(From Grolier Multimedia Encyclopedia)


Obesity, one of the most common health problems in developed countries. In the United States, for example, it is estimated that between 20% and 35% of the population would be considered overweight. Although obesity can occur at any age, it is most prevalent among men and women in their 40s and 50s.

Life-insurance records indicate that people who are overweight do not live as long as people of normal weight. Among the disorders that occur more commonly among the obese are high blood pressure, heart disease, strokes, diabetes, varicose veins, back and joint pains, gallbladder problems, and certain forms of cancer. Also, overweight people tend to have more accidents and be greater risks for surgery. Research has indicated also that overweight women may have more fertility problems and a reduced chance of survival from breast cancer. In addition, because of their size, many overweight people, especially children and teenagers, suffer social and emotional problems.

Diagnosing Obesity

People are said to be obese when their weight is more than 20% over their ideal weight. For example, a man whose ideal weight is 180 pounds (81 kg) would be considered obese if he weighed more than 216 pounds (97 kg). The accompanying tables list desirable weights for men and women according to height and body build.

Another way of diagnosing obesity is related to the proportion of fat in the body. Ideally, a man's body-fat percentage should range between 12% and 20%, while a woman's should range between 20% and 27%. Thus, a muscular athlete may weigh over 250 pounds (112.5 kg) but not be considered obese because the proportion of fat in his body is only 15%.

One of the most accurate ways to measure body fat involves immersing a person in a special water-filled tank to determine the body's volume. This, together with the weight, is then used to calculate the percentage of body fat.

A simpler method, known as the pinch test, measures skinfold thickness. This is done by grasping the skin and underlying tissues at the bottom of the ribcage. A skinfold more than 1 inch (2.5 cm) thick indicates an excess of body fat.

In addition to differing in their ideal fat percentages, men and women differ in the sites where their excess fat is deposited. A potbelly, for example, is more likely to occur in men because their fat cells are concentrated in that area. In women, fat cells are more or less evenly distributed. Contrary to some advertisements, there is no special fat tissue called cellulite. The dimpling that sometimes occurs in fatty regions is related to the elasticity of the skin. The underlying fat tissue in these areas is no different from the fat tissue elsewhere in the body.

Calories and Weight Control

Overweight people generally fall into three main groups: those who freely admit that they overeat and may be compulsive eaters; those who say they eat no more than the average person; and those who insist they remain overweight despite a low food intake. Each group may well be speaking the truth. The intake of food is only part of the equation. The expenditure of energy is the other part.

Food energy is measured in units called calories, or kilogram-calories. Individuals who consume 2,000 calories a day will remain at the same weight as long as they expend 2,000 calories a day. However, if their intake of calories exceeds their expenditure of calories, the excess calories will be stored as fat. Every pound of fat in the body stores 3,500 calories. Therefore, an extra 24 calories a day, as in 1.5 teaspoons of sugar, will add 25 pounds (11.7 kg) of fat over a period of ten years.

The tendency to attribute overweight to an excessive intake of sugars and starches, however, arises from the illusion that obese people usually eat large amounts of candy and sweets. This theory, though, ignores total calories as well as the important role of fat in the diet. Obese people actually derive their excess calories largely from fat. A single gram of fat provides 9 calories, while the same amount of protein or carbohydrate provides only 4 calories.

Do lean people never overeat? Some may actually eat far more than their fat companions, but their physical activity, muscle tone, or a physiological mechanism may ensure that they have no excess calories to be laid down as fat. Many obese people may work and move about in a relaxed, motion-saving manner that keeps their calorie expenditure to a minimum. Lean people may use many more calories performing the same task and thus maintain a normal weight despite a generous intake of food. Whatever the reasons, lean people remain lean because they do not eat beyond their calorie expenditure.

Causes of Obesity

Sometimes, the cause of obesity can be traced to an underlying medical disorder, such as diabetes, but most often a direct physical cause cannot be found. Some of the body's physiological mechanisms are suspected of playing a role in weight control, but research on these mechanisms is not conclusive.

