulose, pectin, gums, hemicellulose, and fibers in oat and wheat bran.

• Functional fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans. Examples include isolated, nondigestible plants (e.g., resistant starches, pectin, and gums) and animal (e.g., chitin and chitosan) or commercially produced (e.g., resistant starch, inulin, and indigestible dextrins) carbohydrates.

• Total fiber is the sum of dietary fiber and functional fiber.

Dietary fiber cannot be digested and metabolized, and consequently it provides a sense of fullness (satiation) during a meal without adding calories (38). This fact has been used by bakeries that lower the number of calories per slice of bread. Pectin and gum are used in food processing to thicken, stabilize, or emulsify the constituents of various food products (161).

Dietary fiber has long been linked to optimal health. Fiber acts as a hydrated sponge as it moves along the large intestine, making constipation less likely by reducing transit time (5, 38, 161). Vegetarian diets high in soluble fiber have been linked to lower serum cholesterol due to the loss of more cholesterol-containing bile acids in the feces. However, the fact that vegetarian diets are also lower in the percentage of calories from fat, which can also lower serum cholesterol, makes the interpretation of the data more complicated (38). Although a high-fiber diet reduces the incidence of diverticulosis, a condition in which outpouchings (diverticula) occur in the colon wall, the role of fiber in preventing colon cancer is mixed (5, 38).

Given the broad role of dietary fiber in normal health, it is no surprise that it has long been recommend that Americans increase their intake of fiber. The AI for total fiber, based on an intake level observed to protect against coronary heart disease, has been set at 38 and 25 g/day for men and women, 19 to 50 years of age, respectively (63). Each of the recommended eating patterns from the Dietary Guidelines for Americans (see later) include food groups to help meet this goal.

Fats

Dietary lipids include triglycerides, phospholipids, and cholesterol. The Acceptable Macronutrient Distribution Range for fat is 20% to 35%. If solid at room temperature, lipids are fats; if liquid, oils. Lipids contain 9 kcal/g and represent about 33% of the American diet, within the AMDR range, and lower than the 42% recorded in 1977 (29, 53, 100, 150, 154, 161). However, part of the decrease in this percentage was due to an increase in total calorie consumption, primarily from carbohydrate (27).

Fat not only provides fuel for energy but it is also important in the absorption of fat-soluble vitamins and for cell membrane structure, hormone synthesis (steroids), insulation, and the protection of vital organs. Most fat is stored in adipose tissue for subsequent release into the bloodstream as free fatty acids (see Chap. 4). Because of fat’s caloric density (9 kcal/g), we are able to carry a large energy reserve, with little weight. In fact, the energy content of one pound of adipose tissue, 3500 kcal, is sufficient to cover the energy cost of running a marathon. The other side of this coin is that because of this very high caloric density, it takes a long time to decrease the mass of adipose tissue when on a diet.

The focus of attention in the medical community has been on the role of dietary fat in the development of atherosclerosis (see Chap. 14). Dietary recommendations from many health organizations include a reduction in salt intake to control blood pressure (see minerals) and a reduction in fat intake, especially saturated fat and trans fat, to alter cholesterol levels. Usually, the cholesterol concentration in the serum is divided into two classes on the basis of what type of lipoprotein is carrying the cholesterol. Low-density lipoproteins (LDLs) carry more cholesterol than do the high-density lipoproteins (HDLs). High levels of LDL cholesterol are directly related to cardiovascular risk, whereas high levels of HDL cholesterol offer protection from heart disease. The concentration of HDL cholesterol is influenced by heredity, gender, exercise, and diet. Diets high in saturated fats and trans fats increase LDL cholesterol. A reduction in the sources of saturated fats would reduce LDL cholesterol. In fact, just substituting unsaturated fats for saturated fats (and not changing the percentage of fat in the diet) will lower serum cholesterol. Past and current Dietary Guidelines for Americans have promoted eating patterns to reduce the saturated fat intake to no more than 10% of caloric intake (see later). In addition, systematic steps are being taken to reduce or eliminate trans fats from the diet. These fats rival saturated fats as a contributor to heart disease, and substituting polyunsaturated fats should reduce cholesterol levels (75).

Protein

Even though protein has the same energy density as carbohydrate (4 kcal/g), it is not viewed as a primary energy source, as are fats and carbohydrates. Rather, it is important because it contains the nine essential (indispensable) amino acids, without which the body cannot synthesize all the proteins needed for tissues, enzymes, and hormones. The quality of protein in a diet is based on how well these essential amino acids are represented. In terms of quality, the best sources for protein are eggs, milk, and fish, with good sources being meat, poultry, cheese, and soybeans. Fair sources of protein include grains, vegetables, seeds and nuts, and other legumes. Given that a meal contains a variety of foods, one food of higher-quality protein tends to complement another of lower-quality protein to result in an adequate intake of essential amino acids (161). The Acceptable Macronutrient Distribution Range for protein is 10% to 35%.

The adult RDA protein requirement of 0.8 g/kg is easily met with diets that include a variety of the aforementioned foods. Although the vast majority of Americans meet this recommendation, an athlete’s protein requirement is higher than this level. The protein requirement for athletes is discussed in Chap. 23.

We are all familiar with the work of the American Heart Association (AHA), the American Cancer Society (ACS), and the American Diabetes Association (ADA) that brings special attention to the importance of diet in the prevention and treatment of cardiovascular diseases, cancers, and diabetes, respectively. As mentioned in Chap. 14, an underlying component associated with these chronic diseases is a chronic low-grade systemic inflammation, which can be altered with diet and physical activity. Foods recommended for good health have a high nutrient density, meaning they contain more nutrients (e.g., vitamins, protein) per calorie, and increase the chance of meeting nutrition standards and achieving a healthy body weight (161).

The Dietary Guidelines for Americans (152) is an evidence-based document that is published every five years by the Department of Health and Human Services and the U.S. Department of Agriculture. The primary purpose of the Guidelines is to inform the development of food, nutrition, and health policy and programs at the national level.

The five overarching Guidelines of the 2015–2020 report are the following:

1. Follow a healthy eating pattern across the lifespan. All food and beverages choices matter. Choose a healthy eating pattern at an appropriate calorie level to help achieve and maintain a healthy body weight, support nutrient adequacy, and reduce the risk of chronic disease.

2. Focus on variety, nutrient density, and amount. To meet nutrient needs within calorie limits, choose a variety of nutrient-dense foods across and within all food groups in recommended amounts.

3. Limit calories from added sugars and saturated fats and reduce sodium intake. Consume an eating pattern low in added sugars, saturated fats, and sodium. Cut back on foods and beverages higher in these components to amounts that fit within healthy eating patterns.

