CHAPTER 136
Overview of Nutrition
Nutrition is the process of consuming, absorbing, and using nutrients needed by the body for growth, development, and maintenance of life.
To receive adequate, appropriate nutrition, people need to consume a healthy diet, which consists of a variety of nutrients—the substances in foods that nourish the body. A healthy diet enables people to maintain a desirable body weight and composition (the percentage of fat and muscle in the body) and to do their daily physical and mental activities.
If people consume too much food, obesity may result. If they consume large amounts of certain nutrients, usually vitamins or minerals, harmful effects (toxicity) may occur. If people do not consume enough nutrients, a nutritional deficiency disorder may result.
To determine whether people are consuming a proper amount of nutrients, doctors ask them about their eating habits and diet and do a physical examination to assess the composition and functioning of the body. Height and weight are measured, and body mass index (BMI) is calculated. BMI is calculated by dividing weight (in kilograms) by the square of the height (in meters). A BMI between 19 and 24 is usually considered normal for men and women.
Body composition, including the percentage of body fat, is sometimes estimated by measuring skinfold thickness or doing bioelectrical impedance analysis. More accurate ways to determine this percentage include weighing people under water (hydrostatic weighing) and doing a dual-energy x-ray absorptiometry (DEXA) scan, but these methods are seldom used.
Levels of many nutrients can be measured in blood and sometimes in tissues. For example, measuring the level of albumin, the main protein in blood, may help determine whether people are deficient in protein. Nutrient levels decrease when nutrition is inadequate.
Components of the Diet: Generally, nutrients are divided into two classes:
Macronutrients: Macronutrients are required daily in large quantities. They include proteins, fats, carbohydrates, some minerals, and water.
Micronutrients: Micronutrients are required daily in small quantities—in milligrams (one thousandth of a gram) to micrograms (one millionth of a gram). They include vitamins and certain minerals that enable the body to use macronutrients. These minerals are called trace minerals because the body needs only very small amounts.
Fat Versus Lean: Body Composition
Maintaining an appropriate weight is important for physical and psychologic health. A standardized height-weight table can be used as a guide. But body mass index (BMI) is more reliable.
A less obvious but important consideration is how much of the body is fat and how much is muscle (body composition). There are several ways to determine body composition:
Hydrostatic weighing: People are weighed underwater in a small pool. Bone and muscle are denser than water, so people with a high percentage of lean tissue weigh more in water and people with a high percentage of fat weigh less. Although this method is considered the most accurate, it requires special equipment, considerable time, and expertise to do.
Skinfold thickness: Body composition can be estimated by measuring the amount of fat under the skin (skinfold thickness). A fold of skin on the back of the left upper arm (triceps skinfold) is pulled away from the arm and measured with a caliper. A skinfold measurement of about 1/2 inch in men and about 1 inch in women is considered normal. This measurement plus the circumference of the left upper arm can be used to estimate the amount of skeletal muscle in the body (lean body mass).
Bioelectric impedance analysis: This test measures the resistance of body tissues to the flow of an undetectable low-voltage electrical current. Typically, people stand barefoot on metal footplates, and the electrical current is sent up one foot and down the other. Body fat and bone resist the flow much more than muscle tissue does. By measuring the resistance to the current, doctors can estimate the percentage of body fat. This test takes only about 1 minute.
Dual-energy x-ray absorptiometry (DEXA): This imaging procedure accurately determines the amount and distribution of body fat. DEXA uses a very low dose of radiation and is safe. However, it is too expensive to use routinely.
WHO IS OVERWEIGHT?
Water is required in amounts of 1 milliliter for each calorie of energy expended or about 2.6 quarts (2,500 milliliters) a day. The requirement for water can be met by the water naturally contained in many foods, and by drinking fruit or vegetable juices and caffeine-free coffee or tea as well as water. Alcoholic beverages and caffeinated coffee, tea, and sodas may make people urinate more, so they are less useful.
Foods consumed in the daily diet contain as many as 100,000 substances. But only 300 are classified as nutrients, and only 45 are classified as essential nutrients: vitamins, minerals, some amino acids (components of protein), and some fatty acids (components of fats). Essential nutrients cannot be synthesized by the body and must be consumed in the diet.
Foods contain many other useful components, including fibers (such as cellulose, pectins, and gums). Foods also contain additives (such as preservatives, emulsifiers, antioxidants, and stabilizers), which improve the production, processing, storage, and packaging of foods (see page 908).
Carbohydrates, Proteins, and Fats
Carbohydrates, proteins, and fats supply 90% of the dry weight of the diet and 100% of its energy. All three provide energy (measured in calories), but the amount of energy in 1 gram (1/28 ounce) differs: 4 calories in a gram of carbohydrate or protein and 9 calories in a gram of fat. These nutrients also differ in how quickly they supply energy. Carbohydrates are the quickest, and fats are the slowest.
