Biology of Humans

15a. Nutrition and Weight Control

In the previous chapter, we learned about how the body digests food so it can be delivered to the cells. In this chapter, we consider the nutrients in a balanced diet—proteins, fats, vitamins, minerals, and water. We see how the body uses these nutrients and examine ways to combine the foods we eat to promote health and fitness.

Planning a Healthy Diet

Planning a healthy diet requires more than simply eating certain foods to avoid deficiencies of particular nutrients. Choosing the right balance of foods can help improve health and reduce the risk of serious chronic diseases, such as heart disease, cancer, and diabetes.

MyPyramid is a food guide released by the U.S. Department of Agriculture (USDA) to help any person plan a well-balanced diet (Figure 15a.1). The easy-to-use website www.MyPyramid.govpersonalizes the nutritional information, guiding each user to one of 12 pyramid-shaped diagrams that summarize the eating pattern considered optimal for someone of his or her age, gender, and level of activity. MyPyramid follows the recommendations in the USDA's Dietary Guide for Americans. This guide promotes a healthy lifestyle that includes wise decisions about food and physical activity. All 12 pyramids share certain features. The human figure shown running up the steps on the side of the pyramid reminds us that daily physical activity is a key to healthy living. There are 12 different pyramids because a healthy dietary plan needs to allow for individual differences in the number of calories each person requires. Each food group is color coded, corresponding to colored bands on the pyramid that remind us to eat a variety of foods. The width of each band suggests what proportion of our daily diets should come from that food group. The bands are wider at the bottom, suggesting that most of the food we eat in each food group should be nutrient dense and contain few "empty" calories (a concept we explain later in this chapter). The pyramid's slogan, "Steps to a Healthier You," reminds us that improvements in diet should happen gradually. Besides being personalized, MyPyramid differs from the old food pyramid by specifying quantities to be consumed in cups or ounces instead of in servings. The Dietary Guide also allows discretionary calories. These are calories from any food group that are allowable after you have met your basic nutrient needs. The flexibility represented by discretionary calories allows you to consume some foods and drinks that contain added fats, added sugar, or alcohol.

·       Eating a balanced diet and maintaining a healthy weight are important ways of safeguarding one's health.

FIGURE 15a.1. The food guide pyramid helps you plan a well-balanced diet, even if you are not an expert nutritionist. The amounts specified in this figure are for a diet totaling 2000 calories per day.

U.S. Department of Agriculture (USDA), http://www.mypyramid.gov/

You are following a diet of 2000 calories a day. If you eat a cup of cereal for breakfast and a turkey sandwich for lunch, how many cups of spaghetti can you eat for dinner?

1-1/2 cups

The Dietary Guide makes three general recommendations for a healthy lifestyle: (1) choose food wisely, (2) stay within your calorie needs, and (3) engage in physical activity. MyPyramid can help you plan a nutritionally balanced diet to satisfy the first recommendation. The remainder of this chapter explains some of the biology that the dietary guidelines are based on.

Nutrients

What does the body do with the food you eat? Food provides fuel, building blocks, molecules needed to carry out chemical processes in the body, and water.

• Food is needed as an energy source for all cellular activities. (Energy is measured in a unit called a calorie, which is the amount needed to raise the temperature of 1 g of water 1°C. In discussions of biochemical reactions, gains and losses of energy are usually reported in kilocalories. A kilocalorie is 1000 calories of energy. In nonscientific discussions, however, the kilo is dropped, and kilocalories are referred to simply as "calories." We will follow this tradition because it is the way most nutritional values are reported to the public.1)

• Building blocks are needed for cell division, maintenance, and repair.

• Molecules, such as vitamins, are needed to coordinate life's processes.

• Water is necessary for maintaining the proper cellular environment and for certain cellular reactions.

The digestive system breaks the complex molecules of carbohydrates, proteins, fats, and nucleic acids into their component subunits. Most cells of the body, especially those of the liver, are able to use these subunits as building blocks by joining them together and sometimes by converting one type of molecule into another. Your body, however, cannot synthesize all of the required kinds of amino and fatty acids, at least not in quantities sufficient to meet bodily needs. These substances that the body cannot synthesize are called essential amino acids and essential fatty acids. Essential means that the substances must be included in the diet because the body cannot produce them.

