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From The New York Times
Scientists call it the obesity paradox, the notion that being overweight or moderately obese lowers the risk of an early death. They have documented the phenomenon in large population studies and in groups of patients with chronic diseases like hypertension and Type 2 diabetes.
But now a new report, published on Wednesday in The New England Journal of Medicine, is calling the obesity paradox into question, at least for patients with Type 2 diabetes.
The study, of nearly 12,000 people with the disease, found that there was no survival advantage for those who had a body mass index that put them in the overweight or obese categories. Instead, the researchers found that the diabetics with the lowest mortality rate were those who were considered normal weight.
The study is among the largest to examine the obesity paradox among people with Type 2 diabetes, an illness that afflicts more than 25 million Americans. The authors argue that previous studies showing a protective effect of a high B.M.I. among diabetics were flawed because they were too small or failed to account for factors like smoking or undiagnosed illnesses that can contribute to low body weight but a shorter life span as well.
The new study found that when smoking and other factors that can contribute to weight loss were accounted for, people in the highest B.M.I. groups had higher mortality rates.
“I think the case is not necessarily closed,” said Deirdre K. Tobias, the lead author of the paper and a research fellow at the Harvard School of Public Health. “But at this point, there is no reason to believe that being overweight or obese would be protective for people with diabetes.”
Other experts disagree, saying that the new study did not adequately take into account fitness levels, a factor that may be a much better predictor of outcomes than being overweight.
Fitness “is the rock star of risk factors,” said Dr. Timothy Church, the author of a 2004 study of men with Type 2 diabetes that found that the better the men performed on treadmill tests, the lower their mortality risk was, regardless of their B.M.I.
“Diabetes is a disease of the muscles,” said Dr. Church, a professor in the preventive medicine laboratory at Pennington Biomedical Research Center in Baton Rouge, La. “The largest consumer of blood sugar in the human body is skeletal muscle. If your muscle is healthy and happy, it consumes more blood sugar. And the best way to do that is to be more physically active.”
In the latest study, the subjects’ physical activity levels declined as B.M.I. rose. But Dr. Tobias said that factoring in physical activity levels made no difference in the results, so the variable was not included in the final published analysis.
Mercedes Carnethon, a diabetes researcher at the Feinberg School of Medicine at Northwestern University, said she was skeptical of an obesity paradox. But in study after study on diabetic patients, she has found evidence of it, which she outlined in a recent report. Dr. Carnethon said the problem with large population studies is that while they show correlations, they do not account for the variability of individual patients.
Lean people with Type 2 diabetes – a group whose numbers are relatively small but rising – may be predisposed to a genetic form of the disease that is particularly severe. Some researchers argue that most of these thin patients actually have a medical condition called latent autoimmune diabetes in adultsthat has been only recognized recently.
It could also be the case, Dr. Carnethon said, that heavier diabetics survive longer because doctors treat them more aggressively than those who are leaner.
Nonetheless, Dr. Carnethon said that patients with diabetes should not be led to believe that they are better off carrying extra weight. But at the same time, they should not be told that the goal of physical activity is simply weight loss.
“The recommendation is to do weight bearing exercises like walking and resistance training,” she said. “Because in addition to increasing aerobic capacity, it increases muscle mass, and that improves glucose control.”