Carbohydrates, proteins, and fats are digested in the intestine, where they are broken down into their basic units: carbohydrates into sugars, proteins into amino acids, and fats into fatty acids and glycerol. The body uses these basic units to build substances it needs for growth, maintenance, and activity (including other carbohydrates, proteins, and fats).
Carbohydrates
Depending on the size of the molecule, carbohydrates may be simple or complex.
Simple carbohydrates: Various forms of sugar, such as glucose and sucrose (table sugar), are simple carbohydrates. They are small molecules, so they can be broken down and absorbed by the body quickly and are the quickest source of energy. They quickly increase the level of blood glucose (blood sugar). Fruits, dairy products, honey, and maple syrup contain large amounts of simple carbohydrates, which provide the sweet taste in most candies and cakes.
Complex carbohydrates: These carbohydrates are composed of long strings of simple carbohydrates. Because complex carbohydrates are larger molecules than simple carbohydrates, they must be broken down into simple carbohydrates before they can be absorbed. Thus, they tend to provide energy to the body more slowly than simple carbohydrates but still more quickly than protein or fat. Because they are digested more slowly than simple carbohydrates, they are less likely to be converted to fat. They also increase blood sugar levels more slowly and to lower levels than simple carbohydrates but for a longer time. Complex carbohydrates include starches and fibers, which occur in wheat products (such as breads and pastas), other grains (such as rye and corn), beans, and root vegetables (such as potatoes).
Carbohydrates may be refined or unrefined. Refined means that the food is highly processed. The fiber and bran, as well as many of the vitamins and minerals they contain, have been stripped away. Thus, the body processes these carbohydrates quickly, and they provide little nutrition although they contain about the same number of calories. Refined products are often enriched, meaning vitamins and minerals have been added back to increase their nutritional value. A diet high in simple or refined carbohydrates tends to increase the risk of obesity and diabetes.
If people consume more carbohydrates than they need at the time, the body stores some of these carbohydrates within cells (as glycogen) and converts the rest to fat. Glycogen is a complex carbohydrate that the body can easily and rapidly convert to energy. Glycogen is stored in the liver and the muscles. Muscles use glycogen for energy during periods of intense exercise. The amount of carbohydrates stored as glycogen can provide almost a day’s worth of calories. A few other body tissues store carbohydrates as complex carbohydrates that cannot be used to provide energy.
Most authorities recommend that about 50 to 55% of total daily calories should consist of carbohydrates.
Glycemic Index: The glycemic index of a carbohydrate represents how quickly its consumption increases blood sugar levels. Values range from 1 (the slowest) to 100 (the fastest, the index of pure glucose). However, how quickly the level actually increases also depends on what other foods are ingested at the same time and other factors.
The glycemic index tends to be lower for complex carbohydrates than for simple carbohydrates, but there are exceptions. For example, fructose (the sugar in fruits) has little effect on blood sugar.
The following also influence a food’s glycemic index:
Processing: Processed, refined, or finely ground foods tend to have a higher glycemic index.
Type of starch: Different types of starch are absorbed differently. For example, potato starch is digested and absorbed into the bloodstream relatively quickly. Barley is digested and absorbed much more slowly.
Fiber content: The more fiber a food has, the harder it is to digest. As a result, sugar is absorbed more slowly into the bloodstream.
Ripeness of fruit: The riper the fruit, the more sugar it contains, and the higher its glycemic index.
Fat or acid content: The more fat or acid a food contains, the more slowly it is digested and the more slowly its sugars are absorbed into the bloodstream.
Preparation: How a food is prepared can influence how quickly it is absorbed into the bloodstream. Generally, cooking or grinding a food increases its glycemic index because these processes make food easier to digest and absorb.
Other factors: The way the body processes food varies from person to person, affecting how quickly carbohydrates are converted to sugar and absorbed. How well a food is chewed and how quickly it is swallowed also have an effect.
GLYCEMIC INDEX OF SOME FOODS
| CATEGORY | FOOD | INDEX |
| Beans | Kidney | 33 |
| Red lentils | 27 | |
| Soy | 14 | |
| Bread | Pumpernickel | 49 |
| White | 69 | |
| Whole wheat | 72 | |
| Cereals | All bran | 54 |
| Corn flakes | 83 | |
| Oatmeal | 53 | |
| Puffed rice | 90 | |
| Shredded wheat | 70 | |
| Dairy | Milk, ice cream, yogurt | 34-38 |
| Fruit | Apple | 38 |
| Banana | 61 | |
| Orange | 43 | |
| Orange juice | 49 | |
| Strawberries | 32 | |
| Grains | Barley | 22 |
| Brown rice | 66 | |
| White rice | 72 | |
| Pasta | — | 38 |
| Potatoes | Instant mashed (white) | 86 |
| Mashed (white) | 72 | |
| Sweet | 50 | |
| Snacks | Corn chips | 72 |
| Oatmeal cookies | 57 | |
| Potato chips | 56 | |
| Sugar | Fructose | 22 |
| Glucose | 100 | |
| Honey | 91 | |
| Refined sugar | 64 |
The glycemic index is thought to be important because carbohydrates that increase blood sugar levels quickly (those with a high glycemic index) also quickly increase insulin levels. The increase in insulin may result in low blood sugar levels (hypoglycemia) and hunger, which tends to lead to consuming excess calories and gaining weight. Carbohydrates with a low glycemic index do not increase insulin levels so much. As a result, people feel satiated longer after eating. Consuming carbohydrates with a low glycemic index also tends to result in more healthful cholesterol levels and reduces the risk of obesity and diabetes mellitus and, in people with diabetes, the risk of complications due to diabetes.