4. Shift to healthier food and beverage choices. Choose nutrient-dense foods and beverages across and within all food groups in place of less healthy choices. Consider cultural and personal preferences to make these shifts easier to accomplish and maintain.

5. Support healthy eating patterns for all. Everyone has a role in helping to create and support healthy eating patterns in multiple settings and support healthy eating patterns in multiple settings nationwide, from home to school to work to communities.

Key Recommendations

Select a healthy eating pattern that accounts for all foods and beverages within an appropriate calorie level.

A healthy eating patterns includes the following:

• A variety of vegetables from all of the subgroups—dark green, red and orange, legumes (beans and peas), starchy, and other

• Fruits, especially whole fruits

• Grains, at least half of which are whole grains

• Fat-free or low-fat dairy, including milk, yogurt, cheese, and/or fortified soy beverages

• A variety of protein foods, including seafood, lean meats and poultry, eggs, legumes (beans and peas), and nuts, seeds, and soy products

• Oils

A healthy eating patterns limits:

• Saturated fats, trans fats, added sugars, and sodium.

Some key recommendations have quantitative goals associated with dietary components that should be limited. These components are of particular public health concern in the United States, and the specified limits can help individuals achieve healthy eating patterns within calorie limits:

• Consume less than 10% of calories per day from added sugars

• Consume less than 10% of calories per day from saturated fats

• If alcohol is consumed, it should be consumed in moderation—up to one drink per day for women and up to two drinks per day for men—and only by adults of legal drinking age.

Americans should aim to achieve and maintain a healthy body weight. The relationship between diet and physical activity contributes to calorie balance and managing body weight. As such, the Dietary Guidelines includes a Key Recommendation to:

• Meet the Physical Activity Guidelines for Americans (see Chap. 16).

A good diet would allow an individual to achieve the RDA/AI for protein, minerals, and vitamins, while emphasizing healthy carbohydrates and minimizing unhealthy fats. Beginning with the 2010 Dietary Guidelines for Americans a variety of food group plans were developed to help individuals meet the Dietary Guidelines.

Healthy Eating Plans

The 2010 Dietary Guidelines for Americans (151) provided eating plans that had abundant vegetables and fruits, whole grains, moderate amounts of foods high in protein (seafood, beans and peas, nuts, seeds, soy products, meat, poultry, and eggs), limited amounts of foods high in added sugars, more oils than solid fats, with some having substantial amounts of low-fat milk and milk products. These diets had a high unsaturated-to-saturated fat ratio, more fiber, and a high potassium content. At the beginning of this section we indicated that various organizations (e.g., AHA) emphasize the importance of diet in the prevention and treatment of various chronic diseases. Table 18.3 shows the consistency in dietary and physical activity recommendations across these organizations (146). These recommendations are clearly in tune with the eating plans described in the 2015–2020 Dietary Guidelines for Americans.

Healthy U.S.-Style Eating Pattern

This food plan is the same as the USDA Food Pattern from the 2010 Dietary Guidelines for Americans. This plan identified daily amounts of foods with a focus on nutrient density from the five major food groups (and subgroups):

• Vegetables—includes dark green (broccoli and spinach) and red and orange (carrots and sweet potatoes) vegetables; beans and peas, starchy vegetables (corn, potatoes), and others (onions, iceberg lettuce)

• Fruits—fresh, frozen, canned, or dried

• Grains—including whole-grain products (whole-wheat bread) and enriched grains (white bread and pasta)

• Dairy products—with the emphasis on low-fat choices

• Protein foods—all meat, poultry, seafood, eggs, nuts, seeds, and processed soy products, with an emphasis on low-fat choices

Details for this plan can be found at http://health.gov/dietaryguidelines/2015/guidelines/appendix-3/.

Healthy Mediterranean-Style Eating Pattern

This eating plan was developed by modifying the Healthy U.S.-Style Eating Pattern. Its development took into account research examining the effect of the Mediterranean style diet on health outcomes. This eating pattern contains more fruit and seafood and less dairy than does the Healthy U.S.-Style Eating Pattern and, as a result, calcium and vitamin D are a little lower. Details for this eating plan can be found at http://health.gov/dietaryguidelines/2015/guidelines/appendix-4/.

Healthy Vegetarian Eating Pattern

This plan was developed by modifying the Healthy U.S.-Style Eating Pattern. This vegetarian plan has more legumes (beans and peas), soy products, nuts and seeds, and whole grains. It is somewhat higher in calcium and dietary fiber and lower in vitamin D due to differences in foods in the protein food group. Details can be found at http://health.gov/dietaryguidelines/2015/guidelines/appendix-5/.

Dietary Approaches to Stop Hypertension (DASH)

The DASH eating plan is an example of a healthy eating plan and has many of the same characteristics as the Healthy U.S.-Style Eating Pattern. The DASH plan was developed (as indicated in the title) to deal with hypertension, both to prevent it and to lower blood pressure in those with the problem. The DASH eating plan has been recognized for some time as an excellent approach to healthy eating consistent with reducing cardiovascular risk factors and achieving and maintaining a normal body weight. In addition, use of the DASH eating plan in a weight-loss program conveys more benefits to patients with metabolic syndrome than does a regular weight-loss diet (6). Details can be found at http://www.nhlbi.nih.gov/health/health-topics/topics/dash.

Evaluating the Diet

Independent of your dietary plan, the question arises as to how well you are achieving the guidelines. How do you analyze your diet? The first thing to do is to determine what you are eating, without fooling yourself. The use of the twenty-four-hour recall method relies on your ability to remember, from a specific time in one day, what you ate during the previous 24 hours. You have to judge the size of the portion you have eaten and make a judgment of whether that day was representative of what you normally eat. Other people use food records, in which a person records what is eaten throughout the day. It is recommended that a person obtain food records for three or four days per week to have a better estimate of usual dietary intake. Because the simple act of recording food intake may change our eating habits, one has to try to eat as normally as possible when recording food intake. It is important to remember that the RDA standards are to be met over the long run, and variations from those standards will exist from day to day (161). The USDA developed an online resource, MyPlate, to promote healthy eating and physical activity to achieve one’s body weight goal. We encourage you to log on to this website (http://www.choosemyplate.gov/MyPlate) to get a feel for its user-friendly nature.

