BMI reconsidered

For decades, the body mass index (BMI) has been the standard for determining whether a person is at a healthy weight, is overweight, or has obesity. Obesity screening has been used to help identify people who are at higher risk of obesity-related diseases, like heart disease and type 2 diabetes. But lately, medical experts have pointed out problems with BMI.

In June, the American Medical Association (AMA) adopted a policy that acknowledges the limitations of BMI and encourages clinicians to use the index in conjunction with other measures of risk when managing adults who are overweight or have obesity.

Although BMI is correlated with body fat levels at the population level, that relationship is less predictable on the individual level. BMI—weight (in kilograms) divided by height (in meters) squared—does not directly assess body fat. At the same BMI, women generally have more fat than men, athletes less fat but greater muscle mass than non-athletes, and older adults more fat than younger ones. Nor does BMI tell you where body fat is located, which matters. Belly fat, aka visceral fat, carries more disease risk than fat around the hips.

Metabolic disorders, including high blood sugar, are one of the well-established risks of obesity. But an estimated 15% of U.S. residents who have obesity, according to BMI, are metabolically healthy. On the other hand, some people with BMIs that are considered “normal” have metabolic disorders.

Finally, most data on BMI have been collected from non-Hispanic white people, so BMI categories based on those data may not be accurate when applied to people from other ethnic groups. At a given BMI, for example, Asian people tend to have higher fat levels than white people.

Insurance companies sometimes use BMI to determine whether certain treatments, such as bariatric surgery or inpatient treatment for anorexia, are medically necessary. In this way, the flaws of BMI can become barriers to health care.

Due to these and other limitations, the AMA suggests using BMI in conjunction with other measurements, such as:

Body adiposity index: It is an estimate of the percentage of body fat based on a formula that divides a person’s hip circumference by their height.

Relative fat mass: It is an estimate of the percentage of body fat based on the ratio of a person’s height to their waist circumference.

Waist circumference: It can be used to estimate visceral fat levels.