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Obesity is a major public health problem and the leading nutritional disorder in the U.S. It is responsible for more than 280,000 deaths annually in this country. A widely accepted definition of obesity is body weight that is 20% or more in excess of ideal weight:height ratio according to actuarial tables. By this definition, 34% of adults in the U.S. are obese. The National Institutes of Health have defined obesity as a BMI of 30 kg/m2 or more, and overweight as a BMI between 25 and 30 kg/m2. By these criteria, two thirds of adults are either overweight or obese. There is strong evidence that the prevalence of obesity is increasing in both children and adults. Increases are particularly striking among African-Americans and Mexican-Americans. More than 80% of black women over the age of 40 are overweight, and 50% are obese. Among factors blamed for the steady increase in the prevalence of obesity are unhealthful eating practices (high-fat diet, overlarge portions) and the decline in physical activity associated with use of automobiles and public transportation instead of walking, labor-saving devices including computers, and passive forms of entertainment and recreation (television, computer games). Despite efforts of public health authorities to educate the public about the dangers of obesity, it is widely viewed as a cosmetic rather than a medical problem. Obesity is an independent risk factor for hypertension, hypercholesterolemia, Type 2 diabetes mellitus, myocardial infarction, certain malignancies (cancer of the colon, rectum, and prostate in men and of the breast, cervix, endometrium, and ovary in women), obstructive sleep apnea, hypoventilation syndrome, osteoarthritis and other orthopedic disorders, infertility, lower extremity venous stasis disease, gastroesophageal reflux disease, and urinary stress incontinence. Lesser degrees of obesity can constitute a significant health hazard in the presence of diabetes mellitus, hypertension, heart disease, or their associated risk factors. Body fat distribution in central (abdominal or male pattern, with an increased waist:hip ratio) versus peripheral (gluteal or female pattern) adipose tissue depots is associated with higher risks of many of these disorders. Obese people are more liable to injury, more difficult to examine by palpation and imaging techniques, and more likely to have unsuccessful outcomes and complications from surgical operations. Not least among the adverse effects of obesity are social stigmatization, poor self-image, and psychological stress. Weight reduction is associated with improvement in most of the health risks of obesity. All treatments for obesity (other than cosmetic surgical procedures in which subcutaneous fat is mechanically removed) require creation of an energy deficit by reducing caloric intake, increasing physical exercise, or both. Basic weight reduction programs involve consumption of a restricted-calorie, low-fat diet and performance of at least 30 minutes of endurance-type physical activity of at least moderate intensity on most and preferably all days of the week. Behavior modification therapy, hypnosis, anorexiant drugs (sympathomimetic agents, sibutramine), the lipase inhibitor orlistat, and surgical procedures to reduce gastric capacity or intestinal absorption of nutrients are useful in selected cases, but the emphasis should be on establishing permanent changes in lifestyle. Weight reduction is not recommended during pregnancy or in patients with osteoporosis, cholelithiasis, severe mental illness including anorexia nervosa, or terminal illness.