Although little is known about the role of heredity in obesity, it is known that if one or both parents are overweight, their children will probably be overweight. Studies of identical twins have shown that even though the twins may have been separated at birth and raised in different households, they still resemble each other in weight many years later.

Medical Disorders. Among the disorders that sometimes cause obesity are hormonal imbalances, low blood sugar, and diabetes. Abnormalities of the brain's appetite control center are also suspected as a cause. Occasionally, obesity may be due to a drug. Some antidepressants, for example, may produce obesity as a side effect.

Thyroid Hormone. Although underfunction of the thyroid gland often is suspected as a cause of overweight, obesity occurs in less than half of all individuals with complete loss of thyroid function. This indicates that such underfunction does not by itself cause obesity. Studies have shown that a shortage of the active form of T3 (triiodothyronine), a thyroid hormone, does develop during periods of low calorie intake. This shortage tends to reduce the body's expenditure of calories and thus may help explain why some obese people have difficulty losing weight even though they are on a low-calorie diet.

Cortisol. The adrenal hormone cortisol also may contribute to obesity. Cushing's syndrome, a disorder caused by excesses of cortisol, is accompanied by weight gain that is caused largely by increased appetite. While most obese people do not have Cushing's syndrome, the secretion of cortisol may increase as overweight develops, thus stimulating appetite. This increase, however, is never great enough to produce Cushing's syndrome, and it promptly disappears when the excess weight is lost.

Sex Hormones. Sex hormones also may be related to overweight. Obesity in girls sometimes leads to the early onset of menstruation. Thereafter, however, the menstrual cycles follow the usual patterns of adolescence.

Menopause, the total cessation of menstruation that normally occurs between the ages of 40 and 50, is marked by a decreased secretion of the female sex hormones estrogen and progesterone. Such decreases would seem to make weight loss easier at this time because estrogen normally increases the amount of fat in the buttocks and other areas and both estrogen and progesterone cause the retention of salts and fluid, resulting in an increase in weight. In fact, however, menopause is often accompanied by an increase in weight. This gain is usually the result of increased food intake, decreased activity, or both.

Hypoglycemia. Hypoglycemia, or low blood sugar, sometimes causes a gain in weight as a result of increased food intake.

Diabetes. Adult-onset diabetes (not the juvenile form that occurs mainly in children and young adults) is almost always accompanied by obesity due to high calorie intake and low physical activity. Exercise together with a restricted diet can virtually cure many causes of this form of diabetes.

Even in people who are not diabetic, obesity adversely affects the body's handling of the simple sugar glucose. In obese people, the rise in blood sugar does not cut off, as it does in nonobese people, the output of stored glucose from the liver. The unduly high glucose levels are accompanied by abnormally high insulin levels, but the insulin is not efficient in controlling glucose levels—possibly because of a shortage of insulin receptors on or in the body's cells. But excessive insulin does facilitate the deposit of extra calories as fat. As a result, fat people may tend to get fatter.

Appetite Control Center. Changes in the brain's appetite control center, which is located in the hypothalamus, may also be related to obesity. Animal studies have shown that damage to a particular area of this center can cause overeating and resulting obesity. Although studies of obese people who overeat have not revealed any abnormalities in their appetite control centers, it is possible that the tests used for this purpose are not sensitive enough to detect abnormalities.

Physiological Mechanisms. Although little is known about the body's physiological mechanism related to weight control, research suggests that some cases of obesity may be due to defects in such mechanisms. Among these mechanisms are those involving the body's fat cells and those involving the activities of certain enzymes.

