From The New York Times:
A few years ago, Mercedes Carnethon, a diabetes researcher at the Feinberg School of Medicine at Northwestern University, found herself pondering a conundrum. Obesity is the primary risk factor for Type 2 diabetes, yet sizable numbers of normal-weight people also develop the disease. Why? In research conducted to answer that question, Dr. Carnethon discovered something even more puzzling: Diabetes patients of normal weight are twice as likely to die as those who are overweight or obese. That finding makes diabetes the latest example of a medical phenomenon that mystifies scientists. They call it the obesity paradox. In study after study, overweight and moderately obese patients with certain chronic diseases often live longer and fare better than normal-weight patients with the same ailments. The accumulation of evidence is inspiring some experts to re-examine long-held assumptions about the association between body fat and disease. Dr. Carl Lavie, medical director of cardiac rehabilitation and prevention at the John Ochsner Heart and Vascular Institute in New Orleans, was one of the first researchers to document the obesity paradox, among patients with heart failure in 2002. He spent more than a year trying to get a journal to publish his findings. “People thought there was something wrong with the data,” he recalled. “They said, ‘If obesity is bad for heart disease, how could this possibly be true?’ ”
But there were hints everywhere. One study found that heavier dialysis patients had a lower chance of dying than those whose were of normal weight or underweight. Overweight patients with coronary disease fared better than those who were thinner in another study; mild to severe obesity posed no additional mortality risks. In 2007, a study of 11,000 Canadians over more than a decade found that those who were overweight had the lowest chance of dying from any cause. To date, scientists have documented these findings in patients with heart failure, heart disease, stroke, kidney disease, high blood pressure — and now diabetes. Experts are searching for explanations. One idea is that once a chronic disease develops, the body becomes catabolic, meaning it needs higher energy and caloric reserves than usual. If patients do not have those reserves, they may become malnourished even though their weight is normal, said Dr. Gregg Fonarow, one of the directors of the preventive cardiology program at the University of California, Los Angeles.