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.
 See the figure Food Guide Pyramid.
Standard healthy diets for children and adults are based on the needs of average people who do not need to lose or gain weight; who do not need to restrict any component of the diet because of disorders, risk, or advanced age; and who 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 disorders, certain 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 specific age groups, such as older people.
Weight Loss Diets
Weight loss requires consuming fewer calories than the body uses. Losing ½ pound of fat by dieting requires 10 days of consuming 200 fewer calories per day than the body uses. If 400 fewer calories are consumed than needed, a dieter can hope to lose ½ pound every 5 to 7 days. One pound of body fat stores about 3,500 calories.
In most conservative weight loss diets, the number of calories consumed is usually reduced to 1,200 to 1,500 a day. Diets containing fewer than 1,200 calories often lack essential nutrients, such as protein, iron, and calcium. 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. Reading food labels makes people aware of the composition and calorie (energy) content of food, including beverages. Counting calories helps people control calorie intake. Using sugar and fat substitutes and eating foods that contain them help some people reduce calorie intake.
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.
Eating small meals frequently 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 may also promote weight loss. 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.
High Protein-Low Carbohydrate Diets: Diets high in protein and low in simple carbohydrates have become popular as a way to lose weight. Such diets usually also restrict fat, because each gram 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 strongly stimulate insulin production, which promotes fat deposition and increases appetite.
Some of these diets recommend avoiding foods with a high glycemic index. Foods that contain carbohydrates are given a glycemic index, which indicates how quickly the carbohydrates are digested and thus how a food affects the level of sugar in the blood. Foods that contain large amounts of sugar (such as maple syrup, honey, and candy) and starchy foods (such as carrots, potatoes, and some cereals) have a high glycemic index because they are quickly digested and quickly increase blood sugar levels. Foods that are high in fiber (such as whole-grain rye bread and all-bran cereals) have a low glycemic index because they are digested more slowly and do not quickly increase blood sugar levels. This index is useful for diabetics who need to control their blood sugar levels and for athletes who need to restore their blood sugar levels after an athletic performance. However, the glycemic index is not useful for dieters. The difference between the speed of carbohydrate digestion for foods with the highest and lowest glycemic indexes is so small that it makes little difference to most dieters. Avoiding foods with a high glycemic index does not promote weight loss, and it eliminates foods with valuable vitamins and minerals.
Some experts do not recommend following a high-protein diet for long periods of 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.
Very low carbohydrate diets (of less than 100 grams a day) can also cause a problem--the accumulation of keto acids (ketosis). When a person does 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. Low carbohydrate diets may be high in fat and thus the total caloric intake may exceed what the body uses; in such cases, weight loss may stop after glycogen is used up.
Low-Fat Diets: This type of diet is the most effective for losing weight and maintaining weight loss. 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 900 calories. For weight to be lost, the amounts of protein and carbohydrates consumed should not increase greatly as the amount of fat decreases. However, the main reason for reducing the amount of fat in the diet is to lower cholesterol levels in the blood--which a dieter may or may not need to do (see Section 3, Chapter 33 and Section 12, Chapter 157).
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. Fiber also 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.
Very low calorie commercial liquid diets contain all needed nutrients. Usually, a drink that contains 220 calories 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 Section 2, Chapter 13), and eating large amounts of grapefruit often causes diarrhea.
Food-Combining and Food-Cycling Diets: Other 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, a person eats nothing but fruits. Later, the person eats only breads, then only protein, then only fats. No scientific evidence supports this approach to weight loss, and the diet is intrinsically unhealthful.
Other Fad Diets: There are many fad diets. Some require extreme reductions in the number of calories consumed; others rely on supplements alleged to help burn fat; and still others are based on eating a single type of food. The Richard Simmons diet (900 calories a day) and the Atkins Diet (2,000 calories a day) are both low in carbohydrates and may result in dehydration if not enough fluids are consumed. Following the Richard Simmons diet for a long time may lead to deficiencies of iron, calcium, and protein as well as vitamins A, B1 (thiamin), B2 (riboflavin), and B3 (niacin). The Atkins diet is particularly high in fat and cholesterol. The Beverly Hills, Rice, and Pritikin diets are low in fat and protein and high in carbohydrates. The Beverly Hills and Rice diets are deficient in protein, iron, calcium, zinc, and vitamin B12. The Pritikin diet is relatively adequate in nutrition, but its low-fat content makes it unpalatable and less likely to be followed. These diets have not been shown to lead to sustained weight loss, and many are dangerous, supplying inadequate amounts of essential nutrients and leading 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.
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