Type 2 diabetes is almost twice as common in African-Americans as it is in whites. Obesity, rather than racial factors, is to blame, a new study in JAMA reports.

Researchers began with 4,251 black and white men and women ages 18 to 30 who were not diabetic. They then followed up with periodic interviews and health examinations over an average of 25 years. Compared to whites, black men were 67 percent more likely, and black women almost three times as likely, to develop diabetes.

When they controlled for a long list of modifiable risk factors — fasting glucose, body mass index, waist circumference, blood pressure, blood lipids, location of residence, socioeconomic status and more — the difference in diabetes incidence between the races disappeared. The key cause, the researchers determined, is obesity, which is tied to all of these risk factors.

“The benefit of capturing these behaviors over time is that we can study how the accumulation of unhealthy risk factors contributes to the development of diabetes,” said the senior author, Mercedes R. Carnethon, an associate professor of preventive medicine at Northwestern University. “Modifiable risk factors matter. The answer is simple, but the strategy to achieve change is complicated.”