Obesity
Obesity is the accumulation of excessive body fat.
For most people, the condition of being overweight is easy to recognize. But medically, a distinction is made between being overweight and being obese. The body mass index (BMI) is used to define these conditions. BMI is weight (in kilograms) divided by height (in meters squared). Overweight is defined as a BMI of 25 to 29.9, and obesity is defined as a BMI of 30 or more.
Body composition--the percentage of fat and muscle in the body--is also considered when obesity is defined. Women who have more than 30% body fat or men who have more than 25% body fat are considered obese (see Section 12, Chapter 152). Thus, people who are very muscular and have low body fat (such as body builders) may have a high BMI without being obese and without increasing health risks.
Obesity is becoming increasingly common throughout the world. In the United States, this increase has been dramatic: Between 1980 and 1999, the percentage of overweight people increased from 47 to 61%, and the percentage of obese people increased from 15 to 26%.
Obesity is more common among women than among men. How common it is (prevalence) varies by age and race. For example, prevalence increases from about 14% at age 25 to 32% at age 55, then decreases to 22% at age 75. Obesity is equally common among black and white men and is slightly more common among Hispanic men. However, obesity is much more common among black and Hispanic women than among white women. About 67% of middle-aged black women are overweight or obese compared with 45% of middle-aged white women.
 See the table Determining Body Mass Index.
Causes
Obesity results from consuming more calories than the body uses. The number of calories needed varies from person to person, depending on age, sex, physical activity, and the person's metabolic rate--the rate at which the body burns calories.
Genetic and environmental factors influence body weight, but precisely how they interact is unclear. One proposed explanation is that body weight is regulated around a set point, similar to a thermostat setting. Some people may have a higher-than-normal set point, which may explain why they are obese and why losing weight and maintaining weight loss are difficult for them.
Obesity tends to run in families. However, families share not only genes but also environment, and separating the two influences is difficult. Genetic factors explain about one third to two thirds of the variability in body weight.
Several genes influence weight. One gene that has been identified--the ob gene--controls the production of leptin. Leptin is a protein made by fat cells. Leptin travels to the brain and acts on receptors in the hypothalamus (the part of the brain that helps regulate appetite). The message carried by leptin is to decrease food intake and increase the amount of calories (energy) burned. Researchers discovered that mutations in the ob gene prevent leptin production and result in severe obesity in mice and in a very small number of children. In these cases, administration of leptin effectively reduces weight to a normal amount. However, most experts think that in most people, many genes influence weight, and each has a very small effect. These genes have not been identified. Thus, genetic treatment of obesity is unlikely in the near future.
Physical inactivity is one of the main reasons for the increase in obesity among people in affluent societies. It is also a common cause of obesity as people age. Sedentary people need fewer calories. When physical activity increases, food intake usually increases. However, when physical activity decreases, food intake does not always decrease accordingly, and for some people, it even increases.
In affluent societies, the diet has become higher in fats. One problem with a high-fat diet is that fats do not appear to trigger the stop-eating (satiety) response as quickly as carbohydrates or proteins. Thus, when a diet is high in fat, more food tends to be eaten. Furthermore, fats have twice as many calories per gram as carbohydrates and proteins.
Drinking alcohol can contribute to obesity. Alcohol tends to increase the number of calories taken in because it is usually consumed in addition to food. A single shot (1 ounce) of liquor has 80 to 90 calories. A 12-ounce regular beer (which is about 8% alcohol) has 150 calories. As soon as alcohol is consumed, it is used as energy, causing the calories from food to be stored as fat. Furthermore, alcohol tends to stimulate the appetite and reduce self-control.
Socioeconomic factors strongly influence obesity, especially among women. In the United States and other developed countries, obesity is more than twice as common among women of lower socioeconomic class as it is among women of higher ones.
People who were obese as children (see Section 23, Chapter 271) are more likely to be obese as adults, largely because when weight is gained during infancy and early childhood, new fat cells form. People who become obese during childhood may have up to 5 times more fat cells than people who maintained a normal weight. Because the number of cells cannot be decreased, weight can be lost only by markedly decreasing the amount of fat in each cell. This fact may limit how much weight can be lost and make maintaining a normal body weight more difficult.
Gaining weight during pregnancy is normal and necessary. However, for a few women, pregnancy is the beginning of weight problems: They gain a large amount of weight and do not lose it afterward. Having several children close together may compound the problem.
After menopause, many women gain weight. At this time, hormonal changes cause fat to be redistributed in the body and to tend to accumulate around the waist rather than the hips and thighs (This redistribution increases health risks (see Section 12, Chapter 156).) Becoming less active, which may occur gradually and unconsciously at this age, also contributes to weight gain.
