The statistical gurus at FiveThirtyEight have looked at the ubiquitous body mass index (BMI) and come to a conclusion that many of us suspected: it stinks as a measure of health. In fact I’d argue that it might be worse than useless, in fact might even be harmful, because it misleads people into thinking they are not at risk of negative health effects because they have a BMI in the “normal” range.

Taken alone as an indicator of health, the BMI is misleading. A study by researchers at UCLA published this month in the International Journal of Obesity looked at 40,420 adults in the most recent U.S. National Health and Nutrition Examination Survey and assessed their health as measured by six accepted metrics, including blood pressure, cholesterol and C-reactive protein (a gauge of inflammation). It found that 47 percent of people classified as overweight by BMI and 29 percent of those who qualified as obese were healthy as measured by at least five of those other metrics. Meanwhile, 31 percent of normal-weight people were unhealthy by two or more of the same measures.² Using BMI alone as a measure of health would misclassify almost 75 million adults in the U.S., the authors concluded…

The researchers analyzed the health data for 15,184 adults who were part of the National Health and Nutrition Examination Survey. Their results,published in the Annals of Internal Medicine, were pretty surprising: They showed that midsection obesity was a killer, even among people with normal BMIs. For example, a man with a BMI of 22 (putting him firmly in the normal range) but too much belly fat according to his waist-to-hip ratio had an 87 percent higher mortality risk than a guy with the same BMI and a healthy waist-to-hip ratio.

What’s more, a man with a normal BMI and disproportionately big belly had more than twice the mortality risk of a man who was overweight or obese by BMI but not by waistline. Among women, those who were normal weight by BMI but had a high waist-to-hip ratio had a 48 percent higher mortality risk than those with a similar BMI but a healthy waist-to-hip ratio, and a 32 percent higher risk compared with those who were obese according to BMI only.

This is the kind of thing that leads me to think BMI has more in common with phrenology than mainstream health care practices.