Numerous studies have shown that depression is associated with an increased risk of heart disease, but exactly how has never been clear.

Now, researchers at Rush University Medical Center have shown that depression is linked with the accumulation of visceral fat, the kind of fat packed between internal organs at the waistline, which has long been known to increase the risk of cardiovascular disease and diabetes.

The study is posted online and will be published in the May issue of Psychosomatic Medicine.

"Our results suggest that central adiposity – which is commonly called belly fat – is an important pathway by which depression contributes to the risk for cardiovascular disease and diabetes," said Lynda Powell, PhD, chairperson of the Department of Preventive Medicine at Rush and the study's principal investigator. "In our study, depressive symptoms were clearly related to deposits of visceral fat, which is the type of fat involved in disease."

The study included 409 middle-aged women, about half African-American and half Caucasian, who were participating in the Women in the South Side Health Project (WISH) in Chicago, a longitudinal study of the menopausal transition. Depressive symptoms were assessed using a common screening test, and visceral fat measured with a CT scan. Although waist size is often used as a proxy for the amount of visceral fat, it is an inaccurate measure because it includes subcutaneous fat, or fat deposited just beneath the skin.

The researchers found a strong correlation between depression and visceral fat, particularly among overweight and obese women. The results were the same even when the analysis adjusted for other variables that might explain the accumulation of visceral fat, such as the level of physical activity. The study found no association between depressive symptoms and subcutaneous fat. The findings were the same for both black and white women.

Powell speculated that depression triggers the accumulation of visceral fat by means of certain chemical changes in the body – like the production of cortisol and inflammatory compounds – but said that more research is needed to pinpoint the exact mechanism.