A nutrient is a substance in food that provides energy; becomes part of a structure; or performs a function in growth, maintenance, or repair. Three nutrients—fats (lipids), carbohydrates, and proteins—can provide energy. Although proteins are included in this list, they are usually used to build cell structures or regulatory molecules, such as enzymes or certain hormones. Vitamins, minerals, water, and fiber do not provide energy, but they are essential to cellular functioning, as we'll see later.

Lipids

Lipid is the more technical name for what we have been calling fat. There are several types of lipids. They include fats, oils, and cholesterol. Even so, 95% of the lipids found in food are triglycerides—the fats that we commonly think of when we hear the term fat. A triglyceride is a molecule made from three fatty acids (hence, tri-) attached to a molecule of glycerol (hence, -glyceride). The fatty acids in the triglyceride give the molecule its characteristics.

An important way that fatty acids can differ is in their degree of saturation, or the extent to which each carbon in the fatty acid is bonded to as many hydrogen atoms as possible. Recall from Chapter 2 that a saturated fatty acid contains all the hydrogen it can hold. A polyunsaturated fatty acid can hold four or more additional hydrogens, and a monounsaturated fatty acid can hold two more hydrogens. In general, saturated fats are solid at room temperature, and they usually come from animal sources. In contrast, unsaturated fats are liquid at room temperature. Oils are unsaturated fats. The oils in our diet usually come from plant sources.

We have a biological need for lipids. Certain lipids, including cholesterol, are essential components of all cell membranes; some are used in the construction of myelin sheaths that insulate nerve fibers. Lipids are also needed for the absorption of the fat-soluble vitamins A, D, E, and K. These vitamins are absorbed from the intestines along with the products of fat digestion. Lipids then carry these vitamins in the bloodstream to the cells that use them. Glands in our skin produce oils that keep the skin soft and prevent dryness. Cholesterol is the structural basis of the steroid hormones, including the sex hormones.

The daily need for dietary fat is a mere tablespoon, yet the average American adult consumes 6 to 8 tablespoons of fat (78 to 196 g) a day, which can quickly add inches to the waistline. Obesity is associated with health problems such as high blood pressure and increased risk of diabetes. In addition, the high consumption of fat is related to certain cancers, including cancer of the colon, prostate gland, lungs, and perhaps the breast.

The blood level of cholesterol, which is partly influenced by the amount and kinds of fat in the diet, affects the risk of developing atherosclerosis, a condition in which fatty deposits form in the walls of blood vessels. The deposits promote an inflammatory response in the artery wall, thereby increasing the risk of heart attack and stroke (see Chapter 12a). The risk of atherosclerosis increases with the blood level of cholesterol. In general, blood cholesterol levels under 200 mg/dl2 are recommended for adults. The bad news is that the average blood cholesterol level for a middle-aged adult in the United States is 215 mg/dl. The good news is that people who lower their blood cholesterol levels can slow, or even reverse, atherosclerosis and, therefore, their risk of heart attack.

Blood cholesterol comes from one of two sources: the diet or the liver. Of the two, cholesterol production by the liver is more significant because most of the cholesterol in blood comes from the liver and not from the food we eat. Diet is still important, and, surprisingly, saturated fat in the diet raises blood levels of cholesterol more than dietary cholesterol does. Moreover, different people's bodies handle cholesterol differently depending on their genetic makeup (as well as the amount of cholesterol consumed in a meal). As a result, there is not a clear relationship between dietary and blood cholesterol.

Total blood cholesterol provides only a partial picture of a person's risk of atherosclerosis. Before cholesterol can be transported in the blood or lymph, it is combined with protein and triglycerides to form a lipoprotein, which makes it soluble in water. Low-density lipoproteins (LDLs) are considered to be a damaging, or bad, form of cholesterol. Although LDLs bring cholesterol to the cells that need it to sustain life, they also deposit cholesterol in artery walls. In contrast, high-density lipoproteins (HDLs) carry cholesterol from the cells, perhaps even from the artery walls, to the liver for elimination. Because HDLs are protective against heart disease, they are considered to be a good form of cholesterol.