Obesity is a major problem in our society, being related to hypertension, elevated serum cholesterol, and adult-onset diabetes (99). In addition, there is growing concern that as the incidence of childhood obesity increases, so will the pool of obese adults. To deal with this problem, we must be able to assess the prevalence of overweight and obesity, as well as describe, in more specific terms, changes in body composition. This section presents a brief overview on how to do that. For those interested in a thorough discussion of body composition assessment issues, we refer you to Heyward and Wagner’s Applied Body Composition Assessment and Heymsfield, Lohman, Wang, and Going’s Human Body Composition in Suggested Readings, and Ratamess’s chapter in the ACSM’s Resource Manual for Guidelines for Exercise Testing and Prescription (115).

Methods of Assessing Overweight and Obesity

In the latter part of the twentieth century, one of the most common ways of making a judgment about whether a person was overweight was to use the Metropolitan Life Insurance height and weight tables (91). This has been replaced with a measure that has now been universally adopted—the Body Mass Index (BMI)—the ratio of body weight (in kilograms) to height (in meters) squared: BMI = wt [kg] ÷ ht [m²]. The BMI is easily calculated, and guidelines for classifying someone as overweight or obese have used percentile rankings or fixed BMI values. Current BMI standards that have been adopted worldwide include (99):

The BMI standards are for adults. Investigators and clinicians have relied on percentile cutoffs (e.g., 85th and 95th percentile) to classify the fatness of children without labeling them as overweight or obese. However, there is now agreement to use a BMI ≥95th percentile (for age and gender) for classifying obesity and a BMI ≥85th percentile but <95th percentile for classifying overweight in children (102, 103).

One of the major problems associated with height/weight tables and the BMI is that there is no way to know if the person is heavily muscled or simply overfat. One of the earliest uses of body composition analysis showed that with height/weight tables, “All-American” football players weighing 200 lbs would have been found unfit for military service and would not have received life insurance (7), even though they were lean. Clearly, there is a need to distinguish overweight from overfat, and that is the purpose of the next section of this chapter.

Methods of Measuring Body Composition

The most direct way to measure body composition is to do a chemical analysis of the whole body to determine the amount of water, fat, protein, and minerals. This is a common method used in nutritional studies on rats, but it is useless in providing information for the average person—since the rats have to be “sacrificed” before analysis! Several methods of measuring body composition are found only in specialized laboratory or hospital settings because of cost, complexity, and the specialized personnel needed; they will only be described briefly with an appropriate reference for someone interested in more detail. The remainder of the methods are used in exercise physiology labs and fitness centers, and they will be addressed in more detail.

When using any of the body composition techniques, it is important to keep in mind that there is an inherent error involved in the estimations of percent body fat. The error is usually given as the standard error of estimate (SEE). If a technique estimates percent body fat as 20% and the SEE is ±2.5%, that means the actual percent body fat lies between 22.5% and 17.5% for 68% of subjects being measured as having 20% fat. Given that we don’t know where in that range an individual is, caution is needed when interpreting percent body fat values. It is best to compare these estimates of percent body fat over time on the same subject, where the change in percent fat is indicative of changes in the fat mass or fat-free mass (115).

The following methods of measuring body composition are expensive and complex, and need specially trained personnel to carry them out.

Isotope Dilution

Total body water (TBW) is determined by the isotope dilution method. In this method, a subject drinks an isotope of water (tritiated water—³H₂O), deuterated water (²H₂O), or ¹⁸O-labeled water (H₂¹⁸O) that is distributed throughout the body water (42, 124).

Photon Absorptiometry

A beam of photons from iodine-125 is passed over a bone or bones, and the transmission of the photon beam through bone and soft tissue is used to determine mineral content and density (25, 83).

Potassium-40

Measures the amount of a naturally occurring radioactive isotope of potassium, ⁴⁰K, that is proportional to the mass of lean tissue (17).

Radiography

An X-ray of a limb allows one to measure the widths of fat, muscle, and bone, and has been used extensively in tracing the growth of these tissues over time (49, 69, 72).

Ultrasound

Sound waves are transmitted through tissues, and the echoes are received and analyzed. This technique has been used to measure the thickness of subcutaneous fat (18).

Nuclear Magnetic Resonance (NMR)

Electromagnetic waves are transmitted through tissues, and the manner in which nuclei of specific tissues absorb and then release energy at a particular frequency (resonance) is used to determine body composition (18).

Total Body Electrical Conductivity (TOBEC)

The subject lies in a large cylindrical coil while an electric current is injected into the coil. The electromagnetic field developed in the space enclosed by the cylindrical coil is affected by the subject’s body composition (112, 130, 155).

Dual Energy X-Ray Absorptiometry (DEXA)

In this technology, a single X-ray source is used to determine whole-body and regional estimates of lean tissue, bone, mineral, and fat with a high degree of accuracy (40, 83, 89, 115, 157). The SEE is approximately ±1.8%.

The following techniques are used more routinely in exercise physiology or fitness center settings: hydrostatic (underwater) weighing and air displacement plethysmography that assess body density; bioelectrical impedance analysis (BIA) that estimates total body water; and sum of skinfolds. They will be addressed in detail after we introduce you to the two-component model of body composition analysis.

Two-Component Model of Body Composition

In some of the conventional methods of determining body composition (e.g., underwater weighing and air displacement plethysmography) the investigator obtains an estimate of whole-body density, and from this calculates the percentage of the body that is fat and the percentage that is fat-free. This is the two-component body composition system described by Behnke that is commonly used to describe changes in body composition (7). The conversion of whole-body density values to fat and fat-free tissue components relies on “constants” used for each of those tissue components. Human fat tissue is believed to have a density of 0.900 g/ml, and fat-free tissue a density of 1.100 g/ml. Using these density values, Siri (137) derived an equation to calculate the percentage of body fat from whole-body density:

This equation is correct only if the density values for fat tissue and fat-free tissue are 0.900 and 1.100 g/ml, respectively. Investigators sensed that certain populations might have fat-free tissue densities different from that of 1.100 g/ml when they observed high values for body fatness for children and the elderly, and extremely low values (<0% body fat) in professional football players (51, 87, 166). Children have lower bone mineral contents, less potassium, and more water per unit fat-free mass, yielding a lower density for fat-free mass (87). Lohman (84) reports density values (g/ml) of 1.080 at age 6, 1.084 at age 10, and 1.097 for boys aged 15½. The lower values in the prepubescent child would overestimate percent body fat by 5%. In contrast to children, who have density values below 1.100 for the fat-free mass, African Americans were shown to have a density of 1.113 g/ml (125). The Siri equation would have to be modified as follows:

In order to determine the correct value for the fat-free mass, scientists use more sophisticated multi-component approaches in which the body is divided into three components (body water, protein + mineral, and fat) or four components (mineral, water, protein, and fat). These approaches are more complex, but they address many of the limitations found in the two-component model (59, 83, 86, 108). Heyward and Wagner’s text (see Suggested Readings) provides guidance in choosing the most appropriate equation depending on age, race, gender, or other factors. The following sections will discuss how whole-body density values are determined by underwater weighing, air displacement plethysmography, BIA, and skinfold procedures.

