BMI alone is an 'imperfect' measure of fat, leading medical association says

Body mass index (BMI) is an "imperfect" measure of body fat with a "problematic history," so the metric should be used in conjunction with other measures of health risk, such as a patient's genetics, blood pressure, cholesterol and other metabolic factors, according to new guidelines from the American Medical Association (AMA).

In a new report presented at the 2023 AMA Annual Meeting in Chicago this month, AMA council members noted that BMI, which roughly estimates a person's body fat based on their weight and height, doesn't distinguish between body fat and lean body mass. Moreover, BMI doesn't capture where on the body people carry excess fat. This is important because upper body fat increases the risk of type 2 diabetes and coronary artery disease more than does lower body fat. 

Moreover, although cheap and easy to calculate, BMI is "inaccurate in measuring body fat in multiple groups" because it doesn't account for differences in the relative body shape and composition of people of different sexes, ages, races and ethnicities, the AMA said in a statement released June 14. 

This is partly because the BMI scale is "primarily on data collected from previous generations of non-Hispanic white populations," according to the statement. In outlining the history of the BMI's advent and use, the association's new report notes that "BMI cutoffs are based on the imagined ideal Caucasian."

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Crucially, this means that the same BMI thresholds don't indicate the same level of disease risk in all patients — for example, evidence suggests that, at the same BMI, Black and Hispanic women have a slightly higher risk of developing type 2 diabetes than white women and Asian women have about double the risk of white women.

In general, the widespread use of BMI in medical research skews scientists' and doctors' understanding of the risk of disease and death linked to obesity.

Frequently, the report states, in studies that find a correlation between high BMI and disease or death, the researchers fail to account for other key factors that may affect people's risk, like a history of smoking, alcohol use, medication use or a family history of disease. In addition, such studies often don't account for the expected fluctuation of weight with age and lack nuance regarding the amount of time participants spend in a given BMI category, and therefore don't capture how those factors shape later disease risk.

Based on the new report, the AMA has adopted a new policy on the use of BMI: The association now recommends that, due to the limitations of the metric, BMI should be used in conjunction with "other valid measures of risk," including but not limited to measures of visceral fat (the fat that surrounds the internal organs), relative fat mass (a body fat estimate that uses a height-to-waist ratio) and waist circumference. Genetic factors, including family history of diabetes and heart disease, and metabolic factors, such as high blood pressure and fasting blood sugar levels, are additional metrics to consider. 

Furthermore, the AMA emphasized that overreliance on BMI can lead to the underdiagnosis and undertreatment of eating disorders because doctors may not flag affected patients with "normal" or "above normal" BMIs. Insurance companies also use BMI to determine whether people's inpatient eating-disorder treatments will be covered, and this can lead to substandard treatment for patients who don't meet the weight cutoffs, the AMA noted.

"There are numerous concerns with the way BMI has been used to measure body fat and diagnose obesity, yet some physicians find it to be a helpful measure in certain scenarios," Dr. Jack Resneck Jr., immediate past president of the AMA, said in the statement. "It is important for physicians to understand the benefits and limitations of using BMI in clinical settings to determine the best care for their patients."