Because of the different roles the two kinds of lipoproteins play in transporting cholesterol, the proportion of HDLs and LDLs in the blood is considered a more important indicator of risk than is the total blood cholesterol (HDL + LDL) level alone. The ratio of total cholesterol to HDLs should not be greater than 4:1. HDL levels higher than 60 mg/dl are considered to reduce the risk of heart disease.

The types of fat in the diet can influence blood cholesterol level and, more importantly, the ratios of LDLs and HDLs. Saturated fats and trans fats are bad for the heart. Saturated fat consistently boosts blood levels of harmful LDL cholesterol, both by directly stimulating the liver to step up its production of LDLs and by slowing the rate at which LDLs are cleared from the bloodstream. Foods high in saturated fat include meat (especially red meat), butter, cheese, whole milk, and other dairy products. Trans fatty acids are formed when hydrogens are added to unsaturated fats (oils) in such a way as to stabilize them or to solidify them, as when margarine is formed from vegetable oil. Trans fats were also found in many packaged foods (Figure 15a.2). Trans fatty acids may behave like saturated fatty acids and raise LDL levels. In addition, they lower good HDLs.

FIGURE 15a.2. Types of dietary fats. The total fat content of the diet should be moderate. The types of fats in the diet can influence the risk of heart attack and stroke.

But not all fats are bad. Monounsaturated fats and polyunsaturated fats, such as omega-3 fatty acids and omega-6 fatty acids,3 are good fats; they lower total blood cholesterol and LDLs. Monounsaturated fats are found in olive, canola, and peanut oils and in nuts. Omega-3 fatty acids are found in the oils of certain fish, such as Atlantic mackerel, lake trout, herring, tuna, and salmon. The most important omega-6 fatty acid is linoleic acid, an essential fatty acid found in corn and safflower oils.

What would you do?

Statins are cholesterol-lowering drugs. Some scientists believe that statins lower cholesterol equally in men and women. Other scientists believe that statins are not effective in women, and they issue warnings about the drug's side effects. If your mother, sister, or aunt had high cholesterol and her physician suggested that she take statins, what would you advise her to do? What information would you want to have before making that decision? How would you find that information?

Carbohydrates

Carbohydrates in the diet come primarily from plant sources and include sugars, starches, and roughage/dietary fiber (Figure 15a.3). The basic unit of a carbohydrate molecule is a monosaccharide. Sugars are simple carbohydrates, meaning they are generally monosaccharides or disaccharides (two monosaccharides linked together). Sugars taste sweet and are naturally present in whole foods such as fruit and milk. In whole foods, sugars are present with other nutrients, including vitamins and minerals. However, refined sugars have been removed from their plant sources and concentrated, so they are no longer mixed with other nutrients. The calories from refined sugars are therefore described as "empty." They provide energy but have no other nutritive value. Refined sugars are found in candies, cookies, cakes, pies, and sodas.

FIGURE 15a.3. Types of carbohydrates. Simple carbohydrates, especially refined sugars, should be minimized in the diet. Most of the carbohydrates in the diet should be complex carbohydrates.

Starches and fiber are complex carbohydrates, which means they are polysaccharides. Plants store energy in starches—long, sometimes branched chains of hundreds or thousands of linked molecules of the monosaccharide glucose. Common sources of starches include wheat, rice, oats, corn, potatoes, and legumes. Fiber is the indigestible part of edible plants. Complex carbohydrates are usually accompanied by other nutrients, but even complex carbohydrates are not all healthful, because they differ in the way they affect blood glucose.