Hydrostatic (Underwater) Weighing

Density is equal to mass divided by volume (D = M/V). Because we already know body mass (body weight), we only have to determine body volume to calculate whole-body density (52). The underwater weighing method applies Archimedes’ principle, which states that when an object is placed in water, it is buoyed up by a counterforce equal to the water it displaces. The volume of water displaced (spilled over) would equal the loss of weight while the object is completely submerged. Some investigators determine body volume by measuring the actual volume of water displaced; others measure weight while the subject is underwater and obtain body volume by subtracting the weight measured in water (M_W) from that measured in air (M_A), or (M_A − M_W). Both methods of determining volume are reproducible, but body fat percentage values are slightly but significantly (0.7%) lower with the volume displacement method (160). The weight of water displaced is converted to a volume by dividing by the density of the water (D_W) at the time of measurement:

This denominator must now be corrected for two other volumes: the volume of air in the lungs at the time of measurement [usually residual volume (V_R)] and the volume of gas in the gastrointestinal tract (V_GI). It is recommended that V_R be measured at the time that underwater weight is measured, but measurement on land with the subject in the same position is a suitable alternative (84). Residual volume can also be estimated with gender-specific regression equations or by taking 24% (males) or 28% (females) of vital capacity. However, the latter two procedures introduce measurement errors of 2% to 3% fat for a given individual (95). V_GI can be quite variable, and although some investigators ignore this measure, others assume a 100 ml volume for all subjects (22, 83).

The density equation can now be rewritten:

Figure 18.2 shows the equipment used to measure underwater weight. Water temperature is measured to obtain the correct water density. The subject is weighed on land on a scale accurate to within 100 grams. The subject puts on a diver’s belt with sufficient weight to prevent floating during the weighing procedure and sits on the chair suspended from the precision scale. The scale can be read to 10 grams and it has major divisions of 50 grams. The subject sits on the chair with the water at chin level and as a maximal exhalation is just about completed, the subject bends over and pulls the head under. When a maximal expiration is achieved, the subject holds that position for about five to ten seconds while the investigator reads the scale. This procedure is repeated six to ten times until the values stabilize. The weight of the diver’s belt and chair (measured under water) are subtracted from this weight to obtain the true value for M_W.

If V_R were to be measured at the time underwater weight is measured, the subject would have to be breathing through a mouthpiece and valve assembly that could be activated at the correct time (111).

The following data were obtained on a white male, aged 36: M_A = 75.20 kg, M_W = 3.52 kg, V_R = 1.43 liters, D_W = 0.9944 at 34°C, V_GI = 0.1 liter.

This density value is now used in Siri’s equation to calculate the percentage of body fat:

The underwater weighing procedure in which V_R is measured (and not estimated) has been used as the “standard” against which other methods are compared. However, remember that due to the normal biological variability in the fat-free mass in a given population, the body fat percentage value is estimated to be within about ±2.0%–2.5% of the “true” value (83, 115).

Air Displacement Plethysmography

Body density can also be calculated from body volume measurements obtained via air displacement plethysmography (in contrast to water displacement that is used in hydrostatic weighing). Small changes in pressure due to the change in the volume of air in a closed chamber are used to calculate the volume of the individual sitting in the chamber. The Bod Pod uses this technology to simplify the measurement of body volume and therefore the calculation of whole-body density (see Fig. 18.3). The information obtained is used in the same way as that collected in hydrostatic weighing (33). Although body fat percentage values derived from this method may not be the same as those from hydrostatic weighing, the differences are small (92). The SEE is approximately ±2.2–3.7% (115).

Bioelectrical Impedance Analysis (BIA)

This is a simple, fast, and noninvasive method of obtaining information about body composition. Electrodes are placed on the end of two limbs, distant from each other (e.g., wrist to ankle, hand to hand, foot to foot) and a small electrical current, undetectable to the person being tested, is applied to one extremity (via source electrode) while the voltage drop (impedance) is measured at the other extremity (via sink or detector electrode) (83, 97). The current flows through all conducting material in the path between the electrodes, with lean tissue (mostly water and electrolytes) being a good conductor (low impedance) and fat tissue being a poor conductor (high impedance). The impedance value provides an estimate of total body water from which the fat-free mass and body fat can be calculated (115).

Early BIA models used a single frequency (50 kHz) and a low-level current (500 mA) that could not distinguish between intracellular and extracellular components of total body water. BIA devices developed to address these limitations use multiple frequencies within a range of 4 to 1000 kHz, and they may provide better estimates of the fat-free mass (83, 115, 128, 158). The SEE is about ±3.5%–5% (115).

However simple the BIA technique is, one must attend to a number of details in order to obtain reliable values (97, 115, 147):

• Clean skin of oil, dirt, and lotions with alcohol prior to electrode placement.

• Place electrodes exactly as described by the manufacturer. Incorrect or inconsistent placement of electrodes from test to test will affect the accuracy and reliability of the measurement.

• If needed for the calculation of body composition, careful measurements of height (to nearest 0.5 cm) and weight (to nearest 0.1 kg) should be obtained.

• Exercise should be avoided for 12 hours prior to the measurement. Exercise affects BIA measurements because of increased blood flow to the skin, the warming of muscles, and sweating (dehydration).

• Alcohol or diuretics (unless prescribed by a physician) should not be used for 48 hours prior to the test because hydration status is very important for an accurate BIA measurement.

• Individuals should be instructed not to eat for four hours before the test and should drink only what is needed to maintain hydration.

• The phase of the menstrual cycle should be noted because of increased variability in the BIA measurement, between tests, in women,

Sum of Skinfolds

Underwater weighing, although a good way to obtain a measurement of body density, is time consuming and requires special equipment and personnel. Paralleling the development of the advanced technologies used in body composition analysis, scientists developed equations that predicted body density from a collection of skinfold measurements. The skinfold method relies on the observation that within any population a certain fraction of the total body fat lies just under the skin (subcutaneous fat), and if one could obtain a representative sample of that fat, overall body fatness (density) could be predicted. Generally, these prediction equations were developed with the underwater weighing method used as the standard. For example, a group of college males and females would have body density measured by underwater weighing, and a variety of skinfold measures would also be obtained. The investigator would then determine what collection of skinfolds would most accurately predict the body density determined by underwater weighing.

