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Obesity is one of the most common problems in clinical practice. Defined as a body mass index (BMI) of greater than 30 kg/m2, over 39% of adult Americans are obese. An additional 35% are overweight, with BMIs between 25 and 30 kg/m2. 18.5% of children are obese and over one-third of children are overweight or obese. Because obesity is at the center of chronic disease risk and psychosocial disability for millions of Americans, its prevention and treatment offer unique patient care and public health opportunities. If all Americans were to achieve a normal body weight, it has been estimated that the prevalence of diabetes would decrease by half, whereas hypertension, coronary artery disease, and various cancers would decrease by 10–20%.

Obesity is often one of the most difficult and frustrating problems in primary care for both patients and physicians. Considerable effort is expended by primary care providers and patients, often with little benefit. Weight-loss diets, for example, even in the best treatment centers, result in an average 5–10% reduction in body weight. This modest clinical success has created a never-ending demand for new weight-loss treatments. Approximately, half of women and one-quarter of men are “dieting” at any one time, spending billions of dollars each year on diet books, diet meals, weight-loss classes, diet drugs, exercise programs, and other weight-loss aids. The challenge for health care providers is to identify those patients with obesity who are most likely to benefit medically from treatment and most likely to maintain weight loss, and to provide them with sound advice, skills for long-term lifestyle change, and support. For patients not motivated to attempt a weight-loss program, health providers must continue to be respectful and empathic and focus on other health concerns. Whenever possible, providers should emphasize prevention of obesity and further weight gain and the importance of physical fitness independent of body size.


Obesity is defined as an excess of body fat. Body fat can be measured by several methods, including isotope-labeled water, underwater weighing, bioelectric impedance, dual-energy X-ray absorptiometry, computerized tomography, and magnetic resonance imaging. In clinical practice, however, obesity is best defined by the BMI—body weight divided by height squared (kilograms per square meter). The BMI correlates closely with measures of body fat and with obesity-related disease outcomes. According to the National Institutes of Health (NIH), an individual with a BMI lesser than 18.5 kg/m2 is classified as underweight, 18.5–24.9 kg/m2 as normal, 25.0–29.9 kg/m2 as overweight, and greater than or equal to 30.0 kg/m2 as obese. Obesity is further classified as class I (BMI 30–34.9 kg/m2), class II (BMI 35–39.9 kg/m2), and class III or extreme obesity (BMI ≥40 kg/m2). The term “morbid obesity” is best avoided for those with class III obesity, since obesity-related morbidity can occur at any obesity level. Measurements of waist circumference, waist-to-hip ratio, ...

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