In a study that looked at data from 6,171 Americans with normal body size, as measured by body mass index, those with a high percentage of body fat were at significantly greater risk of future heart problems than those with low amounts of fat. Their bodies "behave like they are obese, but they are not," Dr. Lopez-Jimenez says.The natural reaction is to get one's body composition tested, despite the fact that "there isn't a consensus among medical experts about what percentage of body fat is 'normal' or what level indicates higher risk."
Some gyms have their own guidelines. At Equinox Fitness Club, trainers consider body composition and waist circumference—another indicator of body fat—more important in assessing fitness of many members than weight. The club's Ms. Coopersmith says that, based on data from the American College of Sports Medicine and the American Council on Exercise, Equinox considers body fat percentages between 25% and 31% for women, and 18% to 26% for men, as "acceptable." Women with body fat of 21% to 24%, and men with 14% to 17%, are "fit." People who reach even lower levels of body fat are considered "athletic," she says.Still, the lesson's clear: Lose fat and gain lean muscle. And don't envy the skinny guy/gal in the next cubicle -- there's a good chance s/he's in worse shape than you.