Body Mass Index

For adults age 18 years and older, the body mass index (BMI), or Quetelet index, is one of the best indirect indicators of degree of obesity. Calculated as weight (in kilograms) divided by height (in meters) squared (kg/m2), adults are typically classified into one of four categories based on criteria established by the World Health Organization (WHO): underweight (BMI < 18.5 kg/m2), normal weight (BMI 18.5 to 24.9 kg/m2), over-weight (BMI 25 to 29.9 kg/m2), or obese (BMI ≥ 30 kg/m2). An additional category, clinically severe or morbid obesity, corresponds to a BMI of 40 kg/m2 or more and represents adults who are at least 100 pounds over their ideal body weight. Current estimates place approximately 22.3% of U.S. adults as obese (19.5% of men and 24.5% of women), with 2% being underweight. The optimal BMI for longevity appears to fall between 20.5 and 24.9 kg/m2 for men and women of all ages, although recent evidence suggests that the longevity advantage may extend to those who are moderately overweight.

Under some conditions, the BMI may underestimate or overestimate degree of obesity. Underestimation may occur in adults who have lost muscle mass such as older adults, whereas overestimation may occur in those who are muscular. Underestimation or overestimation may also occur because the BMI calculation often relies on self-reported height and weight, which can change with age and with an individual's willingness to report this information correctly; for example, adults typically overestimate height and underestimate weight. The BMI also does not consider body fat distribution. For adults with an excess of body fat around the abdominal region (android or central obesity), other indirect measures may be more appropriate, including waist-to-hip ratio and waist circumference. For example, as adults age, body fat tends to shift from peripheral to central sites, causing an associated increase in waist-to-hip ratio with little or no increase in BMI.

Despite these potential limitations, the literature indicates good agreement between BMI and indexes of health such as mortality. The BMI–mortality relationship, often reported as U-shaped or J-shaped, shows mortality risk to increase in underweight (BMI < 18.5) and obese [Page 83](BMI ≥ 30) adults. Adjusting for factors that can potentially confound this relationship, such as smoking status and underlying disease, reduces the strength of the BMI–mortality relationship but does not eliminate it. Epidemiological data further show a consistent relationship between increasing BMI and a variety of chronic illnesses, including type 2 diabetes, hypertension, coronary artery disease (e.g., heart attack), high cholesterol, sleep apnea, degenerative joint disease, and certain cancers. Increasing BMI has also been associated with depression, low self-esteem, physical disability, social discrimination, and unemployment.

In summary, the BMI has proven to be an important tool in clinical and epidemiological studies. Because its calculation is simple and inexpensive and can be applied generally to all adults, the BMI has been used successfully to estimate body composition, develop reference standards, establish baseline data for longitudinal studies, monitor trends in specific populations, and assess risks for adverse health outcomes in all age groups.