White Fat. There are approximately 30 billion fat cells in the body. The vast majority are found in white tissues whose function is to store energy in the form of fat. These so-called white fat cells can increase in number at any age. When new fat cells are formed during a period of high calorie intake, however, they do not act like other fat cells. During a period of low calorie intake, as when a person is on a diet, the new cells do not lose their fat as easily as other fat cells. In addition, they tend to form new fat quickly when normal eating habits are resumed. This may be one reason why obese people who lose weight during a diet find it difficult to maintain their new weight and tend to gain back the lost pounds.

Brown Fat. In addition to ordinary white fat, there is another kind of fat tissue, called brown fat. This tissue, which contains the brownish pigment cytochrome oxidase, makes up about 1% of the total body fat. Although very little is known about the functions of brown fat in humans, in animals it is known to increase the expenditure of calories during periods of high food intake. It burns the excess calories, converting them to heat. Because the amount of brown fat in adult animals decreases with age, the animals tend to form more fat and gain weight as they get older. It is possible that this same mechanism exists in humans, perhaps explaining why people tend to gain weight during their middle years.

The burning of excess calories by brown fat may also help explain why some people gain weight more easily than others. Those who gain weight easily may have a brown-fat defect that reduces the conversion of excess calories into heat.

Enzymes. Certain enzymes also may provide clues to the cause of obesity in some people. One such enzyme is lipoprotein lipase (LPL), which removes fatty substances from the blood and stores them in fat cells. Studies have shown that when obese people lose weight through low-fat diets, their blood levels of LPL rise above normal, helping the depleted fat cells regain their lost fat. This, also, may help explain why some overweight people tend to have difficulty maintaining a normal weight after dieting.

Another enzyme that may prove to be important in weight control is sodium-potassium ATPase. This enzyme acts as a chemical pump, moving sodium and potassium into and out of cells. It has been estimated that this pumping action may burn up as much as half of the body's heat energy. Studies suggest that obese people may have abnormally low levels of this enzyme, possibly indicating that the pumping activity in their cells does not burn up as many calories as in other people.

Eating Patterns. As a rule, nonobese people experience hunger contractions at times when it is appropriate to eat. Similarly, they experience a feeling of fullness when it is appropriate to stop eating. Many obese people either do not feel hunger contractions at all or tolerate them too well. They overeat as if it were a learned bad habit. At the same time, they are less able to perceive a feeling of fullness.

External food signals are also very important. Lean persons generally do not eat when their stomachs are full, even when food is available. Obese people, on the other hand, are easily stimulated to eat by external cues. Hungry or not, with their stomachs full or empty, the obese are tempted to eat anything within their sight, smell, or reach.

Psychological Aspects. Little is known about the psychological aspects of obesity, but it is clear that massively obese people are not simply normal individuals with an excess of fat. As a group, obese people tend to be passive, dependent, and depressed. However, it is not clear whether this pattern develops before or after the obesity occurs. Moreover, scientists have found that very obese people tend to have an altered sense of time. Alone in a room, they estimate the hour to be later than it is.

Treatment of Obesity

Just as a gain in weight is usually due to an increase in food intake, a decrease in activity, or both, a loss of weight can usually be achieved by reversing this pattern—that is, by reducing the intake of calories and increasing their expenditure. Since 1 pound (0.45 kg) of fat stores the equivalent of 3,500 calories, a person must expend 3,500 calories for every lost pound of weight. Thus, a reduction of 500 calories a day over a prolonged period should result in the loss of 1 pound a week, given that the same level of activity is maintained.

Usually, a permanent weight loss can be achieved only through a long-term regimen that includes a calorie-restricted but nutritionally balanced diet, increased exercise, and a retraining of the individual's eating patterns. Other methods, such as fad diets, spot-reducing exercises, and surgery are often unsuccessful and may actually be harmful. Special devices, such as mechanical or electric massagers and vibrators, are of no value in a weight-reduction program.

