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By Adrienne Youdim, MD, Associate Professor of Medicine;Associate Professor of Medicine, David Geffen School of Medicine at UCLA;Cedars Sinai Medical Center

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. People are usually advised to consume 500 to 1000 fewer calories per day to lose about 1 to 2 pounds per week. One pound of body fat stores about 3,500 calories. However, there is no guarantee that each 3,500 calories eliminated from the diet will cause a pound of weight loss because many factors affect how much weight is lost (or gained), as in the following examples:

  • As people lose weight, the body starts using energy more efficiently (possibly intended to guard against starvation), so that fewer calories are burned and less fat is broken down for energy.

  • How much fat and weight are lost, even when the same number of calories is eliminated, varies a great deal from one person to another.

Thus, predicting how much fat and weight a person will lose is difficult.

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. Such diets should be used only if prescribed and supervised by a doctor. Then the doctor can make sure that the diet contains enough essential nutrients, including protein. Consuming fewer than 800 calories is hard to tolerate and is not recommended.

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. For example, beans and legumes provide many nutrients without providing many calories. These foods are also high in fiber and bulk and thus help people feel full and satisfied. Eating fruits and vegetables in a variety of colors (such as strawberries, peaches, broccoli, spinach, and squash) is a way to get a variety of recommended nutrients and antioxidants.

  • 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.

  • 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. Physical activity helps preserve the amount of muscle tissue (muscle mass) people have, and resistance exercises may increase muscle mass. Because muscle tissue, even at rest, burns more calories than fat tissue, having more muscle increases the metabolic rate (the amount of calories the body burns while at rest) and the number of calories people need.

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. Also, 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 and have no carbohydrates stored in the body to use for energy, 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. A reduction of only 10 grams of fat per day saves about 90 calories. Thus, reducing the amount of fat rather than the amount of protein or carbohydrate might seem a faster way to lose weight. However, such a diet may not work. For example, people on low-fat diets may consume more carbohydrates and protein than normal and thus ultimately consume more calories.

In addition, not all fats are alike. Reducing the amount of saturated and trans fat in the diet is a good idea because doing so helps lower cholesterol levels in the blood (see Dyslipidemia). Lowering cholesterol levels benefits most dieters because excess weight increases their risk of atherosclerosis, which can lead to heart attacks or stroke. However, greatly reducing polyunsaturated and monounsaturated fats may be harmful because doing so decreases the level of high-density lipoprotein (HDL)—the good—cholesterol. Having a low HDL level can increase risk of atherosclerosis.

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, as are most liquid diets obtained from a doctor. Using such products to replace one to three meals daily can help people limit the number of calories they consume and thus lose weight or maintain their weight.

However, other liquid diets may 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.

Some Popular 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 B12





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





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

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.

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. However, 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: 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. Some 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.

Diets backed by science

Some diets have been scientifically studied and shown to be effective in promoting health—for example, by reducing the risk of atherosclerosis (including coronary artery disease), which can lead to heart attacks or stroke. These diets include the Mediterranean diet and the DASH diet.

The Mediterranean diet contains large amounts of olive oil, fruits, vegetables, nuts, and grains. It includes fish and poultry (but in smaller amounts than fish). Consumption of dairy, meats such as beef and pork, and sweets is limited. Drinking wine in moderation is encouraged.

Studies have shown that the Mediterranean diet helps reduce the risk of atherosclerosis, including coronary artery disease. This diet significantly reduces the risk of deaths due to coronary artery disease, as well as the risk of having a heart attack, a stroke, and angina (chest pain that occurs when the heart does not get enough blood and oxygen).

When the number of calories consumed is reduced, the Mediterranean diet may result in greater weight loss than a low-fat diet. This type of diet may be especially helpful for people who are overweight, have diabetes or a heart disorder, or have risk factors for atherosclerosis (and coronary artery disease).

The DASH (Dietary Approaches to Stop Hypertension) diet emphasizes eating lots of fruits and vegetables and using low-fat dairy products. Thus, the diet is low in saturated fat and cholesterol. It includes poultry, fish, whole grains, and nuts and limits consumption of red meats, sweets, and salt.

The DASH diet can reduce blood pressure in people with and without high blood pressure, even if weight is not lost, and can reduce blood sugar levels and improve cholesterol levels. This diet can benefit people with high blood pressure and may benefit those who are overweight, have diabetes or a heart disorder, or have risk factors for atherosclerosis (and coronary artery disease).

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