A higher BMI was not linked to greater risk of heart attack or death in a new study of more than 4,000 twins. The researchers came to their findings after analyzing 4,046 monozygotic - or identical - twin pairs, using data from the nationwide Swedish twin registry and the Screening Across Lifespan Twin (SALT) study.

The authors explain that because monozygotic twins are genetically identical, they enable the assessment of obesity-related health risks independent of genetic factors. In all twin pairs, one twin was heavier than the other, with a higher BMI. Overall, the leaner twins had an average BMI of 23.9, while the heavier twins had an average BMI of 25.9. During an average 12.4 years of follow-up, the team assessed the incidence of heart attack, death, and type 2 diabetes onset among each twin pair, and looked at whether this might be associated with BMI.

For more than 3,200 of the twin pairs, the team also gathered data from questionnaires that detailed changes in BMI 30 years prior to study baseline. Overall, the researchers found that in each twin pair, the heavier twin - even those who had a BMI of 30 or more - was at no higher risk of heart attack or death during follow-up than the leaner twin. .

When it came to onset of type 2 diabetes, however, the researchers found that the heavier twins were at greater risk than the leaner twins; a total of 345 cases of type 2 diabetes occurred among the heavier twins during follow-up, compared with 224 cases among the leaner twins.

Furthermore, they found that twins who had experienced an increase in BMI in the 30 years before study baseline were more likely to later develop type 2 diabetes, regardless of their baseline BMI. Based on these latest findings and those of previous studies, Prof. Nordström and colleagues say the importance of obesity and the development of type 2 diabetes is "indisputable."

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