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A study of U.S. women found that those with the lowest levels of melatonin had more than twice the risk of type 2 diabetes compared to women with the highest levels of the hormone. This association held true even after the researchers controlled for other risk factors for type 2 diabetes, such as body weight and dietary habits.
But whether too little melatonin actually causes type 2 diabetes isn't clear. "We found an association between melatonin and type 2 diabetes; what we haven't got from this study is causality," said study lead author Dr. Ciaran McMullan, a research fellow at Brigham and Women's Hospital in Boston. "That's the next step of research."
The findings shouldn't change clinical management. "There's no evidence that taking melatonin will improve someone's chances of avoiding diabetes," McMullan said.
Results of the study, which was funded by the National Institutes of Health, appear in the April 3 issue of the Journal of the American Medical Association.
Melatonin is a hormone most commonly linked to sleep and the body's biological clock. The production of melatonin peaks about three to five hours after you fall asleep in the dark, and almost no melatonin is produced during the day, according to background information in the study. Sleep disruptions can affect melatonin production, as can exposure to more or less daylight, McMullan said.
Melatonin receptors are found throughout the body, including in the islet cells of the pancreas, which produce insulin. This connection suggests melatonin may also play a role in glucose metabolism, according to the study.
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.
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