INACCURATE BMIs

Several of my patients are very muscular because they exercise at the gym regularly. When I calculate a BMI using their height and weight, they fall within the overweight or obese category. But their weight is due to their muscle, not fat. Is there a more precise way to measure BMI in such patients?
—Elizabeth George, MD, Dallas

BMI (weight in kg/height in meters2) is easily obtained and is the standard predictor of morbidity and mortality in obesity. However, BMI does not distinguish fat from muscle weight and may overestimate the risk of a patient who is heavy from muscle mass. Alternatives, which may better measure adiposity, include waist circumference, skinfold measurement, and bioimpedance measures. Waist circumference, also easily obtained, will often be normal in muscular patients (40 inches for men, 35 inches for women). Skinfold measurement provides an estimation of body fat but requires use of calipers by an experienced practitioner to measure the thickness of a pinch of skin, usually at the triceps. Bioimpedance measures the proportion of different tissue types based on their impedance of electrical current but requires specialized equipment.
—Seonaid Hay, MD, assistant professor of medicine, Yale University School of Medicine, New Haven, Conn. (109-21)

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