Hefty and healthy

Americans are heavier than they used to be and worried sick about it. In fact, people's negative perceptions about their bodies may affect their health more than their actual weight, according to an analysis in the March 2008 issue of the American Journal of Public Health. Columbia University researchers who surveyed more than 170,000 people found that regardless of their size, those who were happy with their weight reported better mental and physical health.

At the core of the dissatisfaction is the assumption that svelte is not only more attractive but also healthier. But whether supermodels are destined to outlive the Average Joe is a matter of debate among experts, who sometimes interpret public-health data in strikingly different ways.

Unfortunately, sensationalized coverage of those discrepancies has overshadowed many of the most relevant details-that extra pounds may be less of an issue for older people, for example, or that fitness is more important than fatness. In fact, the most common measure of excess weight—the body mass index, or BMI—is probably an inaccurate measure of overall health precisely because it doesn't reflect fitness very well.

We talked with several experts and found that while they see some different patterns in the numbers they largely agree on the core advice. And despite the hullabaloo over weight, the real emphasis should rest on healthy habits, not the bathroom scale.

The finding that normal-weight people don't outlive their overweight peers had comedians calling for second helpings of mashed potatoes when it was released a few years ago. Researchers from the Centers for Disease Control and Prevention based their analysis on the well-respected National Health and Nutrition Examination Survey. "The findings really should not have been such a surprise," says Katherine Flegal, Ph.D., the study's lead author. Indeed, more than a dozen other large studies have found either no difference or a slightly lower mortality rate among the moderately overweight.

Other experts, however, have reached different conclusions. They point, for example, to two large studies published in the New England Journal of Medicine in 2006. One, of over 1.2 million Korean men and women, found that mortality risk began to increase even in those who were only moderately overweight. The other, an analysis of more than 500,000 members of AARP, had similar findings.

But with time comes perspective. The researchers still point to differences in study design, but say that their findings match on a few key issues:

Researchers have long relied on the body mass index, since it factors in not just weight but also height. But as obesity science evolves, it's becoming increasingly clear that there is simply no one-size-fits-all definition of a healthy BMI.

To begin with, age, ethnicity, and gender affect how the numbers add up. For example, a moderately high BMI appears to increase mortality risk more strongly in Asian men and women—as seen in the large Korean study—than it does in African-American women. And "BMI is not a precise health measure in older people," says Michael Leitzmann, M.D., an investigator with the National Cancer Institute. Being underweight is more likely to be a concern in that group because aging affects both appetite and the ability to absorb nutrients from food.

BMI also doesn't tell you much about fitness. Some couch potatoes have a fine BMI, while many moderately overweight people are very active.

In fact, the inadequacy of our language about weight became clear recently when researchers coined the term "normal-weight obesity" to describe supposedly healthy-weight people with a high percentage of body fat. That group is much more likely than muscular people of any weight to be at high risk for type 2 diabetes and heart disease, according to a recent Mayo Clinic study.

Other measures, such as waist circumference or the ratio of waist to hip size, may more accurately reflect body fat and health risk than weight or BMI. But none of those tools predicts longevity or quality of life as effectively as does a careful assessment of lifestyle factors such as diet and exercise habits.

People with strong cardiovascular function, regardless of their size, are healthier and live longer than their sedentary counterparts. For example, in a study of 2,603 people age 60 and older published last year in the Journal of the American Medical Association, physically fit individuals had roughly the same mortality rate whether they were underweight, normal weight, or overweight. Being fit can even trump being thin: People who were fat but fit had a survival edge over those who were skinny but inactive.

Exercise can improve health even if the scale doesn't budge. Researchers from Harvard University and Brigham and Women's Hospital tracked physical activity and cardiovascular health in more than 27,000 women ages 45 and older. After a decade, the physically active people slashed their risk of heart attack and stroke by as much as 40 percent. The researchers determined that only about 10 percent of that decline could be attributed to a reduced BMI.

The rest stemmed from improvements in markers for inflammation, blood pressure, and LDL (bad) cholesterol. "The message to people who exercise but don't lose weight is that they are still getting most of the benefit," says Samia Mora, M.D., lead author of the Harvard study.

The relative importance of exercise and weight, however, doesn't give you license to specialize in junk food. Study after study has found lower disease rates in people who eat healthfully, with an emphasis on a variety of fruits and vegetables, whole grains, lean protein, and small amounts of unsaturated fat. That diet can lead to improved health even when it doesn't lead to substantial weight loss.

British researchers, for example, recently told a group of overweight women not to diet. Instead, they emphasized good eating habits, enrolled them in exercise classes, and provided social support. Though the women lost only a small amount of weight, they were significantly fitter and reported feeling less stress and better about their bodies.

"We've been shouting from the rooftops about how bad it is to be even a little overweight, but that hasn't solved the problem," says Steven N. Blair, P.E.D., professor of exercise science at the University of South Carolina. "We've become obsessed with BMI when we should be focusing on healthful behaviors."

This article first appeared in the July 2008 issue of Consumer Reports on Health.