Table 60-1 represents the relationship between weight and height. Body mass index (BMI) is calculated as weight in kilograms divided by height in square meters or as weight in pounds multiplied by 704.5 and divided by height in square inches. The National Institutes of Health has issued guidelines for the classification of weight status by BMI that separates patients by risk: Those with a BMI of 25.0 to 29.9 kg/m² are considered overweight; those with a BMI more than 30 kg/m² are considered obese. Extreme obesity is defined as a BMI more than 40 kg/m² and carries a much higher risk for morbidity and mortality. The optimal BMI to minimize the consequences of obesity-related diseases is probably in the range of 19 to 21 kg/m² for women and 20 to 22 kg/m² for men. It is reported that American adults, especially women, who weigh 15% less than their age-matched, normal-weight peers have a significant reduction in projected mortality. Additional factors such as fat distribution and recent weight gain also modify the risk within each BMI category. Persons with increased abdominal fat have increased risk for hypertension, ischemic heart disease, dyslipidemia, diabetes mellitus, and insulin resistance syndrome over those with increased gluteal and femoral fat.

Weight gain during adulthood is an additional risk factor for medical complications. Weight gain of 75 kg in body weight since the age of 12 to 20 years increases the relative risk for cholelithiasis, diabetes mellitus, hypertension, and ischemic heart disease.

correlates adequately with abdominal fat distribution. Deposition of fat in the abdomen, particularly if it is out of proportion to fat distribution elsewhere in the body, represents a health risk for morbidity and mortality that is independent of being overweight or obese. Measuring waist circumference, best taken at the level of the umbilicus with the patient in the supine position, is a reasonable method for assessing a patient’s health risk and monitoring weight-reduction interventions (Table 60-2).