(TIME.com) -- Being overweight or obese is a risk factor for developing Type 2 diabetes, but it turns out that these heavier patients may have an advantage: people who are overweight when they are diagnosed with diabetes live longer than their thinner peers.
The so-called obesity paradox, in which being overweight appears to be protective against early death, has been seen before in heart failure and chronic kidney disease.
But, says study author Mercedes Carnethon, associate professor of preventive medicine at Northwestern University, that doesn't necessarily mean that gaining excess weight is a healthy strategy; rather, it may be that people who are thin when they develop diabetes are already be vulnerable to worse health.
"We hypothesized that their diabetes may be different," she says. "They may have developed diabetes for reasons unrelated to obesity.
Overall, about 85% of people with diabetes are heavy. Gaining too much weight is a major contributor to Type 2 diabetes, since excess fat cells can affect the way the body breaks down glucose and produces insulin, but some normal weight individuals can develop the disease as well.
The elderly and people of Asian descent are more likely to be at normal weight when diagnosed, for example.
For the new study, published in the Journal of the American Medical Association, Carnethon and her team reviewed data on five previous studies that were tracking people for heart disease risk factors.
The studies, which were conducted between 1990 and 2011, included 2,625 people who were recently diagnosed with diabetes, about 12% of whom were at normal weight.
The lean patients looked metabolically similar to those who were obese, with the exception of their weight, the researchers found. But they were twice as likely to die at any point than their heavier counterparts. Even after adjusting for other risk factors known to contribute to diabetes, such as high blood pressure, high cholesterol levels and smoking, the higher mortality rates remained.
Further, to account for the possibility that the normal weight patients could have been losing weight due to other underlying illnesses, the researchers excluded people who died within two years of their diabetes diagnosis. The doubling of early death risk persisted.
Overall, the death rate in overweight and obese people with diabetes was 1.5% per year, compared with 2.8% in thin patients.
So, what sets the disease apart in normal weight people? More studies are needed to figure that out, but the authors have a few guesses.
Genetic predisposition to improper insulin production may be part of the problem, says Carnethon.
"We need to explore the genetic variations related to insulin secretion," says Carnethon. "It's possible that genetic factors could be promoting insulin resistance, and influencing something else related to mortality. We really don't know."
It's also possible that body fat may still play a role. The studies measured the participants' body mass index (BMI), a ratio of their height and weight, but it wasn't able to take into account their body fat composition, or how much of their overall body weight was made up of fat versus muscle.
Many seemingly thin people carry more fat than muscle, making them trim on the outside, but fat on the inside.
Even with a healthy BMI, for example, such people may harbor a lot of visceral fat, deep in their abdomen, a type of fat that is particularly dangerous to health, since it secretes hormones and substances that can hamper insulin's ability to break down sugar. Because many of the thin diabetes patients included in the new study were elderly, they likely had less muscle mass and more fat.
The findings raise the alarm that diabetes isn't just a disease of the overweight or obese, says Carnethon, and that physicians should be looking for signs even among their leaner patients, particularly those who are elderly.
"Doctors' concern about normal weight people with diabetes should potentially be even higher than that for their counterparts who develop diabetes at a heavier weight," she says.
This story was originally published on TIME.com