Being overweight is less likely to cause a heart attack or kill you than it is to increase your risk of diabetes, according to a new study of identical twins.

Researchers used a nationwide Swedish twin registry to find twin pairs with identical DNA but different body mass indexes, or BMIs. While conventional medical wisdom has long suggested that heavier people are more likely to have premature heart attacks than lean people, the new study suggests otherwise.

The research, published online Monday by JAMA Internal Medicine, followed more than 4,000 genetically identical twin pairs with differing BMIs from March 1998 to January 2003, and followed up with them through 2013. The study compared the risk of heart attack, death and Type 2 diabetes.

“The fatter twin actually had a lower risk of myocardial infarction (heart attack) or death, although the risk of diabetes was higher,” said the lead researcher of the study, Peter Nordström, Ph.D., of the Umeå University in Sweden.

“After adjusting for genetic factors, obesity does not seem to be associated with cardiovascular disease or death, at least not to an increased risk,” Nordström said.

The study reported 203 heart attacks (5 percent) and 550 deaths (13.6 percent) among the heavier twins, who had an average BMI of 25.9. But the leaner twins, with an average BMI of 23.9, had slightly higher numbers, with 209 heart attacks (5.2 percent) and 633 deaths (15.6 percent).

Even twins who had a BMI of 30 or higher – the threshold to be considered obese – still didn’t have a greater risk of heart attack or death than the leaner twin.

Yet when it came to Type 2 diabetes, the heavier twins did have a greater risk, leading investigators to conclude “lifestyle interventions to reduce obesity may be more effective in reducing the risk of diabetes than the risk of cardiovascular disease or death.”

“This study has important clinical implications,” wrote David J. Davidson, M.D., of NorthShore University HealthSystem, and the University of Chicago’s Michael H. Davidson, M.D., in an editorial accompanying the study. “The findings confirm the causal link between obesity and diabetes, which is a growing epidemic throughout the world. Therefore, weight reduction should remain the cornerstone for the prevention of diabetes.”

Even so, they wrote, the study has some important limitations. Waist size — a better indicator of whether someone is “metabolically obese” despite having a normal BMI — was not measured, and the 12-year follow-up period may not have been long enough for the higher diabetes rate among the heavier twins to result in more cases of heart disease.

Although the Swedish study suggests losing weight may not reduce the chances of a heart attack, it reinforces previous research that has found smoking is bad for the heart — no matter your weight. The twin who smoked had higher odds of both heart attacks and death.

While Nordström would like to see more studies conducted to clarify the link between weight, heart disease and diabetes, he said the takeaway from the current findings is that quitting smoking is the best way for people to improve their heart health.

“Stopping smoking is for sure the best and most important advice, based on this and all previous research,” Nordström said.