In spite of the association between foods with a low glycemic index and improved health, using the index to choose foods does not automatically lead to a healthy diet. For example, the glycemic index of potato chips and some candy bars—not healthful choices—is lower than that of some healthful foods, such as brown rice. Some foods with a high glycemic index contain valuable vitamins and minerals. Thus, this index should be used only as a general guide to food choices.
Glycemic Load: The glycemic index indicates only how quickly carbohydrates in a food are absorbed into the bloodstream. It does not include how much carbohydrate a food contains, which is also important. Glycemic load, a relatively new term, includes the glycemic index and the amount of carbohydrate in a food. A food, such as carrots, bananas, watermelon, or whole-wheat bread, may have a high glycemic index but contain relatively little carbohydrate and thus have a low glycemic load. Such foods have little effect on the blood sugar level.
Proteins
Proteins consist of units called amino acids, strung together in complex formations. Because proteins are complex molecules, the body takes longer to break them down. As a result, they are a much slower and longer-lasting source of energy than carbohydrates.
There are 20 amino acids. The body synthesizes some of them from components within the body, but it cannot synthesize 9 of the amino acids—called essential amino acids. They must be consumed in the diet. Everyone needs 8 of these amino acids: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Infants also need a 9th one, histidine. The percentage of protein the body can use to synthesize essential amino acids varies from protein to protein. The body can use 100% of the protein in egg and a high percentage of the proteins in milk and meats.
The body needs proteins to maintain and replace tissues and to function and grow. If the body is getting enough calories, it does not use protein for energy. If more protein is consumed than is needed, the body breaks the protein down and stores its components as fat.
The body contains large amounts of protein. Protein, the main building block in the body, is the primary component of most cells. For example, muscle, connective tissues, and skin are all built of protein.
Adults need to eat about 60 grams of protein per day (0.8 grams per kilogram of weight or 10 to 15% of total calories). Adults who are trying to build muscle need slightly more. Children also need more because they are growing.
Fats
Fats are complex molecules composed of fatty acids and glycerol. The body needs fats for growth and energy. It also uses them to synthesize hormones and other substances needed for the body’s activities (such as prostaglandins). Fats are the slowest source of energy but the most energy-efficient form of food. Each gram of fat supplies the body with about 9 calories, more than twice that supplied by proteins or carbohydrates. Because fats are such an efficient form of energy, the body stores any excess energy as fat. The body deposits excess fat in the abdomen (omental fat) and under the skin (subcutaneous fat) to use when it needs more energy. The body may also deposit excess fat in blood vessels and within organs, where it can block blood flow and damage organs, often causing serious disorders.
Fatty Acids: When the body needs fatty acids, it can make (synthesize) certain ones. Others, called essential fatty acids, cannot be synthesized and must be consumed in the diet. The essential fatty acids make up about 7% of the fat consumed in a normal diet and about 3% of total calories (about 8 grams). They include linoleic acid and linolenic acid, which are present in certain vegetable oils. Eicosapentaenoic acid and docosahexaenoic acid, which are fatty acids essential for brain development, can be synthesized from linolenic acid. However, they also are present in certain marine fish oils, which are a more efficient source.
WHERE’S THE FAT?
| TYPE OF FAT | SOURCE |
| Monounsaturated | Avocado, olive, and peanut oils Peanut butter |
| Polyunsaturated | Canola, corn, soybean, sunflower, and many other liquid vegetable oils |
| Saturated | Meats, particularly beef Full-fat dairy products such as whole milk, butter, and cheese Coconut and palm oils Artificially hydrogenated vegetable oils |
| Omega-3 fatty acids | Flaxseed Lake trout and certain deep-sea fish, such as mackerel, salmon, herring, and tuna Green leafy vegetables Walnuts |
| Omega-6 fatty acids | Vegetable oils (including sunflower, safflower, corn, cottonseed, and soybean oils) Fish oils Egg yolks |
| Trans fats | Commercially baked foods, such as cookies, crackers, and doughnuts Some french fries and other fried foods Margarine Shortening Potato chips |
Linoleic acid and arachidonic acid are omega-6 fatty acids. Linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid are omega-3 fatty acids. A diet rich in omega-3 fatty acids may reduce the risk of coronary artery disease. Lake trout and certain deep-sea fish contain large amounts of omega-3 fatty acids. In the United States, people tend to consume enough omega-6 fatty acids, which occur in the oils used in many processed foods, but not enough omega-3 fatty acids.