Calorie-Restricted Diets. The best way to lose weight permanently is to eat all the usual foods in a balanced diet—including such items as candy and ice cream—but in smaller quantities than usual. Such a diet requires no special planning or preparation, and the dieter still receives all the necessary vitamins and minerals. However, avoiding fatty foods is usually helpful, because they provide more than twice as many calories per gram as do proteins and carbohydrates. For some people, increasing the amount of fiber in the diet helps suppress the appetite. Fiber, which is indigestible and nonnutritious, is most plentiful in fresh and frozen fruits and vegetables, whole-grain cereals, and nuts.

The spacing of meals is also important in losing weight. Eating many smaller meals throughout the day is more effective than eating an entire day's allotment of food at one sitting. Studies show that eating only one main meal a day may actually increase one's weight.

Calorie counting is helpful in planning a restricted diet. For example, it is important to know that a medium-sized baked potato jumps in calories from 90 to 115 when it is eaten with a pat of butter. Two tablespoons of sour cream raise the calorie content to nearly 150. A table of common foods and their calorie contents is included in the article Diet.

Many people on a weight-reduction diet find that they must greatly reduce or completely eliminate alcohol from their diet. A gram of pure alcohol provides 7 calories, and 2 ounces (60 ml) of 100-proof whiskey provide 210 calories. Similarly, 12 ounces (360 ml) of beer provides about 170 calories, and 4 ounces (120 ml) of dry red or white wine provides from 75 to 100 calories.

Some obese people may fail to lose weight even though they expend more calories than they consume. In many cases, the body retains water in place of the lost fat so that the body weight does not decrease. This is a short-term effect, however, which soon disappears.

Exercise. Exercise should be an important part of any weight-control program. However, exercise alone is not enough. A practical approach to weight loss uses up fat deposits through a combination of calorie restriction and exercise.

Besides burning up calories, exercise helps suppress the appetite and provides a way of releasing anxiety and tension. In addition, it has an insulin-like effect on glucose—that is, it facilitates the burning of this simple sugar—and it lowers the levels of cholesterol and triglycerides in the blood. Studies also show that exercise elevates the body's calorie expenditure, not only during the exercise but for as long as 15 hours afterward. Thus, exercise during the day may increase the body's normal energy expenditure even during the night.

Strenuous exercise is the most effective. Among the best kinds of exercise are brisk walking, running, bicycling, and swimming. These activities use up large numbers of calories. Spot-reducing exercises, such as sit-ups, are seldom helpful. They may increase the muscle tone of a particular area, but they do little to burn up fat.

Fad Diets. The fact that there are so many different fad diets attests to their ineffectiveness over the long run. Most of the popular fad diets either restrict or increase one or more kinds of foods. Not only are these diets nutritionally unbalanced, but they give the false impression that dietary manipulations of fats, sugars, starches, or proteins can cause weight loss without a reduction in calories. There are no scientific data to support claims that certain kinds of foods, such as those containing starches and other complex carbohydrates, are more likely to cause weight loss than others, such as ordinary sugar, which are equal in calories.

What often does happen during a fad diet is that there is a loss of fluid—and thus weight—during the first week or two. Not only is this unhealthy, but as soon as normal eating habits are resumed, the fluid is regained and so is the lost weight.

Drugs. In cases where obesity is caused by underfunction of the thyroid gland, the administration of thyroid hormone can help in weight reduction. However, such cases make up only a tiny fraction of the obese population.

Appetite-suppressing drugs (amphetamines or their derivatives) were once commonly prescribed to help people lose weight. Although these drugs can suppress appetite temporarily, they produce a number of unpleasant side effects, including dryness of the mouth, restlessness, insomnia, rapid heartbeat, and lightheadedness. For this reason, and because the drugs are subject to widespread abuse, the U.S. Food and Drug Administration advises doctors to avoid prescribing them for weight control. Over-the-counter preparations, such as PPA (phenylpropanolamine), share some of the dangers of amphetamines and also should be avoided.

Surgery. Two kinds of surgical operations have been used to help in the quest for weight loss. A jejunoileal bypass shortens the small intestine and can produce a weight loss of 1 to 2 pounds (0.45 to 0.9 kg) per week. This operation, however, may cause serious complications and generally is not recommended.

