You don't have to look fat to be fat, a new study suggests. A report from the Mayo Clinic finds that people of a normal weight with a high percentage of body fat are at greater risk for heart problems than those with lower fat. In cases of "normal weight obesity," healthy-looking bodies essentially "behave like they are obese," says the lead researcher. Up to 30 million Americans may fall into this category—but many won't know it.
"This is a culture obsessed with weight, but very little attention is paid to the composition of that weight," one exercise physiologist tells the Wall Street Journal. In other words, body mass index, the calculation generally used to determine whether someone is overweight or obese, may not be enough to spot problems; your percentage of body fat matters, too, though there isn't agreement on what is "normal." But the findings suggest that increasing lean muscle and lowering body fat is the way to lower heart risk—which likely means hitting the gym.