Kinds of Fat: There are different kinds of fat: monounsaturated, polyunsaturated, and saturated (see box on page 403). In general, saturated fats are more likely to increase cholesterol levels and increase the risk of atherosclerosis. Foods derived from animals commonly contain saturated fats, which tend to be solid at room temperature. Fats derived from plants commonly contain monounsaturated or polyunsaturated fatty acids, which tend to be liquid at room temperature. Palm and coconut oil are exceptions. They contain more saturated fats than other plant oils.
Trans fats (trans fatty acids) are a different category of fat. They are man-made, formed by adding hydrogen atoms (hydrogenation) to monounsaturated or polyunsaturated fatty acids. Fats may be partially or fully hydrogenated (or saturated with hydrogen atoms). In the United States, the main dietary source of trans fats is partially hydrogenated vegetable oils, present in many commercially prepared foods. Consuming trans fats may adversely affect cholesterol levels in the body and may contribute to the risk of atherosclerosis.
Fat in the Diet: Authorities generally recommend that fat be limited to less than 30% of daily total calories (or fewer than 90 grams per day) and that saturated fats and trans fats should be limited to less than 10%. When possible, monounsaturated fats and polyunsaturated fats, particularly omega-3 fats, should be substituted for saturated fats and trans fats. People with high cholesterol levels may need to reduce their total fat intake even more. When fat intake is reduced to 10% or less of daily total calories, cholesterol levels tend to decrease dramatically.
Vitamins and Minerals
Vitamins and minerals are essential nutrients. That is, they cannot be synthesized by the body and so must be consumed in the diet.
Vitamins are classified as water soluble—vitamin C and the eight members of the vitamin B complex—or fat soluble—vitamins A, D, E, and K (see page 917). Only vitamins A, E, and B12 are stored to any large extent in the body.
Some minerals are required in fairly large quantities (about 1 or 2 grams a day) and are considered macronutrients (see page 932). They include calcium, chloride, magnesium, phosphorus (occurring mainly as phosphate in the body), potassium, and sodium. Minerals required in small amounts (trace minerals) are considered micronutrients. They include chromium, copper, fluoride, iodine, iron, manganese, molybdenum, selenium, and zinc. Except for chromium, all of these minerals are incorporated into enzymes or hormones required in metabolism. Chromium helps the body keep blood sugar levels normal. Trace minerals such as arsenic, cobalt, fluoride, nickel, silicon, and vanadium, which may be essential in animal nutrition, have not been established as requirements in human nutrition. Fluoride helps stabilize the mineral content of bones and teeth by forming a stable compound with calcium and thus helps prevent tooth decay. All trace minerals are toxic at high levels, and some (arsenic, nickel, and chromium) can cause cancer.
COMPARING SOLUBLE AND INSOLUBLE FIBER
| TYPE OF FIBER | SOURCES | FUNCTIONS |
| Soluble | Apples Barley Beans Citrus fruits Lentils Oat bran Oatmeal Pectin (from fruit) Psyllium Rice bran Strawberries |
Helps moderate the changes in blood sugar and insulin levels that occur after eating a meal Helps reduce cholesterol levels May reduce the risk of coronary artery disease |
| Insoluble | Apples Brown rice Pears Prunes Many vegetables, including cabbage, root vegetables, and zucchini Whole grains and whole-grain breads and pastas |
Provides bulk to feces and thus helps food move through the digestive tract, preventing constipation Helps eliminate cancer-causing substances produced by the bacteria in the large intestine Reduces pressure in the intestine, helping prevent diverticular disease Is helpful in losing weight because the body processes it slowly |
Some vitamins (such as vitamins C and E) and minerals (such as selenium) act as antioxidants, as do other substances in fruits and vegetables (such as beta-carotene). Antioxidants protect cells against damage by free radicals, which are by-products of the normal activity of cells. Free radicals readily participate in chemical reactions—some useful to the body and some not—and are thought to contribute to such disorders as heart and blood vessel disorders and cancer. People who eat enough fruits and vegetables, which are rich in antioxidants, are less likely to develop heart and blood vessel disorders and certain cancers. However, whether these benefits are due to antioxidants, other substances in the fruits and vegetables, or other factors is not known.
Getting enough vitamins and minerals from foods is usually preferable to getting them from supplements. Foods, unlike supplements, contain other substances necessary for good health. However, always eating a healthy, well-balanced diet may be difficult. So taking a multivitamin that contains the recommended daily allowances for vitamins and minerals is a good idea, particularly when a healthy diet may not be possible.