Investigators found that subcutaneous fat represents a variable fraction of total fat (20%–70%), depending on age, sex, overall fatness, and the measurement technique used. At a specific body fatness, women have less subcutaneous fat than men, and older subjects of the same sex have less than younger subjects (86). Given that these variables could influence an estimate of body density, it is no surprise that of the more than 100 equations developed, most were found to be “population specific” and could not be used for groups of different ages or sex. This obviously creates problems for professionals in adult fitness programs or elementary or high school physical education teachers when they try to find the equation that works best for their particular group. Fortunately, a good deal of progress has been made to reduce these problems.

Jackson and Pollock (65) and Jackson, Pollock, and Ward (67) developed “generalized equations” for men and women—that is, equations that could be used across various age groups. In addition, these equations have been validated for athletic and nonathletic populations, including postpubescent athletes (84, 135, 136). In these equations, specific skinfold measurements are obtained and the values are used along with age to calculate body density. Jackson and Pollock (66) simplified this procedure by providing tabulated body fatness percentage values for different skinfold thicknesses across age. All that is needed is the sum of skinfolds to obtain a body fatness percentage value. Appendixes E and F show the percent body fat tables for men and women respectively, using the sum of three skinfolds. For example, a woman aged 25 with a sum of skinfolds equal to 50 mm has a body fat percentage equal to 22.9% (look to the right of the sum of skinfold column where 48–52 is shown, over to the second column—ages 23–27). Body fatness percentage derived from skinfold measures is estimated to have an error of about ±3.5% relative to the “true” value (85, 115).

The use of skinfold measurements was extended to children in the schools for early identification of obesity. Currently, the FITNESSGRAM uses the calf and triceps skinfolds to estimate percent body fat and to set criterion-referenced standards (see Chap. 15) for health. As an alternative, when skinfold measurements cannot be taken, the BMI is used with appropriate criterion-referenced cutoff values for health (164). For more on this, visit the FITNESSGRAM website (http://www.fitnessgram.net).

Body Fatness for Health and Fitness

We have already seen how the BMI is used to classify individuals relative to health and disease. How do we use information on body composition (e.g., percentage of body fat) to make judgments about one’s status relative to health and fitness. Lohman, Houtkooper, and Going (82) recommend the following range of percent fat values for health and fitness for men and women:

Now that we know how to obtain a percent body fatness measure and what the healthy and fitness percent body fat standards are, how can we determine the body weight associated with those values? In the following example, a female college student has 30% body fat and weighs 142 lbs. She wants to be at the low end of the healthy range, so her goal is 20% fat.

• Step 1. Calculate fat-free weight:

  • 100% − 30% fat = 70% fat-free

  • 70% × 142 lb = 99.4 lb fat-free weight

• Step 2. $Goal weight=fat-free weight(1−goal % fat)′$, with goal % fat expressed as a fraction:

  For $20%=99.4 lb(1−.20)=124 lb$

Her weight at her 20% goal is 124 lb.

Lohman (85) also provides values for body fat percentages that are below the optimal range: For boys, 6% to 10% is classified as low and <6% as very low. Comparable values for girls are 12% to 15% and <12%, respectively. There is great pressure in our society to be thin, and this can be carried to an extreme. A far-too-common problem in our high schools and colleges is an eating disorder known as anorexia nervosa, in which primarily young women have an exaggerated fear of getting fat. This fear leads to food restriction and increased exercise in an attempt to stay thin, when they are, in fact, already thin (48). Bulimia nervosa is an eating disorder in which large quantities of food are taken in (binging), only to be followed by self-induced vomiting or the use of laxatives to rid the body of the food that was eaten (purging). While anorexia nervosa is characterized by the cessation of the menstrual cycle in women and the development of an emaciated state, the majority of the bingers/purgers are in the normal weight range (161).

It is clear that to stay in the optimal body fat percentage range, one must balance the dietary consumption of calories with energy expenditure. We will now consider that topic.

In Chap. 14 we discussed the major risk factors associated with degenerative diseases. Although high blood pressure, cigarette smoking, elevated serum cholesterol, and inactivity have been accepted as major risk factors, more and more evidence points to obesity as being a separate and independent risk factor for coronary heart disease (CHD), and one directly tied to two of the major risk factors. A variety of diseases are linked to obesity: hypertension, type 2 diabetes, CHD, stroke, gallbladder disease, osteoarthritis, sleep apnea and respiratory problems, and some types of cancer (endometrial, breast, prostate, and colon). In addition, obesity is associated with complications of pregnancy, menstrual irregularities, hirsutism, stress incontinence, and psychological disorders (e.g., depression) (23, 99, 114, 117).

It should be no surprise that obesity is linked to an increased morbidity due to cardiovascular disease and some types of cancers. However, being overweight is not. Overweight was associated with a lower mortality from all causes, including cardiovascular disease (45). This information provides special urgency to promote strategies, including doing physical activity on a regular basis, to reduce the chance of a person moving from the overweight to the obese category. Let’s take a more detailed look at obesity.

Obesity

If we use a BMI of ≥30 as a classification for obesity, the prevalence of obesity in U.S. adults increased from 15% in the 1976–1980 reporting period, to 23.3% in 1988–1994, to 30.9% in 1999–2000, to 32.2% in 2004, and to 35.9% for 2009–2010 (96, 104). When you include those who are classified as overweight (BMI 25.0–29.9), the prevalence of overweight and obesity was 69.2%. Consequently, more than two-thirds of the U.S. adult population is overweight, and one-third is obese. In addition, some ethnic groups were over-represented. More than half of non-Hispanic Black women were obese, and almost 80% were overweight. The good news is that the prevalence of obesity did not change between 2009 and 2010 and 2011 and 2012 (105).

All obesity is not the same. Studies suggest that not only is the relative body fatness related to an increased risk of CVD but the distribution of that fatness must also be considered. Individuals with a large waist circumference compared to hip circumference have a higher risk of CVD and sudden death (138, 139). These data suggest that in addition to skinfold or underwater weighing estimates of body density, measurements of waist and hip circumferences should be obtained (78, 79). Ratios of waist to hip circumference >0.95 for men and >0.8 for women are associated with the CVD risk factors of insulin resistance, high cholesterol, and hypertension, and such individuals are treated even if they are only borderline obese (9, 21, 26, 138). Given that the risk of these problems is associated with abdominal obesity, the BMI guidelines mentioned previously (99) use only waist circumference and recommend values of 102 cm (40 in) and 88 cm (35 in) to be used for men and women, respectively, when classifying those at high risk. Although various reviews support the continued use of circumference measurements to assess health risk (132), there is renewed interest in developing appropriate cutoff values for different racial groups since existing standards were derived primarily from Caucasian populations (60, 113, 142).

