Obese heart attack and angina patients are more likely to survive after treatment, a study suggests.
Obesity is linked to a risk of heart disease
Researchers found obese patients were less than half as likely to die in the three years after treatment as patients with a normal body mass index.
The German and Swiss study - involving 1,676 patients and in the European Heart Journal - could not explain why.
And experts warned obese people were more likely to develop heart problems in the first place.
The study followed 1,676 patients who were hospitalised with unstable angina or a specific type of heart attack called a non-ST-segment elevation myocardial infarction.
Standard treatment was a coronary angiography to diagnose the extent of the problem, followed by a process to unblock the arteries called coronary revascularisation or a coronary artery bypass graft.
Lead researcher Dr Heinz Buettner said: "Although there is no doubt that people who are overweight, obese and very obese have a higher risk of developing diabetes, hypertension and coronary artery disease, the evidence from our study shows once a coronary event has occurred and been optimally treated, obese patients switch to a more favourable prognosis compared to normal weight patients."
The obese patients tended to be younger than the normal weight patients, and were more likely to be given prescriptions for heart drugs such as statins and beta-blockers on discharge from hospital.
But Dr Buettner said his analysis suggested that neither of these two factors could fully explain the lower mortality rate.
He said it was possible that differences in body chemistry caused by obesity might play a role.
For instance, levels of blood platelets, which can affect clotting, are lower in obese patients, while levels of fat in the heart tissue, which might have a protective effect, are higher.
Another theory is that the higher levels of endogenous cannabinoids in obese people might be key. There is a growing body of evidence to suggest that these chemicals have a protective effect during a heart attack.
Dr Buettner said it was important that obese people made efforts to lose weight.
He said: "Not all patients can be treated with early revascularisation because an acute coronary syndrome always has the risk of sudden cardiac death.
"It is well known that even a modest intentional weight loss can improve or prevent obesity-related cardiovascular risk factors."
June Davison, of the British Heart Foundation, said the study raised more questions than it answered.
"Further research is needed for us to understand the reasons for this link," she said.
"What we are sure of, is that if you are overweight you have a higher risk of developing high blood pressure, coronary heart disease and diabetes in the first place."