The most important function of carbohydrates is to provide fuel for the body. Recall that the digestive system breaks down all carbohydrates (except fiber) to simple sugars, primarily glucose, which is absorbed into the bloodstream. Glucose is the carbohydrate that cells use for fuel most of the time. A measure called the glycemic response describes how quickly a serving of food is converted to blood sugar and how much the level of blood sugar is affected. The glycemic index is a numerical ranking of carbohydrates based on their glycemic response. The scale of the glycemic index ranges from 0 to 100, with pure glucose serving as a reference point of 100. Sugar and starchy foods such as white bread, potatoes, and white rice have a high value on the glycemic index because they cause the blood sugar level to rise sharply. Foods with a low value on the glycemic index, including whole fruit, whole-grain foods, brown rice, and barley, cause a more modest and gradual increase in blood sugar. Generally, foods with a low value on the glycemic index are high in fiber.

The glycemic response is important because it influences how the body reacts to different foods. After you eat a carbohydrate with a high glycemic index value, your blood glucose rises. As discussed in Chapter 10, a rise in blood glucose level causes the pancreas to release more of the hormone insulin, which lowers blood glucose (thus inducing hunger) by converting excess glucose to fat. In short, in a healthy person, consumption of a high glycemic food is followed by fat formation and by increased hunger. In people with diabetes mellitus, rising blood sugar levels due to high-glycemic foods may cause medical problems, because these people are unable to produce or to use insulin (see Chapter 10a).

Nutritionists recommend that you limit your intake of simple carbohydrates, especially refined sugars, and instead choose sources of carbohydrates that contain more than just calories. For example, fruits, vegetables, grains, and milk are food groups that have low glycemic index values and supply many other nutrients besides carbohydrates.

Dietary fiber is found in all plants that are eaten for food, including fruits, vegetables, dried beans, and whole grains. Fiber is a form of carbohydrate that humans cannot digest into its component monosaccharides. Because only monosaccharides can be absorbed from the small intestine, dietary fiber is passed along to the large intestine. Some of the fiber is digested by the bacteria living in the large intestine, and the remaining fiber gives bulk to feces.

Although fiber cannot be digested or absorbed, it is still an important part of a healthful diet. Fiber is good for the heart and blood vessels, because it lowers LDLs but does not lower the beneficial HDLs. Intestinal disorders such as constipation and hemorrhoids improve when the amount of fiber in the diet is increased (discussed in Chapter 15). Fiber can absorb an amazing amount of water, thereby softening stools and making them easier to pass.

Proteins

A protein consists of one or more chains of amino acids. Human proteins contain 20 different kinds of amino acids. The protein in the food you eat is digested into its component amino acids, which are then absorbed into the bloodstream and delivered to the cells, creating a pool of available amino acids. Your cells then draw the amino acids needed to build proteins in your body from those available in the pool. In addition, the body is able to synthesize 11 of the amino acids from nitrogen and molecules derived from carbohydrates, fats, or other amino acids. The 9 remaining amino acids that the body cannot synthesize— called essential amino acids—must be supplied by the diet.

When your body makes a protein, the amino acids are strung together in a specific order, depending on which protein is being made. If a particular amino acid is needed for a certain protein but is not available, then that protein cannot be synthesized. Consider this analogy: A sign maker with a bag containing only 1 copy of the letter R and 100 copies of the other 25 letters in the alphabet could make only one NO PARKING sign. There is simply no way to substitute for the limiting letter, R. The same principle applies to protein synthesis; if a needed amino acid is lacking, protein production stops or the body breaks down existing proteins to get that amino acid.

In short, the pool of amino acids available for protein synthesis must always contain sufficient amounts of all the essential amino acids. Dietary proteins described as complete proteins contain ample amounts of all the essential amino acids. Animal proteins are generally complete. Plant proteins are generally incomplete proteins and are low in one or more of the essential amino acids. Eating certain combinations of incomplete proteins from two or more plant sources ensures that the pool of amino acids available for protein synthesis contains ample amounts of all the essential amino acids (Figure 15a.4). Such combinations are called complementary proteins because if the combinations are correct, they supply enough of all the essential amino acids (Figure 15a.5). A vegetarian must be sure to consume complementary proteins.

FIGURE 15a.4. Types of protein in the diet

FIGURE 15a.5. Complementary proteins are combinations of two or more incomplete proteins that together supply all the essential amino acids.

