Researchers at Harvard have found that diabetes and depression are often interlinked health conditions. Those who were depressed tended to have higher risk of developing the deadly disease.

"This study indicates that these two conditions can influence each other and thus become a vicious cycle," said study co-author Dr. Frank Hu, a professor of nutrition and epidemiology at the Harvard School of Public Health in Boston. "Thus, primary prevention of diabetes is important for prevention of depression, and vice versa."

In the U.S. alone, about 10 per cent of the population has diabetes, and about 6.7 over 18 years of age tend to experience clinical depression, every year.

Nearly 95 per cent of diabetes diagnoses are Type 2, and very often it is the obese that later develop as diabetics.

55,000 nurses were studied for ten years. Of these 7,400 became depressed with a 17 per cent greater risk of developing diabetes. Those taking antidepressants had an increased risk of 25%. About 2,800 of them who actually diagnosed diabetic were 29 per cent more likely to get depressed.

Tony Z. Tang, adjunct professor in the department of psychology at Northwestern University, said "None of these treatments are cures, unlike antibiotics for infections. So, depressed patients on antidepressants and diabetic patients on insulin still frequently suffer from their main symptoms. These patients fare worse in the long run because they were much worse than the other patients to start with."

However, he noted that when obesity and inactivity was factored in, such correlations didn’t tend to exist.

"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 [also] more likely to have diabetes." But if research establishes a strong connection between the two illnesses it could advance treatment, Tang added.

With the two of them sharing similar risk factors, researchers often link the two.

Depression could also lead to higher levels of blood sugar and increased cortisol due to depression can lead to other health hazards.

"On the other hand, management of diabetes can cause chronic stress and strain, which in the long run, may increase risk of depression," said Hu. The two "are linked not only behaviorally, but biologically."