BMI, or body mass index, has long been a part of health evaluations. A calculation that takes into account a person’s weight and height, BMI is supposed to reflect how much excess fat a person might be carrying and give doctors a sense of a patient's risk for obesity-related health conditions.

For years, however, doctors have found that BMI isn’t the best measure of obesity-related health, since it can’t distinguish between fat and muscle weight.

In a study published Nov. 11 in Nature , researchers led by Francesco Rubino at King’s College London report that a new framework for assessing obesity that combines BMI with other measures can dramatically improve doctors’ ability to determine whether a person’s excess weight is a health concern or not.

The study included more than a decade's worth of data from more than 844,000 people in China, Spain, the U.K., and the U.S.…