Psychologic factors, such as emotional disturbances, are no longer considered an important cause of obesity. However, stress can affect weight. When under stress, some people eat more, and some people eat less.
Hormonal disorders rarely cause obesity. Excess production of cortisol by the adrenal glands (Cushing's syndrome) causes an unusual type of obesity in which fat accumulates only in the trunk while the arms and legs remain thin. Polycystic ovary syndrome (see Section 22, Chapter 244) may be associated with obesity. Occasionally, an increased level of insulin in the blood (hyperinsulinemia) causes obesity.
Many drugs used for common disorders promote weight gain. Examples are drugs used to treat psychologic and neurologic disorders (including many antidepressants and antipsychotics), some antihypertensives (such as beta-blockers), corticosteroids, and some drugs used to treat diabetes (such as insulin).
Stopping smoking usually results in weight gain. Nicotine decreases appetite and increases the metabolic rate. Thus, when nicotine is stopped, food intake increases and the metabolic rate decreases, so that fewer calories are burned. As a result, body weight may increase by 5 to 10%.
Symptoms
Accumulation of excess body fat changes overall appearance. Severely obese people often walk abnormally to accommodate their weight. They widen their stance, making walking less steady and stressing the joints. As a result, osteoarthritis may develop or worsen, particularly in the hips, knees, and ankles, and walking may become even more difficult. Low back pain may also result. Fatigue is common. Physical and social activities may be decreased because of fatigue, lack of mobility, or other complications. The feet and ankles often swell because fluid accumulates (a condition called edema).
Because obese people have relatively little body surface for their weight, they cannot eliminate body heat efficiently and they sweat more than thinner people. Skin disorders are particularly common because moisture is trapped in skin folds.
Obese people may have difficulty breathing and may become short of breath, even when exertion is minimal. These problems occur when the lungs are compressed by accumulation of excess fat below the diaphragm (the muscle that divides the chest from the abdomen) and in the chest wall. Furthermore, airflow may be reduced if excess fat accumulates in the tissues that line the throat, narrowing the airway. Sleeping on the back makes breathing even more difficult (regardless of weight). Breathing problems may disturb sleep, and breathing may stop momentarily but repeatedly (a condition called sleep apnea (see Section 6, Chapter 81)). Sleep apnea can lead to daytime sleepiness and other problems, such as high blood pressure and strokes.
Obesity increases the risk of developing many disorders. For example, because the heart has to work harder, heart failure is more common among obese people. Certain cancers--of the breast, uterus, and ovaries in women and of the colon, rectum, and prostate in men--are more common among people who are obese than among those who are not. Menstrual disorders, osteoarthritis, gout, and gallbladder disease are also more common.
The risk of developing some disorders is affected by the location of excess fat. In men and in women after menopause, fat tends to accumulate in the abdomen (abdominal obesity), producing a so-called apple shape. In women, fat tends to accumulate in the thighs and buttocks (described as lower-body obesity), producing a so-called pear shape. Abdominal obesity has been linked with a high risk of coronary artery disease, stroke, high blood pressure, type 2 diabetes, and high levels of fats (lipids) in the blood. For people with abdominal obesity, losing as little as 5 to 10% of their weight dramatically reduces these risks. Blood pressure decreases in most people who have high blood pressure, and more than half of the people with type 2 diabetes can discontinue insulin or other drugs that lower blood sugar. Losing weight and changing to a low-fat diet can reduce levels of fats in the blood.
Obesity doubles or triples the risk of premature death. The more severe the obesity, the higher the risk. In the United States, 300,000 deaths a year are attributed to obesity.
In a culture that values thinness, being obese can cause psychologic or emotional problems. Many young obese women have a poor body image, which leads to self-consciousness and discomfort in social situations. People who are obese may experience prejudice and job discrimination, causing feelings of rejection and low self-esteem. However, depression does not appear to be more common among obese people than among other people.
Diagnosis and Treatment
Obesity is readily diagnosed, and its severity is determined by the BMI. Other tests, such as those used to determine body composition, are rarely needed (see Section 12, Chapter 152).
Untreated obesity tends to worsen. Even though progress has been made in helping people lose weight and maintain weight loss, most people who lose weight regain it, usually within 3 years. The concern that losing and regaining weight (weight cycling) causes health problems is unfounded and should not prevent obese people from trying to lose weight.
To lose weight, people must consume fewer calories than they burn. They can reduce their intake of calories or exercise more (to burn more calories). Usually, both should be done.
