McMullan and his colleagues reviewed data from the U.S. Nurses' Health Study. The researchers found 370 women who developed type 2 diabetes during the study period, from 2000 to 2012. They also selected 370 women without diabetes for comparison. Melatonin levels were obtained through urine samples.
When researchers compared women with the lowest levels of melatonin to those with the highest, they found that low levels increased the risk of developing type 2 diabetes by 2.17 times.
McMullan said it's not yet clear how melatonin might affect the risk of type 2 diabetes. "The next step is to look at whether we can adjust melatonin secretion in people, and to confirm whether or not melatonin is a modifiable risk factor for type 2 diabetes," he said.
Dr. Joel Zonszein, director of the clinical diabetes center at Montefiore Medical Center in New York City, said the study illustrates the complexity of type 2 diabetes. "But it doesn't prove that taking melatonin would cause a person to be more insulin sensitive," he said.
"While there is a correlation between melatonin and insulin resistance, we cannot tell from this study if low melatonin is the cause," Zonszein said. "We are far away from melatonin as a treatment."
In the United States, about 8 percent of residents have diabetes, but many don't know it. The disease, a leading cause of heart disease and stroke, is typically associated with obesity, sedentary lifestyle and older age.
Learn more about preventing type 2 diabetes from the American Diabetes Association.
SOURCES: Ciaran McMullan, M.D., research fellow, Brigham and Women's Hospital, Boston; Joel Zonszein, M.D., director, clinical diabetes center, Montefiore Medical Center, New York City; April 3, 2013,
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