A 10-year study by Harvard University scientists found that diabetes puts people at risk for depression and that depression puts people at risk for type 2 diabetes. The two-way connection between the diseases was discovered in 55,000 nurses surveyed over the decade.
The main findings: The 7,400 nurses who suffered from depression were 17 percent more likely to develop diabetes. And the 2,800 surveyed who had diabetes were nearly 30 percent more likely to become depressed.
"This study indicates that these two conditions can influence each other and thus become a vicious cycle," said Dr. Frank Hu of the Harvard School of Public Health and one of the study's co-authors. "Thus, primary prevention of diabetes is important for prevention of depression, and vice versa."
Severity of illness seemed to make a difference, too. If depressed patients were on antidepressants, they were 25 percent more likely to develop diabetes. Diabetic patients on insulin were more than 50 percent more likely to be depressed.
Sounds bad, right? The study certainly raises provocative questions, but it might not be as simple as the stark numbers and percentages suggest.
Just because diabetics are more likely to be depressed and depressed people are more likely to have diabetes doesn't mean either disease directly causes the other. Researchers suggest that more work is needed.
Tony Tang, an adjunct professor in the Northwestern University psychology department, pointed to weight and activity as important contributing factors in the study. Once those were taken into account, the links between depression and diabetes were looser. "This suggests that much of the observed correlation between depression and diabetes comes from confounding variables," he said. "In layman's terms, being fat and having an unhealthy lifestyle makes people more likely to be depressed and more likely to have diabetes."
Harvard's Hu defended the work. He said that if weight and activity could put people at risk for both diseases, "We can still say that the conditions are linked." He said that depression can increase patients' levels of cortisol, a stress hormone that increases blood sugar and can lead to weight gain. Diabetes could be a result.
Managing diabetes, on the other hand, can be stressful. And that long-term emotional strain can take a toll in the form of depression, Hu said.
The study was published in the November 22 issue of the Archives of Internal Medicine.
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