In a study of genetically identical twins, researchers have found that higher body mass index (BMI) may not put the heavier sibling at an increased risk of heart attack and death risk.

Higher BMI, however, was associated with increased risk for the onset of diabetes for that twin, the findings showed.

“The results revealed a significant association between obesity and diabetes after accounting for genetic factors. The association between obesity and diabetes was significantly stronger than the association with CVD [cardiovascular disease] and death,” a researcher said.

Peter Nordstrom from Umea University in Sweden, and co authors used a nationwide Swedish twin registry to identify 4,046 monozygotic twin pairs with differing body mass indexes.

Monozygotic twins are genetically identical so they provide a unique tool for evaluating risks associated with obesity independent of genetic factors.

The study — conducted from March 1998 to January 2003 with follow-up on outcomes through 2103 — compared the risk of heart attack, death and Type-2 diabetes in the twin pairs.

During an average follow-up of 12.4 years, there were 203 heart attacks (five per cent) and 550 deaths (13.6 per cent) among the heavier twins (average BMI 25.9) compared with 209 heart attacks (5.2 per cent) and 633 deaths (15.6 per cent) among leaner twins (average BMI 23.9), according to the results.

Even in twin pairs where the heavier twin had a BMI of 30 or more, the risk of heart attack or death was not greater in the heavier twin.

However, the risk of the onset of diabetes was greater in the heavier twins, the study reported.

The findings were published online in the journal JAMA Internal Medicine.