There are many theories, but Lavie says the answer is probably not simple.

It could be that lean heart patients have more aggressive disease because they are genetically predisposed to develop heart disease and have worse outcomes, he says.

Obese people may also be diagnosed and placed on treatment earlier in the course of their disease because symptoms closely linked to obesity -- such as becoming easily winded -- are similar to those seen with heart disease.

But there is also some evidence, mostly from animal studies, suggesting a direct protective benefit for obesity, UCLA professor of cardiology and obesity paradox researcher Gregg C. Fonarow, MD, tells WebMD.

If such a benefit is proven, Fonarow says it could have profound implications for the management of overweight and obese heart patients.

"We will have to ask ourselves if uniformly recommending weight loss in these individuals is advisable," he says. "It may be that we can't take the body weight standards for a healthy population and automatically apply them to people with cardiovascular disease."

But Lavie says studies suggest that weight loss improves outcomes among obese heart patients.

"Overweight heart patients do better than thin patients, but overweight patients who lose weight do best of all," he says.

Studies also show that whether patients lose weight or not, engaging in regular exercise is one of the best things they can do for their hearts.

Fonarow says the obesity paradox should reassure obese patients with heart disease who have tried to lose weight and failed.

"These patients often feel that there is nothing they can do to turn things around, but if they take their medication and stay physically active their prognosis is actually very good," he says.