Unfortunately, obesity is not just a problem for adults. The prevalence of overweight and obesity in children and adolescents increased dramatically (from ~6% to ~15%) between the late 1970s and 2000; however, the rate of increase has slowed between 2000 and 2009–10 for both 6–11-year-olds (from 15.1% to 18.0%) and 12–19-year-olds (from 14.8% to 18.4%) (74, 96, 103, 106). Data for 2011–12 show no significant increase. However, a major concern is that the prevalence of those at the high end of the BMI scale (97th percentile for BMI) continues to increase at a fast rate (103). The increase in overweight and obesity in children over the past 35 years carries with it a disease burden (e.g., type 2 diabetes), formerly reserved for those over age 40 and overweight. What causes obesity?

Causes of Obesity

There is clearly no single cause of obesity. Obesity is related to a wide variety of genetic and environmental factors that can alter energy balance, which, in turn, leads to weight gain and obesity. Figure 18.4 provides a snapshot of some of the factors that can alter energy balance (98):

• Family history includes both the genetics involved as well as the lifestyle (physically active, planned meals vs. fast food, emotional environment, job security, etc.). Bouchard (13) and colleagues have determined, on the basis of an analysis of the relationships between and among nine types of relatives (spouses, parent-child, siblings, uncle-nephew, etc.), that 25% of body fat and fat mass is tied to genetic factors, and 30% is due to cultural transmission.

• Eating Plan—Is one of the healthy eating plans described earlier used in meal planning, or does fast food contribute substantial calories? Also, is alcohol consumption an issue?

• Physical activity—Is the person meeting the U.S. Physical Activity Guidelines (see Chap. 16)? Does the person have a sedentary job? How much time is spent watching TV or playing computer games?

• Health conditions such as having insufficient thyroid hormone (i.e., being hypothyroid) or having Cushing’s syndrome (i.e., too much cortisol due to overproduction of ACTH)

• Medications like antidepressants and the corticosteroids (linked to Cushing’s syndrome)

• Lack of sleep affects hormones involved in eating behavior (e.g., leptin) and provides more opportunities for eating.

Although there are many factors that can alter energy balance and cause obesity, the focus of attention is on two of them: diet and physical activity. The next section will address both relative to weight control.

The Framingham Heart Study showed that body weight increases as we age. A reasonable question to ask is whether this gain in weight was due to an increase in caloric intake. Interestingly, caloric intake decreased over the same age span (15). We are forced to conclude that energy expenditure decreased faster than the decrease in caloric intake, and as a result, weight gain occurred (11). This weight gain problem can be corrected by understanding and dealing with one or both sides of the energy balance equation. We will deal with the energy balance equation first, and then discuss how modifications in energy intake can affect weight loss. Finally, we will explore the variables on the energy expenditure side of the equation.

Energy Balance

Weight gain occurs when there is a chronic increase in caloric intake compared to energy expenditure. A net gain of about 3500 kcal is needed to add 1 lb (454 grams) of adipose tissue. We are all familiar with the energy balance equation:

What is implied by this equation is that an excess energy intake of 250 kcal/day will cause body weight to increase 1 pound in 14 days (250 kcal/day × 14 day = 3500 kcal = 1 pound). At the end of one year, the person will have gained about 24 pounds. As reasonable as this equation may appear, we know that a weight gain of that magnitude will not occur. The equation is a “static” energy balance equation that does not consider the effect that the weight gain will have on energy expenditure (143).

The energy balance equation can be expressed in a manner to account for the dynamic nature of energy balance in biological systems:

When body weight increases as a result of a chronically elevated energy intake, there is a compensatory increase in the amount of energy used at rest, as well as during activity when that heavier body weight is carried about. At some point, then, the additional 250 kcal/day of energy intake will be balanced by a higher rate of energy expenditure brought about by the higher body weight. Body weight will stabilize at a new and higher value, but it will be more like 3.5 pounds higher, rather than 24 pounds higher (143).

Diet and Weight Control

A good diet provides the necessary nutrients and calories to provide for tissue growth and regeneration and to meet the daily energy requirements of work and play. In our society, we are fortunate to have a variety of foods to meet these needs. However, we tend to consume more than the recommended amount of fat, which some believe is related to our country’s obesity problem. The hypothesis that the fat content of the diet is an important aspect of weight control revolves around the factors driving energy intake. There is a mandatory need for carbohydrate oxidation by the nervous system, and we are driven to eat what is used (43, 57, 126). If we eat a high-fat diet, we will take in a considerable amount of fat while consuming the necessary carbohydrates to refill the carbohydrate stores. This fat is stored, and body weight will increase. As mentioned earlier, as body weight increases, daily energy expenditure increases until an energy balance is achieved at a new and higher body weight. In this sense, the elevation of body weight and fat mass due to a high-fat/high-calorie diet is a compensatory mechanism that results in weight maintenance.

However, while a great deal of attention is focused on the composition of the diet, we should not forget that calories count in any weight-loss or weight-maintenance program. For example, studies in which subjects were switched from a high-fat to a low-fat diet, while maintaining a constant caloric intake, showed no change in energy expenditure or body weight (57, 81, 121, 149). In addition, in conditions in which a negative caloric balance was imposed to achieve weight loss, the composition of the diet (high-fat vs. low-fat) did not matter (56). In this sense, one should not get carried away with a high-carbohydrate diet and consume calories in excess of what is needed (44). In fact, when a variety of popular diets (Atkins, Ornish, Weight Watchers, and Zone) were compared over the course of one year, each modestly reduced body weight, with no difference between diets. However, as Figure 18.5 shows the best predictor of weight loss was the degree to which individuals adhered to the diet—no matter which one (32, 47, 101, 122).

Consistent with these findings, in the newly released set of guidelines from the American Heart Association and the American College of Cardiology for the management of overweight and obesity, a wide variety of diets are recommended for weight loss (39, 68). The diets are to be prescribed within a framework of a comprehensive lifestyle intervention that includes exercise (see later section) and counseling. These diets include high-protein, low-fat, low or high glycemic load, vegetarian, and Mediterranean types with the primary focus on energy (calorie) restriction to achieve weight loss. The guidelines recommend using “trained interventionists” to deliver the program, which included a maintenance phase when weight-loss goals were achieved. The trained interventionists included a broad range of health professionals, including registered dietitians, psychologists, exercise specialists, health counselors, and professionals in training (68). In contrast to dietary strategies to achieve weight loss in which the primary focus is on energy restriction, a parallel set of guidelines were provided to reduce the risk of cardiovascular and metabolic disease. Those guidelines focused on the health-related quality of the diet as described earlier in this chapter, including the use of the DASH and Mediterranean-style diets (39).