Sources of animal protein usually contain a lot of fat, whereas plant protein sources usually contain less fat and are good sources of carbohydrates, too. The tendency in the United States to make meat the focal point of every meal is the main reason that the typical diet here contains excess fat. For a healthier diet, choose lean, low-fat, or fat-free sources of protein. Eating a variety of plant proteins will supply all the essential amino acids and will help reduce the percentage of fat calories in the diet as well as boost the percentage of calories from complex carbohydrates. When choosing among animal sources of protein, reduce the amount of fatty red meat in favor of chicken (with the fatty skin removed) or fish.

Vitamins

A vitamin (vita, life) is an organic (carbon-containing) compound that, although essential for health and growth, is needed only in minute quantities—milligrams or micrograms. All the vitamins you need in a day would fill only an eighth of a teaspoon. These tiny amounts are sufficient because vitamins are not broken down or destroyed during use. Most function as coenzymes, which are nonprotein molecules necessary for certain enzymes to function. Enzymes and coenzymes are continuously recycled and, therefore, can be used repeatedly by the body.

There are two categories of vitamins: water-soluble vitamins, which dissolve in water, and fat-soluble vitamins, which are stored in fat. Of the 13 vitamins known to be needed by humans, 9 are water soluble (C and the various B vitamins) and 4 are fat soluble (A, D, E, and K). Table 15a.1 lists the vitamins, their functions, good sources for them, and the problems associated with deficiencies or excesses.

Except for vitamin D, our cells cannot make vitamins, so we must obtain them in our food. A varied, balanced diet is the best way to ensure an adequate supply of all vitamins. No one food contains every vitamin, but most contain some. Vitamins are often more easily available for absorption when the foods containing them are cooked. Cooked carrots, for instance, are a better source of vitamin A than are raw carrots. However, water- soluble vitamins are likely to be lost if the vegetables containing them are cooked by being boiled in water. Steaming those vegetables is a better way to preserve their vitamin content.

TABLE 15a.1. Vitamins

Certain vitamins have to be consumed in adequate amounts every day. Folic acid, one of the B vitamins that is particularly abundant in dark leafy greens, plays a role in preventing birth defects, such as spina bifida, that involve the brain and spinal cord. It now seems that folic acid, along with vitamins B6 and B12, may also help prevent heart disease. Five daily servings of fruits and vegetables should provide enough of these vitamins to protect the heart. Antioxidant vitamins, including vitamin C, vitamin E, and beta-carotene, protect against cell damage due to oxidation. Some researchers believe that antioxidants slow the aging process and protect against cancer, atherosclerosis, and macular degeneration (the leading cause of irreversible blindness in people over age 65). These, too, should be ingested every day. Spinach, collard greens, and carrots are good sources of antioxidant vitamins.

Stop and think

Some weight-loss medications work by preventing fats from being absorbed from the digestive system. Why might these drugs lead to deficiencies in vitamins A, D, E, and K?

Minerals

The minerals needed in our diet are inorganic substances essential to a wide range of life processes. We need fairly large amounts, although not what would be described as "megadoses," of seven minerals: calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium. In addition, we need trace amounts of about a dozen others. Table 15a.2 on page 308 lists selected minerals, their functions, good sources for them, and the problems associated with deficiencies or excesses.

TABLE 15a.2. Selected Minerals

We can obtain the necessary minerals from the foods we eat, as long as we prepare them in ways that do not reduce their mineral content. Like certain vitamins, many minerals are water soluble and can be lost during food preparation.

Sodium, a component of table salt, is essential to health, but most Americans consume too much of it. High salt intake causes high blood pressure in some people. The Dietary Recommendations for Americans advises that salt intake should not exceed 2400 mg (slightly more than 1 teaspoon) a day. Processed foods are especially high in salt, which is added to preserve food and enhance the taste. Salt is found in nearly every processed food product, including canned vegetables, cheese, bread, and processed meats. To reduce your salt intake, use salt sparingly when cooking fresh food, and read the labels on prepared food.

