Leah Dawson is unhealthy—at least that's what public health guidelines would have you believe. The 41-year-old mother of three, who manages her husband's athletic career (he's Olympic skier Toby Dawson), works out four days a week and carefully monitors her diet. Her cholesterol and blood pressure are enviable. But never mind all that: Leah Dawson is considered "unhealthy" because at 5'8", her weight has hovered near 170 pounds. That puts her at 26 BMI (body mass index—the height-to-weight formula doctors use to diagnose weight problems), which is in the overweight category. "My doctor would tell me that I needed to lose weight," she says. "My lab work would come out fine; I've even passed a stress test. But I'm never 'okay.'"
Thirty-nine-year-old Teresa Holler, on the other hand, is considered "healthy." As a physician assistant, Holler knows she needs to monitor her weight. But after a tough pregnancy six years ago, she slowed down her normally rigorous exercise program. Initially, the trim 5'3", 128-pound, size-5 Holler wasn't too concerned: Her BMI remained at 23, putting her in the healthy range of 18.5 to 24.9. Still, she began to worry that something was amiss. "I felt like I was getting this ring of fat around my midsection," she says. "And I was just less energetic." This led her to request a body-fat test from her doctor. She was shocked to find the amount of fat she was carrying on her diminutive frame—30 percent—qualified her as borderline obese, dramatically elevating her risk for diabetes and heart disease. "Here I was, spending my days telling other people how to get healthy, and I'd missed this," she remembers.
Health experts have been too strict with heavy people like Dawson and too lax with normal-weight people like Holler: Researchers at the Mayo Clinic have found that about half of American adults in the normal-body-weight category still have too much body fat—and given that these people also tend to have many of the risk factors associated with heart disease, that assessment is probably correct. "I'm a cardiologist, and I was seeing too many patients whose BMI checked out as normal but who didn't look right," says Francisco Lopez-Jimenez, MD, the Mayo doctor who headed the study. "Some had a big belly, some had low muscle mass. And they had high blood sugar and cholesterol. It became clear to me that the measurements we were using weren't working for these people."
How risky is it for a normal-weight person to carry too much fat? The answer lies in the way the body stores the stuff. "All fat isn't created equal," says Timothy Church, MD, PhD, a professor at Louisiana State University's Pennington Biomedical Research Center. "The deeply deposited kind—visceral fat—produces three times more bad chemicals than other types, and it drains directly into your liver."
Visceral fat bulks up the trunk of the body, and it has been directly tied to increased risk of diabetes and heart disease. (People who carry fat on the hips and thighs seem to have a lower risk.) Importantly, visceral fat tends to be the type normal-weight people have. It sneaks up on a person because it's not especially visible and it can build up without altering weight—replacing muscle that is naturally lost with aging. And that stealthy buildup of fat may help explain the results of a 2005 study in the Journal of the American Medical Association (JAMA): Researchers analyzed mortality data of people in different weight categories, from underweight to obese, and found that there were fewer deaths among overweight people than those in the normal-weight group.