Doctors call it the "Obesity Paradox" -- where overweight patients with chronic diseases outlive their thinner counterparts.
Researchers at Northwestern University analyzed data from 5 studies of heart disease, then zeroed in on those who developed type II diabetes.
Those who had normal body mass indexes were about twice as likely to die during the course of the studies than patients who were overweight or obese.
It's unclear why this is; doctors think it may be an issue of the ratio of fat to muscle in body composition. It may be that the thinner patients have less muscle, which is critical to controlling blood sugar.