Water

Water is perhaps the most important nutrient; it transports materials through our bodies, lubricates and cushions organs, helps in temperature regulation, and provides a medium for many vital chemical reactions. We can live without food for about 8 weeks, but without water for only about 3 days. Nutritionists recommend that we consume eight 8-ounce glasses (2 quarts) of water a day. Water in fruits and vegetables makes up about half of that requirement for the average adult. The rest of the requirement does not have to be consumed as plain water, but it should not come from carbonated sweet drinks, caffeinated beverages, or alcoholic beverages. Carbonation interferes with water absorption, and sugar adds empty calories. Caffeine and alcohol increase water loss in urine.

Food Labels

Grocery stores generally offer several choices for essentially the same product. Food labels and the knowledge you now have about nutrients can help you make healthy choices.

Figure 15a.6 shows a food label from a box of baked whole wheat crackers. It provides some useful general pointers for reading food labels. First, note the serving size described on the label, and remember that the amount you actually eat, whether larger or smaller than what is described on the label, will be what determines the number of calories and amount of nutrients you consume.

FIGURE 15a.6. Tips for reading food labels

Next, notice the number of calories reported per serving and the number of calories from fat. In the example shown in the figure, 40 of the 120 calories in a serving, or nearly 34% of the calories, come from fat. Dietary guidelines recommend that you consume less than 30% of your total calories as fat. However, if you crave these crackers, you can make up for the amount of fat in them, to some extent, by eating low-fat food at another time during the day. Also note the amount of saturated fat and trans fat reported on the label, because these kinds of fat should be minimized in your diet.

In considering the percent daily values of various nutrients reported on the label, keep two things in mind. First, these values are based on a 2000-calorie diet. Your own diet may require more or fewer calories. Second, as becomes clear from the daily values provided at the bottom of the label, you should be attempting to keep dietary fat and sodium consumption below those amounts.

Food labels can also help you increase your intake of nutrients that are important to consume. Be sure to get enough vitamin A, vitamin C, calcium, iron, and phosphorus. The daily values reported for carbohydrates and dietary fiber will also help you consume enough of those substances.

Energy Balance

The body requires energy for maintenance of basic body functions, for physical activity, and for processing the food that is eaten. The energy that is needed strictly for maintenance is called the basal metabolic rate (BMR); it is the minimum energy needed to keep an awake, resting body alive, and it generally represents between 60% and 75% of the body's energy needs. A male usually has a higher metabolic rate than does a female because a male's body has more muscle and less fat than a female's. Muscles use more energy than fat does. So, while a man and a woman of equal size sit on the couch and watch television together, he burns 10% to 20% more calories than she does. As you age, muscle mass and metabolic rate both decline. Together, these factors reduce caloric needs. If other adjustments in lifestyle are not made to compensate, these metabolic changes can add extra pounds each year after age 35. Besides gender and age, a person's BMR is influenced by age, health, food intake, and genetics.

The second largest use of energy is physical activity. Exercise is an excellent way to burn calories. It not only boosts your energy needs during the activity but also speeds up metabolic rate for a while afterward. When you determine your caloric needs, the calories you use in exercise are added to those needed to maintain your BMR.

Exercise helps keep the body in good working order. The Dietary Guide for Americans encourages adults to engage in at least 30 minutes of moderate-intensity physical activity on most days of the week. Aerobic exercise reduces the risk of diseases of the heart and blood vessels and lowers blood pressure (discussed in Chapter 12). Weight-bearing exercise reduces the risk of osteoporosis, a loss of bone density (discussed in Chapter 5). In general, regular physical exercise reduces stress and the risk of certain chronic diseases, including diabetes. Regular exercise also manages body weight, helping to prevent the unhealthy weight gain that can occur in adulthood.

Obesity

Although overweight and obese are both terms used to describe people who have excess body weight, they do not have exactly the same meaning. An obese person is overweight because of excess fat. An athletic person who because of well-developed muscle weighs more than the weight listed as desirable on height-weight tables is not obese. However, most people who are overweight have too much body fat. Overweight and obese generally refer to ranges of weight that are considered to be unhealthy.

