How much can one simple number tell you about your health? A growing body of research over the past few years has highlighted the shortcomings of the body mass index (BMI), a basic measure of rotundity, as a predictor of well-being. The latest—and in some ways most comprehensive—of these reports appeared in August in the journal Science.
The BMI formula, developed in the 1800s by a Belgian statistician and sociologist, divides a person's weight, in kilograms, by the square of his or her height, in meters. As the new study points out, a normal BMI can mask metabolic abnormalities; even people with a normal weight-to-height ratio can harbor disorders in the way the body handles nutrients. Increasingly, researchers are documenting the many ways a metabolic condition called insulin resistance, for example, elevates the risk for heart disease, Alzheimer's disease and other ailments. According to a 2008 analysis, nearly one in four people with a normal BMI were metabolically unhealthy.
Conversely, an elevated BMI does not necessarily reflect poor metabolic health. About half of overweight individuals are metabolically normal. In recent years Katherine Flegal of the U.S. Centers for Disease Control and Prevention and her colleagues have found that people in the overweight (but not obese) BMI category tend to live longer, as a group, than folks in the normal BMI range. Flegal cautioned against overinterpreting the results, however. After all, there is no way for a few BMI groupings to account for the diversity of bodies.
Take, for instance, the location of excess body fat. Fatty deposits around the abdomen are much more hazardous than fat under the skin of the arms or legs. Some researchers think it makes sense to incorporate a third measurement to better classify body types. Waist circumference, along with height and weight, yields the so-called ABSI (a body shape index), for which numerous calculators are available online.
But there is no need to ditch BMI, which is still a decent approximation of health risk for most people. The point is that good health depends on a lot of things—physical fitness, diet, smoking, and even our surroundings and the company we keep—many of which cannot be quantified.