Importance: Observational studies have shown that obesity is a major risk factor for cardiovascular disease and death. The extent of genetic confounding in these associations is unclear.
Objective: To compare the risk of myocardial infarction (MI), type 2 diabetes, and death in monozygotic (MZ) twin pairs discordant for body mass index (BMI).
Design, Setting, and Participants: A cohort of 4046 MZ twin pairs with discordant BMIs (difference >0.01) was identified using the nationwide Swedish twin registry. The study was conducted from March 17, 1998, to January 16, 2003, with follow-up regarding incident outcomes until December 31, 2013.
Main Outcomes and Measures: The combined primary end point of death or MI and the secondary end point of incident diabetes were evaluated in heavier compared with leaner twins in a co-twin control analysis using multivariable conditional logistic regression.
Results: Mean (SD) baseline age for both cohorts was 57.6 (9.5) years (range, 41.9-91.8 years). During a mean follow-up period of 12.4 (2.5) years, 203 MIs (5.0%) and 550 deaths (13.6%) occurred among heavier twins (mean [SD] BMI, 25.9 [3.6] [calculated as weight in kilograms divided by height in meters squared]) compared with 209 MIs (5.2%) and 633 deaths (15.6%) among leaner twins (mean [SD] BMI, 23.9 [3.1]; combined multivariable adjusted odds ratio [OR], 0.75; 95% CI, 0.63-0.91). Even in twin pairs with BMI discordance of 7.0 or more (mean [SE], 9.3 [0.7]), where the heavier twin had a BMI of 30.0 or more (n = 65 pairs), the risk of MI or death was not greater in heavier twins (OR, 0.42; 95% CI, 0.15-1.18). In contrast, in the total cohort of twins, the risk of incident diabetes was greater in heavier twins (OR, 2.14; 95% CI, 1.61-2.84). Finally, increases in BMI since 30 years before baseline were not associated with the later risk of MI or death (OR, 0.97; 95% CI, 0.89-1.05) but were associated with the risk of incident diabetes (OR, 1.13; 95% CI, 1.01-1.26).
Conclusions and Relevance: In MZ twin pairs, higher BMI was not associated with an increased risk of MI or death but was associated with the onset of diabetes. These results may suggest that lifestyle interventions to reduce obesity are more effective in decreasing the risk of diabetes than the risk of cardiovascular disease or death.