A gastric bypass reduces the size of the stomach. This operation does not always produce a weight loss, and even when it does, the loss may be only temporary or even replaced by a gain.

Changing Eating Patterns. The ultimate answer to the control of overweight entails the retraining of the individual's eating patterns. If overeating is caused by anxiety or insecurity, the person must learn to relieve this discomfort by means other than eating. If appetite and hunger are rampant, they must be restrained. If a feeling of fullness is not achieved with reasonable amounts of food, behavior must be altered.

Some people have been helped by behavior modification therapy, based on the principle that overeating is a learned bad habit that can be unlearned, and others have been helped by psychotherapy. Group programs, such as Weight Watchers, can be successful for many overweight people who need the emotional support and encouragement of others.

T. S. Danowski, M.D.
University of Pittsburgh


Beers, Mark H., and Robert Berkow, eds., The Merck Manual of Diagnosis and Therapy, 17th ed. (Merck Res. Labs. 1999).

"From the Centers for Disease Control and Prevention: Prevalence of Leisure-Time Physical Activity among Overweight Adults—United States, 1998," JAMA 283 (2000):2650–2651.

Jeffcoate, W., "Obesity Is a Disease: Food for Thought," Lancet 351 (1998):903–904.

Macready, N., "The Stubborn Enigma of Obesity," Lancet 351 (1998):888.

Schwartz, Hillel, Never Satisfied: A Cultural History of Diets, Fantasies, and Fat (Anchor Bks. 1990).

Thompson, J. Kevin, ed., Body Image, Eating Disorders, and Obesity: An Integrative Guide for Assessment and Treatment (Am. Psychological Assn. 1996).

Willett, W. C., et al., "Guidelines for Healthy Weight," New England Journal of Medicine 341 (1999):427–434.

Appended Material

Desirable Weights for Women

(Weights at ages 25–29, for optimum longevity)

Weight in pounds (with 3 pounds of clothing)
Height (in shoes*) Small frame Medium frame Large frame
4'10" 102–111 109–121 118–131
4'11" 103–113 111–123 120–134
5'0" 104–115 113–126 122–137
5'1" 106–118 115–129 125–140
5'2" 108–121 118–132 128–143
5'3" 111–124 121–136 131–147
5'4" 114–127 124–138 134–151
5'5" 117–130 127–141 137–155
5'6" 120–133 130–144 140–158
5'7" 123–136 133–147 143–163
5'8" 126–139 136–150 146–167
5'9" 129–142 139–153 149–170
5'10" 132–145 142–156 152–173
5'11" 135–148 145–159 155–176
6'0" 138–151 148–162 158–179

*1-inch heels.

Note: Weights are somewhat higher than in previously published tables and are not universally accepted as ideal.

Source: Metropolitan Life Insurance Company, published in 1983.

Desirable Weights for Men

(Weights at ages 25–29, for optimum longevity)

Weight in pounds (with 5 pounds of clothing)
Height (in shoes*) Small frame Medium frame Large frame
5'2" 128–134 131–141 138–150
5'3" 130–136 133–143 140–153
5'4" 132–138 135–145 142–156
5'5" 134–140 137–148 144–160
5'6" 136–142 139–151 146–164
5'7" 138–145 142–154 149–168
5'8" 140–148 145–157 152–172
5'9" 142–151 148–160 156–176
5'10" 144–154 151–163 158–180
5'11" 146–157 154–166 161–184
6'0" 149–160 157–170 164–188
6'1" 152–164 160–174 168–192
6'2" 155–168 164–178 172–197
6'3" 158–172 167–182 176–202
6'4" 162–176 171–187 181–207

*1-inch heels.

Note: Weights are somewhat higher than in previously published tables and are not universally accepted as ideal.)

Source: Metropolitan Life Insurance Company, published in 1983.