Researchers at the Harvard School of Public Health have discovered a correlation between depression and type 2 diabetes, two serious conditions in the United States.

After following nearly 55,000 nurses over ten years and documenting their conditions of depression and diabetes through questionnaires, the researchers observed a possible biological and behavioral linkage between the conditions.

The study found a 17 percent greater risk of developing type 2 diabetes among the 7,400 nurses that had depression symptoms, and a 29 percent greater risk of developing depression among the 2,800 nurses that developed diabetes.

“It is the first study to indicate the bidirectional relationship, also the largest one and longest duration of time,” said Frank B. Hu, senior author of the study and professor of medicine at Harvard Medical School.

Hu said he believes that one of the important factors contributing to the biological and behavioral linkages between depression and diabetes is stress.

In terms of the biological connection, people with depression symptoms often have chronic stress, and certain stress hormones such as cortisol—a hormone released by the brain upon increased stress levels—may lead to abnormal glucose metabolism, a risk factor for type 2 diabetes, according to Hu.

Behaviorally, long-term stress may result in diminished quality of life through overeating and an unhealthy diet, thus increasing risks for both diabetes and depression, Hu said.

The chronic nature of diabetes may also increase stress level, as it often requires daily monitoring and management of high blood sugar.

Hu said he hopes that the results of this study will help improve current clinical treatments of diabetes and depression.

“For people who have clinical depression, I think clinicians should pay attention to blood sugar and potential risks for diabetes,” Hu said. “On the other hand, for people who have diabetes, I think we should pay attention to their mental health and the psycho-social aspect of their diabetes management.”

“[Depression and diabetes] may become a vicious cycle if you don’t pay attention to the bidirectional relationship,” he added.

The study was published in the journal Archives of Internal Medicine earlier this week.