It’s no secret that being overweight or obese increases one’s risk for heart disease. The research keeps coming in to show that obesity above all other variables is the key predictor of heart risk. But for people who have already had heart trouble in the past, a new study shows that fitness level may actually predict one’s risk of dying better than his or her weight.

The research team followed over 850 men and women who were in a cardiac rehabilitation program — in other words, they had already suffered from heart disease and were at risk of suffering from it again. To determine fitness levels, the participants were asked to walk on treadmills, measuring how long they could walk and how well their bodies took in oxygen, both of which are key markers for fitness.

The picture was grimmer for thin and unfit people. Overweight, unfit people had seven times the risk of dying; and thin, unfit participants had 10 times the risk.

The participants’ body fat or "adiposity" was determined through two measures: body mass index (BMI) and waist-to-hip ratio (WHR). Typically, people who carry more weight around their waists (abdominal fat) are at higher risk for heart disease.

Not surprisingly, thinner and fitter people were least likely to die during the average 10 year follow-up. This group of people served as controls. People who were fit and obese or fit and overweight (when measured by BMI) had a greater chance of dying than controls (two to three times). But the differences were not significant, meaning that theoretically they could have been due to chance.

But the picture was grimmer for thin and unfit people. Overweight, unfit people had seven times the risk of dying; and thin, unfit participants had 10 times the risk. Therefore, fitness level may actually be a better gauge of mortality risk than obesity itself.

That was the picture when BMI was used to measure weight. But when WHR was used, heavier still had a greater risk of dying, even if they were fit.

The authors write that after both measures of weight (BMI and WHR) were controlled for, fitness was still found to be a strong predictor of death risk. This suggests that regardless of weight, fitness alone is a good predictor of mortality.

Why would overweight people be less likely to die in the years following heart problems than normal weight people? There are several possibilities, including the fact that thinner people may simply have fewer reserves than overweight people — and they are more likely to be smokers. The fact that overweight and obese people tend to develop cardiovascular disease earlier in life than normal weight people might also give them better odds of survival.

While research has shown several mechanisms that might explain this "obesity paradox", more studies will be needed to get to the heart of the issue. The study should not, of course, be thought of as license to throw in the towel on weight loss if you are relatively fit. Because of the many health issues that are associated with being overweight and obese, shedding the extra pounds is generally a good idea.

This study was conducted at the Mayo Clinic in Minnesota, and published in the March 8, 2011 online issue of American Heart Journal.