Energy Expenditure and Weight Control

The other side of the energy balance equation involves the expenditure of energy and includes the basal metabolic rate, thermogenesis (shivering and nonshivering), and physical activity/exercise. We will examine each of these relative to its role in energy balance.

Basal Metabolic Rate

Basal metabolic rate (BMR) is the rate of energy expenditure measured under standardized conditions (i.e., immediately after rising, 12 to 18 hours after a meal, in a supine position in a thermoneutral environment). Because of the difficulty of achieving these conditions during routine measurements, investigators have measured resting metabolic rate (RMR) instead. In this latter procedure, the subject reports to the lab about four hours after eating a light meal and after a period of time (30 to 60 minutes), the metabolic rate is measured (94). Given the low level of oxygen uptake measured for BMR or RMR (200 to 400 ml · min^-1), a variation of only ±20 to 40 ml · min^-1 represents a potential ±10% error. In the following discussion, both BMR and RMR will be discussed interchangeably, except where a specific contrast must be made for clarification.

The BMR is important in the energy balance equation because it represents 60% to 75% of total energy expenditure in the average sedentary person (109). The BMR is proportional to the fat-free mass, and after age 20 it decreases approximately 2% and 3% per decade in women and men, respectively. Women have a significantly lower BMR at all ages, primarily due to their lower fat-free mass (30, 37, 107, 163). Consistent with this, when RMR is expressed per unit of fat-free mass, there is no gender difference. At any body weight, the RMR decreases about 0.01 kcal/min for each 1% increase in body fatness (107). Although this may appear to be insignificant, this small difference can become meaningful in the progressive increase in weight gain over time. For example, a 5% difference in body fatness at the same body weight results in a difference of 0.05 kcal · min^–1, or 3 kcal · hr^–1, which is equal to 72 kcal · d^–1. It must be emphasized that the percentage of body fat makes only a small contribution to the BMR. This was confirmed in a study examining the relationship of body composition to BMR; fat mass did not improve the prediction of BMR based on the fat-free mass alone (30).

Part of the variation in BMR between individuals is due to a genetic predisposition to be higher or lower (13). The range (±3 SD) in normal BMR values is about ±21% from the average value, and such variation helps to explain the observation that some have an easier time maintaining body weight than others. If an average adult male has a BMR of 1500 kcal · d ^–1, those at +3 SD can take in an additional 300 kcal · d ^–1 (21% of 1500 kcal) to maintain weight, whereas others at the low end of the range (−3 SD) would have to take in 300 fewer kcals (37, 50).

The fat-free mass is not the only factor influencing the BMR. In 1919, Benedict (8) showed that prolonged dieting (reduction from about 3100 kcal to 1950 kcal) was associated with a 20% decrease in the BMR expressed per kilogram of body weight. This observation was confirmed in the famous Minnesota Starvation Experiment (73) and is shown in Figure 18.6. In this figure the BMR is expressed as a percent of the value measured before the period of semi-starvation. The percent decrease in BMR is larger “per man” because of the loss of lean tissue (as well as fat tissue); however, when the value is expressed per kilogram of body weight or per unit of surface area (m²), the BMR is still shown to be reduced. A decrease in the concentration of one of the thyroid hormones (T₃) and a reduced level of sympathetic nervous system activity have been implicated in this lower BMR due to caloric restriction (14). What these data mean is that during a period of low caloric intake, the energy production of the tissues decreases in an attempt to adapt to the lower caloric intake and reduce the rate of weight loss. This is an appropriate adaptation in periods of semi-starvation but is counterproductive in weight-reduction programs. This information bears heavily on the use of very-low-calorie diets as a primary means of weight reduction (36, 46, 110, 156, 159).

The BMR is also responsive to periods of overfeeding. In the dieting experiment of Benedict (8) mentioned earlier, when the subjects were allowed a day of free eating, the BMR was elevated on the following day. Further, in long-term (14 to 20 days) overfeeding to cause obesity, increases in resting and basal metabolic rates have been recorded. In essence, during the dynamic phase of weight gain (going from a lower to a higher weight), more calories are required per kilogram of body weight to maintain the weight gain than to simply maintain normal body weight (50, 134). This increased heat production due to an excess caloric intake, called thermogenesis, will be discussed in the next section. However, before leaving this discussion of the BMR, we need to mention the effect of exercise training. When careful measurements are made in which the RMR is expressed per kilogram of fat-free mass, the RMR is similar for trained and untrained individuals, and resistance and endurance training do not affect the value (19, 20).

Thermogenesis

Core temperature is maintained at about 37°C by balancing heat production with heat loss. Under thermoneutral conditions, the BMR (RMR) provides the necessary heat, but under cold environmental conditions, the process of shivering is actuated, and 100% of the energy required for involuntary muscle contraction appears as heat to maintain core temperature. In addition, some animals (including newborn humans) produce heat by a process called nonshivering thermogenesis, involving brown adipose tissue (BAT). This type of adipose tissue is rich in mitochondria and increases heat production in response to norepinephrine (NE) and other hormones. Recently, BAT has been discovered in adult humans, and a variety of studies indicate that it may play a role in preventing weight gain with age. The interest has now shifted to the “browning” of white adipose tissue. Some white adipose tissue has brown-like (beige) adipocytes that are responsive to stimuli used to engage brown adipose tissue. Clearly, this is an area we will hear more about in the future (12, 80, 123).

Thermogenesis involves more than brown adipose tissue. The heat generated due to the food we consume accounts for about 10% to 15% of our total daily energy expenditure; this is called the thermic effect of food (15, 109). Generally, this is determined by having a subject ingest a test meal (700−1000 kcal), and the elevation in the metabolic rate is measured following the meal. This portion of our daily energy expenditure is influenced by genetic factors (13, 109), is lower in obese than in lean individuals (71, 129), and is influenced by the level of spontaneous activity and the degree of insulin resistance (144). However, because it represents such a small part of the overall daily energy expenditure, it is not a good predictor of subsequent obesity.

Physical Activity and Exercise

Physical activity constitutes the most variable part of the energy expenditure side of the energy balance equation, being 5% to 40% of the daily energy expenditure (24, 109). There are those who have sedentary jobs and who do little physical activity during their leisure time. Others may have strenuous jobs or expend 300 to 1000 kcal during their leisure time every day or two. Just how important is physical activity in weight control? Epidemiological evidence suggests an inverse association between physical activity and body weight, with body fat being more favorably distributed in those who are physically active (34, 118). Consistent with these observations, various studies have shown an inverse relationship between the number of steps accumulated throughout the day and the BMI (see Fig. 18.7) (58, 148).