The body mass index (BMI) is a number that provides a reliable indicator of body fat because it evaluates your weight in relation to your height (Figure 15a.7). A BMI greater than 30 is generally considered unhealthy and an indication of obesity. However, just as it is possible for a very muscular person to have a BMI above 30 and not be considered obese, it is possible for a person in the healthy weight range to have too much fat and little muscle. The Centers for Disease Control and Prevention estimates that 68% of Americans are overweight and that almost 34% are obese. The number of obese people in the United States has been steadily rising since 1985, but the rate of increase is slowing (Figure 15a.8).

FIGURE 15a.7. The body mass index (BMI) evaluates body weight relative to height. A BMI over 30 is usually considered to be unhealthy and a sign of obesity.

FIGURE 15a.8. Obesity trends among adults in the United States. Obesity is defined as a high amount of body fat relative to total body weight. There has been an alarming increase in the number of obese people over the last decade.

(Centers for Disease Control and Prevention)

Although most dieters are motivated to slim down for cosmetic reasons, more important reasons are the health risks associated with obesity. For instance, obesity leads to disease of the heart and blood vessels. Even individuals who are just slightly overweight are at increased risk of having a heart attack. Obesity also raises total cholesterol levels in the blood while lowering levels of the beneficial HDL cholesterol. In addition, it increases the risk of high blood pressure, which can lead to death from heart attack, stroke, or kidney disease. Obesity has harmful effects besides those on the heart and blood vessels. It can induce diabetes, which results in elevated blood glucose levels; it is a major cause of gallstones; and it can worsen degenerative joint diseases.

Weight-Loss Programs

Successful weight-loss programs generally have three components: (1) a reduction in the number of calories consumed, without departing from the recommended nutritional guidelines; (2) an increase in energy expenditure; and (3) behavior modification. Gradual changes in eating habits are most likely to lead to permanent lifestyle changes.

To determine the number of calories needed each day to maintain a desirable weight, a person must take activity level and age into account. A pound of fat contains approximately 3500 calories, so to lose 1 pound a week, a person should reduce calorie consumption by 500 calories a day (500 calories x 7 days = 3500 calories), or increase calorie use by 500 calories a day, or any equivalent combination. The lowest daily caloric intake recommended is 1200 calories for an adult female and 1500 calories for an adult male, unless they are in a medically supervised program.

The easiest way to reduce calorie intake while continuing to eat healthily is to cut back on fatty foods, especially those containing saturated fat, because fat contains more than twice as many calories as an equivalent weight of carbohydrate or protein. In addition, it is easier for the body to store the unused fat from foods as body fat than as protein or carbohydrate. Indeed, most of us do end up wearing the fat we eat.

Another recommended way of reducing calories is to avoid sugar. As we have seen, foods that taste sweet are packed with sugar and, therefore, calories. Recall that sugar is a high-glycemic carbohydrate.

A third healthful diet tip is to increase the amount of fiber in the diet. High-fiber foods, such as fruits and vegetables, tend to be low in calories and fat but also high in vitamins and minerals. Because fiber is bulky, these foods are also filling.

Approximately 60% to 90% of dieters who lose weight will later regain all the weight they lost. Often, the weight is regained because the weight loss was achieved by drastically cutting back calories, which can be unhealthy and is very difficult to sustain for long periods. As old eating habits return, so do the pounds. When the determination to shed some pounds returns, the diet begins again. This process is commonly known as "the yo-yo effect."

What causes the yo-yo effect? When there is a severe restriction in calories, as occurs in the typical crash diet, the body adopts a calorie-sparing defense that reduces the resting metabolic rate by as much as 45%. This response conserves energy and evolved long ago to help our ancestors survive during times of food scarcity. Today, it makes weight loss from successive diets progressively more difficult. Furthermore, with each diet, a person usually loses both lean and fat tissue, especially if increased exercise is not part of the weight-loss program. When the weight is regained, most of it is fat. Fat is less metabolically active than lean muscle tissue, so the person's metabolic rate drops even lower. Thus, repeated crash dieting may be a "no-lose situation."

