Diseases & Conditions



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.

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 B 12 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 B 12 even though they consume enough in foods because the stomach and intestine become less able to remove vitamin B 12 from food or to absorb it. Older people with this problem can absorb vitamin B 12 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.

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 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.

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 Cholesterol Disorders: Dyslipidemia ). 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.

Some Fad Diets

Type of Diet

Weight Loss Approach





2,000 calories a day

Is particularly high in fat and cholesterol

Beverly Hills




Is deficient in protein, iron, calcium, zinc, and vitamin B 12





Unpalatable and less likely to be followed because it is so low-fat





Is deficient in protein, iron, calcium, zinc, and vitamin B 12

Richard Simmons

Low-calorie (900 calories a day)

Causes deficiencies in iron, calcium, protein, and vitamins A, (thiamin (B 1 ), riboflavin (B 2 ), and niacin (B 3 ) if it is followed a long time

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 Factors Affecting Response to Drugs: Some Drug-Food Interactions ), 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.

Last full review/revision July 2008 by Margaret-Mary G. Wilson, MD

Source: The Merck Manual Home Edition