Different approaches help different people. Some people work on their own or join a group of like-minded people. These groups include Overeaters Anonymous (OA), Take Off Pounds Sensibly (TOPS), and community-based and work-site programs. Books and magazine articles, special diet plans, and weight loss products such as meal replacement formulas may be used. There is little information about the success of these approaches.
Other people choose organized programs. Typically, weekly meetings are conducted by counselors and supplemented by instructional and guidance materials. Counselors may be licensed health care practitioners or not. Such programs tend to be limited in duration, and many people drop out of them. Costs vary from about $15 per week to $3,000 per 6 months of treatment. There is little information about their effectiveness. Nevertheless, their ready availability and the desire for treatment have made them popular.
Most weight management programs focus on diet with nutritional counseling and exercise. Most programs include behavior modification techniques to facilitate diet and exercise. These techniques help people identify and change the behaviors that trigger overeating (such as shopping for food when they are hungry or keeping high-calorie snacks at home). Doctors may prescribe drugs that help reduce body weight as part of a weight management program.
Diet and Nutritional Counseling: Dieting is useful only when it includes permanent changes in eating habits. Reputable programs teach people how to make safe, sensible, gradual changes in eating habits to increase the consumption of complex carbohydrates (fruits, vegetables, breads, and pasta) and decrease the consumption of fat. Fad diets may be dangerous and are best avoided (see Section 12, Chapter 152). For mildly obese people, only a modest restriction of calories and dietary fat is recommended. Usually, the number of calories consumed is reduced to 1,200 to 1,500 a day. Very low calorie diets of 800 calories per day, or even fewer, have been used in the past but have been largely abandoned. After these diets, body weight is usually rapidly regained.
Exercise: Regular moderate exercise can help with weight loss. One commonly suggested goal is 30 minutes or more 5 to 7 days a week (see Section 1, Chapter 6). Exercise increases the number of calories the body uses. Yet, exercise cannot substitute for controlling caloric intake. It takes about 1 hour of walking to burn the calories in one alcoholic drink and about 1 hour of running to burn those in one piece of cheesecake.
Aerobic exercise, such as jogging, walking briskly (3 to 4 miles an hour), biking, singles tennis, skating, and cross-country skiing, burn more calories than less active exercises (see Section 1, Chapter 6). 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 more effective. It burns about 6 to 8 calories per minute.
Drugs: The combination of fenfluramine and phentermine (often called fen-phen) was the most effective treatment used so far. However, fenfluramine was removed from the market because of heart valve problems in people who took this combination. Seven weight loss drugs are currently available by prescription: orlistat, sibutramine, phentermine, benzphetamine, diethylpropion, mazindol, and phendimetrazine. Orlistat limits the breakdown and absorption of fats in the intestine, producing, in effect, a low-fat diet. Sibutramine, phentermine, benzphetamine, diethylpropion, mazindol, and phendimetrazine are all believed to reduce appetite by affecting chemical messengers in the part of the brain that controls appetite. Weight loss with these drugs is rarely more than 10%. Some nonprescription diet aids, including medicinal herbs, claim to enhance weight loss by increasing metabolism or by increasing a feeling of fullness. Although usually harmless, they are ineffective and, if they contain stimulants (such as ephedra), should be avoided. New drugs for the treatment of obesity are being developed.
Surgery: For severe obesity (BMI of more than 40), surgery is the treatment of choice. There are two major types of surgery. In vertical banded gastroplasty, rows of staples and a band that limits the entrance of food into the stomach are used to form a 1-ounce stomach pouch, which drastically reduces the amount of food that can be eaten at one time. In gastric bypass surgery (which is somewhat more effective), some of the small intestine is bypassed, thus reducing the absorption of food. These operations were originally performed by opening the abdomen. More and more commonly, they are being performed through a laparoscope, a viewing tube that is inserted into the abdominal cavity through a small incision just below the navel. Surgery using a laparoscope is much less traumatic and recovery after surgery is much more rapid.
 See the figure Bypassing Part of the Digestive Tract.
These operations result in a large weight loss--half or even more of the person's excess weight and as much as 80 to 160 pounds. Weight loss is rapid at first, then slows gradually over a period of about 2 years. The loss is often maintained for years. The loss greatly reduces obesity-related complications (such as high blood pressure and diabetes) and improves the person's mood, self-esteem, body image, activity level, and ability to work and interact with other people.
When conducted at specialized centers, surgery is well tolerated. Fewer than 10% of people develop complications from surgery, and fewer than 1% die.
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