Scientists are stumped and warn overweight people that the results should not be used as an excuse 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.
There are various theories for why obese people seem to survive heart attacks more often than normal weight patients. There may be physiological differences in the hearts of the obese, or perhaps it depends on where the fat is on their bodies. But the paradox has been shown in several studies.
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, an assistant professor in medicine at Duke University who led the 2005 study. "We need to understand scientifically what's happening in these folks before we can develop new therapies."
Some doctors think there may be physiological reasons for the discrepancy in survival rates.
"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.
Others suggest that not all fats are equal.
"Only the fat around the abdomen seems to be dangerous," said Dr. Don Poldermans, a professor of medicine at Erasmus University in the Netherlands.
Differences between the thin and fat heart attack patients, including a higher prevalence of smokers in thin patients, could help explain the survival edge that fat patients seem to have. But even after statistically adjusting for these differences, fat patients still had a distinct advantage.
Doctors do not think the findings suggest major changes to patient care, but say it is important to better understand the paradox so that they can provide better treatment.
"This is not a big enough factor to impact clinical treatment," said Dr. Rob Califf, director for translational medicine at Duke University. Other signs such as the size of the heart attack and whether patients have kidney problems are more important in predicting survival, Califf said.
Doctors say 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, director of interventional cardiology at the Herz-Zentrum in Germany. "Obese patients have a better chance to correct their situation compared to thin patients who may just have bad genes," he said.
Because obesity can lead to other dangerous conditions — including high blood pressure, diabetes, and cancer — the apparent survival advantage fat people have after a heart attack might easily be erased by something even 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."