From a weight-loss perspective, one would think that a caloric deficit that results from an increase in energy expenditure through physical activity is equivalent to a caloric deficit due to a decrease in caloric intake. However, as Ross et al. (120) pointed out, leading authorities (99) have stated that the addition of exercise makes only a modest contribution to weight loss. How can this be the case, given the equivalency of the caloric deficit induced by either diet or exercise? Ross et al. (120) showed that in the majority of weight-loss studies in which a diet treatment was compared to an exercise treatment, the energy deficit caused by exercise was only a fraction of that caused by diet. In this situation, it is not surprising that weight loss due to diet was greater than that due to exercise; however, the weight loss due to exercise was as expected. The exercise-induced weight loss, 30% of that due to diet, was equivalent to the caloric deficit associated with the exercise (28% of that due to diet). In a study to determine whether an exercise-calorie is the same as a diet-calorie, Ross et al. (119) did a controlled experiment designed to achieve a 700 kcal/day energy deficit due to either exercise alone or diet alone. Both dietary intake and the structured exercise programs were carefully monitored to ensure compliance. Both groups lost 7.5 kg over 12 weeks—exactly what was expected from the 58,800 kcal deficit (700 kcal/day times 84 days). However, the exercise group lost more total fat, thus preserving muscle. Another group in this study did the same amount of exercise (700 kcal/day) but were told to increase their caloric intake by 700 kcal/day to maintain weight! Not only did they increase their $V˙O2$ max like the other exercise group, they also decreased their abdominal and visceral fat in spite of no weight loss (119). This is additional evidence to support the proposition that persons who are overweight or obese gain health benefits from exercise, even when weight is not lost.

These results were supported in a more recent study that compared subjects who underwent a six-month 25% caloric restriction with those who had a 12.5% caloric restriction plus an exercise intervention equal to a 12.5% reduction in caloric intake (116). Both groups lost the same amount of weight, similar to what was mentioned earlier. There is little question that a calorie of aerobic exercise is really equal to a calorie of diet restriction as far as change in body weight is concerned; however, the exercise intervention yields results (e.g., increase in $V˙O2$ max) that cannot be realized by a diet-alone approach to weight loss (116, 119).

In both of these studies, caloric intake was carefully monitored, as demanded by the research design. What happens when an exercise intervention is used and the subjects are simply told to not change their dietary intake? In a recent study, overweight and obese men and women were randomly assigned to either a control group or one of the following exercise groups: those who did 400 kcal/session or 600 kcal/session. The participants did five sessions a week for ten months. Although the exercise sessions were carefully monitored to make sure the subjects expended the necessary calories, they were simply told to not change their diet. Weight loss averaged 3.9 kg and 5.2 kg for the 400 and 600 kcal/session groups, respectively, while the control group gained 0.5 kg. The weight loss was due entirely to loss of body fat and represented a clinically significant weight loss in terms of risk of chronic diseases (35). These results were consistent with an earlier study that examined the effect of different amounts of exercise (energy expenditures equivalent to jogging 20 miles/wk vs. jogging 12 miles per week vs. walking 12 miles per week) on weight loss in subjects who were told to maintain a stable body weight. Weight loss over the nine-month study for the three groups was 2.9 kg vs. 0.6 kg vs. 0.9 kg, respectively, compared to a weight gain of 1.0 kg for the control group (140). More body fat than weight was lost, signifying an increase in fat-free mass. The emphasis in these studies was on vigorous exercise, except for the walking group in the last study.

Two well-controlled physical activity interventions focused on moderate-intensity physical activity, with half done in a structured setting and half done at home. Doing 180–370 minutes of physical activity per week (with no dietary restriction) resulted in a weight loss of about 1.5 kg over the 12-month studies (64, 90). Again, fat loss exceeded weight loss. Overall, weight loss follows a dose-response pattern, being greater with higher levels of energy expenditure.

Weight Loss vs. Weight Maintenance

Strange as it may seem, given what has just been presented, exercise is not required to achieve weight loss. All that is necessary is a caloric deficit, the magnitude of which is easily controlled by diet (56). However, as mentioned previously, the use of exercise as a part of a weight-loss program might maintain a higher lean body mass and RMR and result in an optimal body fatness at a higher body weight.

Although exercise might not be an essential ingredient in a weight-loss program, it is in a weight-maintenance program (141). A major unresolved question is, how much? However, some of the general exercise recommendations for health and fitness presented in Chap. 16 would be considered reasonable for weight-maintenance programs. The primary factor involved in energy expenditure is the total work accomplished, so low-intensity, long-duration exercise is as good as high-intensity, short-duration exercise in expending calories. For the sedentary, overweight person, moderate-intensity exercise is the proper choice because it can be done for longer periods of time in each exercise session and can be done each day. Recent well-controlled physical activity interventions showed that regular participation in moderate-intensity physical activity of 180 minutes or more per week (with no dietary restriction) was associated with a weight loss of about 1.5 kg over 8 to 12 months, indicating that the subjects were weight stable over that time (64, 90, 140). Although the magnitude of weight change was small, the message it conveys is important: Only 30 minutes or more of moderate-intensity physical activity per day is enough to prevent migration from one BMI level to the next—an important effect considering the prevalence of overweight and obesity in our society. However, as we saw in Chap. 16, “more is better” when it comes to physical activity and health, including maintaining a healthy body weight. It is clear that virtually any form of exercise contributes to fat loss and the maintenance of body weight. The important thing is to just do it. See Clinical Applications 18.2 for information on “successful losers.”

1. What is the difference between an RDA standard and a Daily Value?

2. Which two minerals are believed to be inadequate in women’s diets?

3. Relative to coronary heart disease, what fats should we limit in our diet?

4. Describe nutrient density in the context of the five food groups in the U.S. Style Healthy Eating Pattern.

5. Describe the two-component model of body composition assessment. Why should a different body density equation be used for children in contrast to adults?

6. What is the principle of underwater weighing?

7. Given a 20-year-old college male, 180 lb, 28% fat, what is his target body weight to achieve 17% fat?

8. Is obesity more related to genetics or the environment?

9. If a person consumes 120 kcal per day in excess of need, what weight gain does the static energy balance equation predict compared to the dynamic energy balance equation?

10. What happens to the BMR when a person goes on a prolonged low-calorie diet?

11. What are the differences in outcomes when someone uses diet alone, versus a combination of diet and exercise, to achieve a weight-loss goal?

12. What is the thermic effect of food?

13. In contrast to the general physical activity recommendation for achieving significant health benefits, how much physical activity may be needed to prevent weight gain or maintain weight once it has been lost?