LONDON - While being fat increases your chances of a heart attack, some studies suggest a puzzling paradox: Obese people seem to have a better chance of surviving one. Scientists are stumped over why that seems to be the case and pose several theories.

There may be physiological differences in the hearts of obese and normal-weight people. Or perhaps it depends on where the fat is on their bodies.

However, experts warn, the results should not be used as an excuse for the overweight to indulge.

"We really don't want people to think that they should put on a bit of weight to have a better chance with their bypass surgery," said Dr. Gerald Fletcher, a cardiologist at the Mayo Clinic in Florida and a spokesman for the American Heart Association.

"These results do not mean it's OK to be fat. Being fat is still dangerous to your health for lots of other reasons," Fletcher said.

A 2005 study published in the American Journal of Medicine by scientists at Duke University examined nearly 16,000 people in 37 countries. The authors found that one year after a heart attack, the death rate for normal-weight patients was 4.3 percent. For obese patients, it was just 2.2 percent.

Several other studies have confirmed those findings, including a paper last month in the European Heart Journal. German and Swiss doctors tracked more than 1,600 patients for three years after their heart attacks, and concluded that only 3.6 percent of fat patients had died, against nearly 10 percent of normal-weight patients.

"We don't have a good explanation for the biological phenomenon that's causing this," said Dr. Eric Eisenstein, leader of the Duke study. "We need to understand scientifically what's happening in these folks before we can develop new therapies."

There is a higher prevalence of smoking among thin patients, one possible explanation. But even after statistically adjusting for that, fat patients still had a distinct advantage, researchers found.

Some experts suggest it depends on where the fat is located, noting that fat around the abdomen is the biggest risk. Other doctors think there may be physiological differences in the heart.

"It could be that the hearts of obese people are 'pre-conditioned' because they're under more stress in the first place," said Dr. Andrew Newby, a professor of vascular biology at Bristol Heart Institute and spokesman for the European Society of Cardiology. Newby said that fat people who had heart attacks might be better able to withstand the initial shock to the system.

Dr. Rob Califf of Duke University said the survival rate difference between fat and thin "is not a big enough factor" to make changes in patient care. Other signs such as the magnitude of the heart attack and whether patients have kidney problems are more important in predicting survival, he said.

But experts say it is important to better understand the fat-thin paradox so doctors can provide better treatment.

Some suggested that fat people who have heart attacks can markedly improve their survival odds if they make some major lifestyle changes, an option that normal-weight patients may not have.

"Even moderate weight loss can have a big impact," said Dr. Heinz Buettner of the Heart Centre in Bad Krozingen in Germany. "Obese patients have a better chance to correct their situation compared to thin patients who may just have bad genes."

Because obesity can lead to other dangers - including high blood pressure, diabetes, and cancer - the apparent survival advantage fat people have after a heart attack might be erased by something worse down the line.

"Obese patients may get lucky after one heart attack, but they are still high-risk patients," said Fletcher. "If they stay fat after their surgery, they could end up back in the hospital soon and more bad things could happen."