Fiber
Some foods contain fiber, which is a tough complex carbohydrate. Fiber may be partly soluble: It dissolves in water, and the body may be able to digest some of it. Or it may be insoluble: It does not dissolve in water, and the body cannot digest it. Eating too much insoluble fiber can interfere with absorption of certain vitamins and minerals.
Authorities generally recommend that about 30 grams of fiber be consumed daily. In the United States, the average amount of fiber consumed daily is about 12 grams because people tend to eat products made with highly refined wheat flour and do not eat many fruits and vegetables. An average serving of fruit, a vegetable, or cereal contains 2 to 4 grams of fiber. Meat and dairy foods do not contain fiber.
Did You Know…
Eating a lot of insoluble fiber (in such foods as brown rice, prunes, and many vegetables) can reduce the absorption of certain vitamins and minerals.
Food Additives and Contaminants
Additives: Substances, such as preservatives, emulsifiers, antioxidants, and stabilizers, are often added to a food to do the following:
Enable it to be processed more easily
Preserve it longer and reduce spoilage
Prevent contamination by microorganisms and thus prevent food-borne disorders
Improve taste, add color, or enhance its aroma, making it more appealing
In commercially prepared foods, the amount of additives that can be included is limited to that shown to be safe by laboratory tests. However, weighing the benefits of additives against the risks is often complex. For example, nitrite, which is used in cured meats, not only improves flavor but also inhibits the growth of bacteria that cause botulism. However, nitrite converts to nitrosamines, which can cause cancer in animals. On the other hand, the amount of nitrite added to cured meat is small compared with the amount of nitrates that occurs naturally in food and that is converted to nitrite by the salivary glands.
Rarely, some additives (such as sulfites) cause allergic reactions. Sulfites, which occur naturally in wines, are added to such foods as dried fruit and dried potatoes as a preservative.
Contaminants: Foods may be contaminated because the air, water, and soil are polluted, for example, by heavy metals (such as lead, cadmium, and mercury) or PCBs (polychlorinated biphenyls). PCBs used to be used as coolants and in many other products and are now present in the air, soil, and water in many places. Foods may be contaminated by pesticides, packaging materials, or during cooking or processing. Foods may also be contaminated by drugs (such as antibiotics and growth hormone) that are given to animals.
Sometimes limited amounts of contaminants are allowed in foods because the contaminants cannot be completely eliminated without damaging the foods. Common contaminants include
Pesticides
Heavy metals
Nitrates (in green leafy vegetables)
Aflatoxins, produced by molds (in nuts and milk)
Growth-promoting hormones (in dairy products and meat)
Levels that have not caused illness or other problems in people are considered safe. However, determining whether a small amount of a contaminant has caused a problem is very difficult. Thus, safe levels are often determined by general agreement rather than by hard evidence. Whether problems can result from consuming a small amount of some contaminants over a long time is unclear, although with very tiny amounts such problems are unlikely. If problems occur, they probably affect only a few people.
Foods may contain animal hairs, animal feces, and insect parts in such tiny amounts that removal is impossible.
Calories
A calorie is a measure of energy. Foods have calories. That is, foods supply the body with energy, which is released when foods are broken down during digestion. Energy enables cells to do all of their functions, including building proteins and other substances needed by the body. The energy can be used immediately or stored for use later.
When the supply of energy—the number of calories consumed in foods—exceeds the body’s immediate needs, the body stores the excess energy. Most excess energy is stored as fat. Some is stored as carbohydrates, usually in the liver and muscles. As a result, weight is gained. An excess of only 200 calories per day for 10 days is likely to result in a weight gain of nearly 1/2 pound, mostly as fat.
Did You Know…
After the first few pounds are lost, weight loss slows down when the body has burned all its stored carbohydrates and starts burning stored fat.
When too few calories are consumed for the body’s needs, the body begins to use carbohydrates stored in the liver and muscle. Because the body mobilizes stored carbohydrates quickly and because water is usually excreted as carbohydrates are mobilized, weight loss tends to be fast initially. However, the small amount of stored carbohydrates provides energy for only a short time. Next, the body uses stored fat. Because fat contains more energy per pound, weight loss is slower as the body uses fat for energy. However, the amount of fat stored is much larger and can, in most people, provide energy for a long time. Only during prolonged, severe shortages of energy, does the body break down protein. If normally nourished people experience total starvation (when no food is consumed), death occurs in 8 to 12 weeks.
How Are Calories in Foods Measured?
Food labels contain the number of calories per serving. But how is this number determined? The answer is surprisingly simple: The food is burned. A sample of the food is placed in an insulated, oxygen-filled chamber that is surrounded by water. This chamber is called a bomb calorimeter. The sample is burned completely. The heat from the burning increases the temperature of the water, which is measured and which indicates the number of calories in the food. For example, if water temperature increases by 20 degrees, the food contains 20 calories. This method of measuring calories is called direct calorimetry.
