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Obesity is one of the most common problems in clinical practice. Defined as a body mass index (BMI) >30 kg/m2, over 30% of adult Americans are obese. An additional 35% are overweight, with BMIs between 25 and 30 kg/m2. 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 57%, hypertension by 17%, coronary artery disease by 17%, and various cancers by 11%.

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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 with little benefit. Weight-loss diets, for example, even in the best treatment centers, result in an average 8% reduction in body weight. This lack of clinical success has created a never-ending demand for new weight-loss treatments. Approximately, 45% of women and 25% 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, "fat farms," 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.

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Obesity is defined as an excess of body fat. Body fat can be measured by several methods including total body water, total body potassium, bioelectrical impedance, and dual-energy x-ray absorptiometry. 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 <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 ≥30.0 kg/m2 as obese. Obesity can be 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.

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The relationship between body weight and mortality is curvilinear, similar to other cardiovascular risk factors. Most studies have demonstrated a J-shaped relationship, suggesting that the thinnest portion of the population also has an excess mortality. This is primarily ...

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