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.

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