The study, published in today's edition of the Archives of Internal Medicine, theorizes that the culprit is a high level of the stress hormone cortisol.
Insulin enables glucose (sugar) to enter the body's cells to be used as fuel. But when people are depressed, or severely stressed, the cells in the pancreas are suppressed and secrete less insulin to enable the body to clear glucose from the bloodstream.
"When you're depressed or under stress your body is trying to keep glucose in the bloodstream because it needs immediate energy," said Mercedes Carnethon, an assistant professor of preventive medicine at the Northwestern University in Chicago, and lead author of the study.
"So it's blocking insulin action, and may even be producing more glucose because your body thinks it needs the sugar."
This, in turn, decreases insulin sensitivity and increases fat deposits around the waist, both risk factors for diabetes.
The new study involved 4,681 women and men over 65 living around the United States. The participants, none of whom had diabetes at the outset, were tracked over 10 years.
Researchers found that seniors who were most persistently depressed, or who suffered bouts of severe depression, were significantly more likely to develop diabetes.
The research team filtered out known risk factors for diabetes such as obesity, physical inactivity and smoking. They also examined the role of inflammation. (People who suffer from depression typically have a high level of C-reactive protein. And the level of CRP typically rises when there is body-wide inflammation.) But depression still appeared to play a key role in the development of diabetes.
Both depression and diabetes are common chronic illnesses, particularly in those 65 and older.
About one in every 11 Canadian adults now suffers from Type 2 diabetes. It is an insidious disease that damages the circulatory system and is a leading cause of heart disease, kidney failure, blindness and circulation problems leading to amputation.
About one in eight adults experience a major depression at some point in their lives.