Energy requirements vary markedly from about 1,000 to more than 4,000 calories a day depending on age, sex, weight, physical activity, disorders present, and the rate at which people burn calories (metabolic rate). However, typically, the number of calories needed per day to maintain body weight is about
1,600 for sedentary women, young children, and older adults
2,000 for older children, active adult women, and sedentary men
2,400 for active adolescent boys and young men
The division of caloric intake by a 24-hour period (daily intake) is arbitrary. Also, the needs of the body vary depending on its activity at any particular time. Vigorous activity, especially aerobic exercise, increases needs substantially, and a lack of activity decreases needs.
Nutritional Requirements
General guidelines for a healthy diet have been developed even though daily nutritional requirements, including those for essential nutrients, vary depending on age, sex, height, weight, physical activity, and the rate at which the body burns calories (metabolic rate). Recommended dietary allowances for protein, vitamins (see table on page 918), and minerals (see table on page 934) are periodically published by The Food and Nutrition Board of the National Academy of Sciences-National Research Council and the U.S. Department of Agriculture. These allowances are intended to meet the needs of healthy people.
The U.S. Department of Agriculture also provides an interactive tool at www.mypyramid.gov. It enables people to enter information about themselves (their age, sex, activity level, and foods usually eaten) so that they can evaluate their diet and get recommendations about healthful foods and portion sizes that can help them reach and maintain a healthy weight. The amount of food needed each day from each food group varies depending on the person’s energy needs.
In general, authorities recommend that fat intake be reduced to about 30% of calories or less and the intake of fruits, vegetables, and cereals be higher than most Americans eat. Drinking enough fluids is also important.
Diets
A diet is whatever a person eats, regardless of the goal—whether it is losing weight, gaining weight, reducing fat intake, avoiding carbohydrates, or having no particular goal. However, the term is often used to imply a goal of losing weight, which is an obsession for many people.
Standard healthy diets for children and adults are based on the needs of average people who have certain characteristics:
They do not need to lose or gain weight.
They do not need to restrict any component of the diet because of disorders, risk, or advanced age.
They expend average amounts of energy through exercise or other vigorous activities.
Thus, for a particular person, a healthy diet may vary substantially from what is recommended in standard diets. For example, special diets are required by people who have diabetes, certain kidney or liver disorders, coronary artery disease, high cholesterol levels, osteoporosis, diverticular disease, chronic constipation, or food sensitivities. There are special dietary recommendations for young children, but little guidance is available for other age groups, such as older people.
WEIGHT LOSS DIETS
Weight loss requires consuming fewer calories than the body uses. Losing 1/2 pound of fat by dieting requires 10 days of consuming 200 fewer calories or 5 to 7 days of consuming 400 fewer calories per day than the body uses. One pound of body fat stores about 3,500 calories.
SPOTLIGHT ON AGING
The best diet for older people has not been determined. However, older people may benefit from changing some aspects of their diet, based on the way the body changes as it ages. No changes are required for some nutrients such as carbohydrates and fats.
Calories: As people age, they tend to be less active and thus use less energy, making it easier to gain weight. If they try to consume fewer calories to avoid weight gain, they may not get all the nutrients needed—particularly vitamins and minerals. If older people stay physically active, their need for calories may not change.
Protein: As people age, they tend to lose muscle. If older people do not consume enough protein, they may lose even more muscle. For older people who have problems eating (for example, because of difficulty swallowing or dental disorders), protein can be consumed in foods that are easier to chew than meat, such as fish, dairy products, eggs, peanut butter, beans, and soy products.
Fiber: Eating enough fiber can help counter the slowing of the digestive tract that occurs as people age. Older people should eat 8 to 12 servings of high-fiber foods daily. Getting fiber from foods is best, but fiber supplements, such as psyllium, may be needed.
Vitamins and minerals: Older people may need to take supplements of specific vitamins and minerals in addition to a multivitamin. Calcium, vitamin D, and vitamin B12 are examples. Getting enough calcium and vitamin D from the diet is difficult. These nutrients are needed to maintain strong bones, which are particularly important for older people. Some older people do not absorb enough vitamin B12 even though they consume enough in foods because the stomach and intestine become less able to remove vitamin B12 from food or to absorb it. Older people with this problem can absorb vitamin B12 better when it is given as a supplement.
Water: As people age, they are more likely to become dehydrated because their ability to sense thirst decreases. Thus, older people need to make a conscious effort to drink enough fluids rather than wait until they feel thirsty.
Older people are more likely to have disorders or take drugs that can change the body’s nutritional needs or the body’s ability to meet those needs. Disorders and drugs can decrease appetite or interfere with the absorption of nutrients. When older people see their doctor, they should ask their doctor whether the disorders they have or the drugs they take affect nutrition in any way.
