For the first time in a single study, Johns Hopkins scientists have found that diabetes contributes to depression and vice versa, confirming long-held assumptions about the intertwined nature of two diseases that affect millions of Americans.
The research, published yesterday in the Journal of the American Medical Association, provides added proof that diabetes plays a role in depression and depression plays a role in diabetes. Previous studies have looked at only one aspect of the link.
For years, researchers had assumed that diabetes led to depression, said University of Michigan epidemiologist Briana Mezuk. The new research provides evidence. "That's what makes this study so great," said Mezuk, an expert on diabetes and depression.
Both ailments affect a significant number of people. Almost 21 million Americans - 7 percent of the population - have diabetes. In Baltimore, the proportion is even higher: One in 10 adults is diabetic, according to the federal Centers for Disease Control and Prevention.
And 30 million adults - about 16 percent of the population - have had at least one bout with serious depression during their lifetime.
"We were able to show that there's a bidirectional association," said the study's lead author, Dr. Sherita Hill Golden, a diabetes expert at the Johns Hopkins School of Medicine.
Golden said she hopes that the study will lead to increased understanding among both doctors and patients that the two illnesses are so closely intertwined. She emphasized in particular that doctors who treat diabetes should watch their patients for signs of depression.
The study looked at an ethnically diverse group of 6,814 men and women between the ages of 45 and 84, spread across the country. Over three years, subjects visited a clinic three times to be examined for symptoms of type 2 diabetes and depression. Type 2 diabetes is a chronic illness in which the body cannot properly remove sugar from the blood. The illness has a genetic component but usually occurs in overweight people or those who eat a high-sugar diet. It differs from type 1 diabetes, which is much less common and is an autoimmune disorder in which the body attacks its own pancreas. The pancreas makes insulin, the hormone that controls blood sugar.
Those with higher levels of depression were almost 50 percent more likely to develop diabetes than those who were not depressed. Subjects who were most depressed were most likely to become diabetic.
Compared with the general population, depressed people tend to smoke more, to overeat, to exercise less and to be overweight. All of these factors increase the risk of diabetes. Golden's study found that these activities contributed to the higher rates of diabetes for subjects with depression.
But there are other factors, too. Golden, an endocrinologist, suspects that depression might cause physiological changes that predispose people to diabetes. Depression raises levels of stress hormones such as cortisol as well as other molecules that lead to inflammation; these chemicals in turn contribute to diabetes.
Depression plays a role in several chronic physical conditions besides diabetes. For years, scientists have known that depression is associated with higher rates of heart disease, stroke, osteoporosis and perhaps dementia. Stress hormones and inflammatory molecules play a role in all of those diseases.
Interestingly, the link between depression and diabetes seems to be closely related to socioeconomic status. A new study by the University of Michigan's Mezuk in the American Journal of Public Health found that people with depression whose level of educational achievement was limited to high school or less had a 200 percent increased risk of becoming diabetic.
But depressed people with at least some college education had no increased risk. Mezuk suspects that the difference has something to do with the fact that in people with less education and income, depression tends to last much longer - probably because this group has less access to treatment. The longer the illness persists, the more chance it has to do harm.
Most researchers think that for diabetic patients who become depressed, worry and anxiety play a central role. The disease is chronic, and patients must think constantly about diet as well as the possibility of complications, which can include heart disease, peripheral nerve problems and blindness.
"Either you have them or you're worried about getting them," said epidemiologist William Eaton. A professor at the Johns Hopkins Bloomberg School of Public Health, he has studied diabetes and depression for years but did not work on the study.
Golden found that diabetic patients getting treatment were more likely to be depressed than patients not in treatment. She suspects that diabetes treatment, which involves regular monitoring and injections, causes psychological stress on patients, thus increasing the risk of depression.
Awareness of the link between the illnesses can make a significant difference: A study last year by University of Pennsylvania researchers found that depressed diabetic patients who received mental health care were half as likely to die after two years compared with depressed diabetic patients who did not receive the extra care.
"Treating depression is not only important for mental health, it's important for physical illness as well," said University of Pennsylvania family medicine specialist Dr. Hillary Bogner, the study's lead author.