Eating Disorders

Obesity can be considered an eating disorder associated with overeating. The obese and overweight would be wise to follow a nutritionally sound weight-loss program. It is important to remember, however, that dieting can be taken too far. Most people with the eating disorders anorexia nervosa and bulimia nervosa began their descent into these disorders by dieting. Anorexia nervosa is a deliberate self-starvation. A person whose body weight is 85% or less than expected for his or her height is considered to be anorexic. In contrast, bulimia nervosa is marked by binge eating large quantities of food and then purging by self-induced vomiting, enemas, laxatives, diuretics, or excessive exercise.

The changes in eating habits associated with these eating disorders are thought to be the result of psychological, social, and physiological factors. Both disorders are associated with a preoccupation with body size and shape. Anxious depression also seems to play a role.

The behavior patterns associated with anorexia nervosa and bulimia differ, but both result in a severe deficit in calories and nutrients. A person with anorexia nervosa eats very little food and, therefore, consumes few calories. But anorexia, which means "lack of appetite," is misnamed. Although people with anorexia nervosa deny feeling hunger, their refusal to eat stems from an intense fear of becoming fat—not from lack of appetite. Excessive exercise is also typical in anorexia nervosa. No matter how much weight is lost, it is never enough, because an anorexic person has a distorted body image. He or she perceives the body as fat even when emaciated (Figure 15a.9). In contrast, a person with bulimia eats a huge amount of food at one time but then eliminates it from the body. During a bulimic binge, which may last as long as 8 hours, as many as 20,000 calories may be consumed. Each binge is followed by attempts to purge the body of the calories, usually by selfinduced vomiting or by laxatives.

FIGURE 15a.9. Anorexia is a form of self-starvation. No matter how emaciated the person becomes, the individual still perceives the body as being fat.

Eating disorders have many negative effects on the body. Negative health effects of bulimia include esophageal injuries, tooth decay, and gum disease resulting from frequent vomiting, as well as dehydration, constipation, and electrolyte imbalance. One major side effect of anorexia nervosa is a severe decrease in bone health. Although the excessive exercise engaged in by most anorexic people may have a slight strengthening effect on bones, many other factors work to weaken the bones. For example, amenorrhea (cessation of menstruation), malnutrition, and low body weight, particularly low body fat, are all consequences of anorexia nervosa, and they can contribute to poor bone health.

The effects of anorexia nervosa are not so different from those of starvation. In the early phase of the illness, an anorexic person typically chooses a diet that is low in energy-dense foods but rather high in proteins and other essential nutrients. Dietary protein, combined with the high activity levels characteristic of a person with anorexia nervosa, has a nitrogen-sparing effect. As a result, the initial weight loss is almost entirely due to loss of fat tissue. However, when fat reserves are exhausted and refusal of food becomes more severe, the body begins to break down its own proteins to use as an energy source. The primary sources of these proteins are skeletal and heart muscle; therefore, skeletal and heart muscle mass decreases. At the same time, water loss is accelerated, especially from within the body cells. This water loss leads to disturbances in metabolism and electrolyte balance.

Without treatment, up to 20% of people with serious eating disorders die. Even with treatment, 2% to 3% die. Heart problems are the most common cause of death in people with anorexia nervosa. Starvation, dehydration, and electrolyte disturbances cause the heartbeat to slow (bradycardia) and blood pressure to fall (hypotension). A potential cause of death associated with either anorexia nervosa or bulimia is hypoglycemia, an abnormally low blood glucose level. Because the brain depends entirely on glucose for its metabolism, hypoglycemia can cause unconsciousness and death.

Help for people with anorexia nervosa and bulimia nervosa is available. Treatment usually involves the family and centers on psychotherapy to help the person develop a healthier body image.

Looking ahead

In this chapter we considered the nutrients that the body needs to stay healthy. In the next chapter, we will examine the kidney's role in eliminating metabolic wastes from the body and maintaining fluid balance.

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1 In technical writing, kilocalories are written as Calories.

2 dl = deciliter (100 milliliters).

3 The number in omega-3 and omega-6 refers to the location of the first double bond in the carbon chain of the fatty acid.