Most conservative weight loss diets involve consuming at least 1,200 to 1,400 calories a day. When rapid weight loss is needed, fewer than 1,200 calories may be consumed, but only for a short time. Such diets often have too little of essential nutrients, such as protein, iron, and calcium. Consuming fewer than 800 calories does not increase the amount of weight lost and is harder to tolerate.
To be healthy, weight loss diets should provide about the same volume of food (by including more fiber and fluids) as the normal diet. They should also be low in saturated fat and sugar and include essential nutrients, including antioxidants. The following general guidelines may help people lose weight:
Reading food labels: People learn what nutrients and how many calories food, including beverages, contains. Then, people can plan their diet more effectively.
Counting calories: People keep track of the number of calories they eat. This strategy helps people control calorie intake.
Choosing nutrient-rich, low-calorie foods: When fewer calories are consumed, getting the needed nutrients—particularly vitamins and minerals—is more difficult. So people should choose foods that contain many nutrients but not many calories. Whole-grain cereals and whole-grain breads that are fortified with vitamins are good choices. Fruits and vegetables that are deeply colored (such as strawberries, peaches, broccoli, spinach, and squash) tend to contain more nutrients than those that are less deeply colored.
Eating small meals frequently: This strategy can help with weight loss for several reasons. Insulin levels usually increase after eating, and more insulin is produced when many calories are consumed, especially when the meal is rich in carbohydrates. High insulin levels promote the deposition of fat and increase appetite. Eating small, frequent meals prevents insulin levels from increasing, thus discouraging fat deposition and helping suppress appetite.
Eating certain types of foods at certain times of the day: For example, fast-energy foods, such as carbohydrates, are best eaten when the body needs a large supply of energy—that is, in the morning and during vigorous exercise. The body’s need for energy is lowest at night, so avoiding carbohydrates in the evening may help.
Using sugar and fat substitutes: Such substitutes and foods that contain them can sometimes help people reduce calorie intake. However, in some cases, sugar substitutes have effects on metabolism that slow the rate of weight loss.
Exercising: Combining increased exercise with dieting greatly enhances weight loss because exercise increases the number of calories the body uses. For example, vigorous walking burns about 4 calories per minute, so that 1 hour of brisk walking per day burns about 240 calories. Running is even better, burning about 6 to 8 calories per minute.
Did You Know…
Regardless of the weight loss diet followed, people must consume fewer calories than the body uses to lose weight.
Many people follow a specific diet to lose weight.
High Protein-Low Carbohydrate Diets: Diets high in protein and low in simple carbohydrates have become popular as a way to lose weight. Most of these diets usually also restrict fat because each gram of fat supplies so many calories. However, some high protein-low carbohydrate diets, such as the Atkins diet, do not restrict fat.
The theory behind these diets is that slower-burning energy sources—protein and fat—provide a steady supply of energy and thus are less likely to lead to weight gain. In addition, people tend to feel full longer after eating protein than after eating carbohydrates because carbohydrates empty from the stomach quickly and are digested quickly. Carbohydrates also stimulate insulin production, which promotes fat deposition and increases appetite. However, the reason that these diets cause weight loss appears to be that people tire of the foods allowed by the diet and thus consume fewer calories.
Experts disagree about whether avoiding foods with a high glycemic index helps with weight loss, particularly in low-carbohydrate diets, or not. The effect of the glycemic index is less important when only a small percentage of total calories is carbohydrates. In a low-carbohydrate diet, the difference between how fast the carbohydrates in various foods (with their different glycemic indexes) are digested is sometimes so small that it makes little difference to most dieters. Avoiding foods with a high glycemic index also sometimes eliminates foods with valuable vitamins and minerals. Experts also disagree on how important the glycemic load (the glycemic index plus the amount of carbohydrate in a food) is for weight loss.
Some experts do not recommend following a high-protein diet for a long time. Some evidence suggests that over years, very high protein diets impair kidney function and may contribute to the decrease in kidney function that occurs in older people. People with certain kidney and liver disorders should not consume a high-protein diet. High-protein diets can speed the body’s processing of certain drugs and thus may affect how well the drug works.
Very low carbohydrate diets (of less than 100 grams a day) can lead to the accumulation of keto acids (ketosis). When people do not consume enough energy for the body’s needs, the body breaks down fats. As part of this process, the body produces keto acids. In small amounts, keto acids are easily excreted by the kidneys without causing symptoms. However, in large amounts, they can cause nausea, fatigue, bad breath, and even more serious symptoms, such as dizziness (due to dehydration) and abnormal heart rhythms (due to electrolyte imbalances). People following a low-carbohydrate diet (or any other weight loss diet) should drink large amounts of water to help flush keto acids from the body.
Low-carbohydrate diets tend to cause large amounts of weight to be lost during the first week or so, as the body converts stored carbohydrates (glycogen) to energy. As glycogen is broken down, the body also excretes large amounts of water, adding to the weight loss. However, once the body begins to use stored fat for energy, weight loss slows. People following a low-carbohydrate diet may substitute fats for the carbohydrates they are avoiding. In such cases, the diet may be so high in fat that the total caloric intake exceeds what the body uses. In such cases, weight loss stops after glycogen is used up.
