The truth behind the BMI

BMI might be the standard for weight assessment these days. But that doesn’t mean it’s the best one. Or even that it’s always accurate.

For one thing, it doesn’t tell you anything about your body composition. That means that people with a lot of lean muscle and little fat may be classified as overweight when they’re anything but.

But more often, the opposite happens. People who are carrying around a lot more fat than they should be, but whose weights are relatively low, will still have BMIs that fall well within the healthy “normal” range.

It’s called normal weight obesity–better known as “skinny-fat.”And a recent analysis of data from more than 1,500 older subjects showed just how common–and potentially deadly–this phenomenon actually is.

As it turns out, one in every five men and one in every three women fell into this category–with body fat levels over 25 percent and 35 percent, respectively. And being “skinny fat” came with serious consequences, too.

Women with excess body fat had a 57 percent higher risk of dying from heart-related events over the next 11 years. “Skinny fat” men, meanwhile, faced a higher death risk over the longer term than their leaner peers.

So what’s the takeaway here? The truth is, like all the one-size-fits-all formulas that came before it, BMI measurements can fail miserably in practice. And it’s one reason why your bathroom scale can only tell you so much.

If you want a true picture of your body’s health, take a look at its fat levels first.

“Normal Weight Obesity and Mortality in United States Subjects >=60 Years of Age (from the Third National Health and Nutrition Examination Survey).” Am J Cardiol. 2013 Aug 29. pii: S0002-9149(13)01533-6.