Low-Fat Diets: Fat supplies a large number of calories per gram and is more readily deposited as body fat than are proteins and carbohydrates. Reducing the amount of fat rather than the amount of protein or carbohydrate may be an easier way to reduce total caloric intake because a small reduction in fat saves so many calories. A reduction of only 10 grams of fat per day saves about 90 calories. However, the best reason for reducing the amount of fat in the diet is to lower cholesterol levels in the blood (see page 961). Lowering cholesterol levels benefits most dieters because weight increases their risk of atherosclerosis, which can lead to heart attacks or stroke. Because lowering cholesterol levels can help prevent or delay atherosclerosis, a low-fat diet tends to be the best weight loss diet for overall health.
High-Fiber Diets: Fiber indirectly helps with weight loss in several ways:
It provides bulk, which makes people feel full faster.
It slows the rate at which the stomach empties so people feel full longer.
It requires more chewing, forcing people to eat more slowly and perhaps less.
High-fiber foods, such as fruits and vegetables, wheat bread, and beans, are filling without providing many calories. Eating more high-fiber foods may enable people to eat fewer less filling, high-calorie foods, such as high-fat foods. However, fiber supplements, such as guar gum and cellulose, are not effective for weight loss.
Liquid Diets: Many people use liquid diets to lose weight, mainly because they are convenient. However, the contents of such liquids vary, and many are unlikely to be of much help in losing weight. Some commercially available liquid diets are well-balanced, with appropriate proportions of protein, carbohydrates, and fat plus supplemental vitamins and minerals. But others contain a large proportion of carbohydrates, producing a sweet and tasty drink, and are not necessarily low in calories. Such liquid diets are more useful as a supplement to other foods for people who are trying to gain weight.
Usually, a commercial liquid-diet serving (a drink) contains 220 calories, and a drink is consumed 4 times a day instead of meals. Such diets are effective for short-term weight loss. For long-term weight loss, two or three meals are replaced with a liquid-diet drink. The remaining one or two meals should be low-fat, low-calorie, and nutritious.
An alternative to commercial diets is the all-milk diet. This diet is simple and inexpensive and may be useful for short-term weight loss.
Grapefruit Diet: One popular fad diet involves consuming large amounts of grapefruit and grapefruit juice. The theory behind this diet is that grapefruit contains an enzyme that helps burn fat, but this theory has never been proved.
Although grapefruits are a healthful food—containing no fat, little sodium, and large amounts of vitamin C, beta-carotene (at least in pink grapefruits), and fiber—a diet based primarily on one fruit is nutritionally unsound. A grapefruit diet may help some people reduce total caloric intake, but it does not supply a balance of nutrients, which is needed for good health. Furthermore, eating grapefruit alters the levels of several drugs in the blood (see table on page 92), and eating large amounts of grapefruit often causes diarrhea.
Food-Combining and Food-Cycling Diets: These fad diets are based on a theory that eating certain kinds of foods at different times promotes weight loss. An example is the Beverly Hills Diet, which recommends cycling different foods, usually over a 6-week period. For part of the time, people eat nothing but fruits. Later, people eat only breads, then only protein, then only fats. No scientific evidence supports this approach to weight loss, and the diet is intrinsically unhealthful.
Fad Diets: There are many fad diets, including some of the above. Many fad diets promise quick weight loss and do not provide any scientific evidence of their effectiveness. Some require extreme reductions in the number of calories consumed. Others rely on supplements alleged to help burn fat. Still others are based on eating a single type of food. These diets have not been shown to lead to sustained weight loss, and many are dangerous. They provide inadequate amounts of essential nutrients and, over time, can lead to serious metabolic disturbances, such as loss of bone density and strength (including osteoporosis), problems with menstruation, abnormal heart rhythms, high cholesterol levels, kidney stones, and worsening of gout.
SOME FAD DIETS
| TYPE OF DIET | WEIGHT LOSS APPROACH | DISADVANTAGES |
| Atkins | High-protein Low-carbohydrate 2,000 calories a day |
Is particularly high in fat and cholesterol |
| Beverly Hills | Low-fat Low-protein High-carbohydrate |
Is deficient in protein, iron, calcium, zinc, and vitamin B12 |
| Pritikin | Low-fat Low-protein High-carbohydrate |
Unpalatable and less likely to be followed because it is so low-fat |
| Rice | Low-fat Low-protein High-carbohydrate |
Is deficient in protein, iron, calcium, zinc, and vitamin B12 |
| Richard Simmons | Low-calorie (900 calories a day) | Causes deficiencies in iron, calcium, protein, and vitamins A, thiamin (B1), riboflavin (B2), and niacin (B3) if it is followed a long time |