MONDAY, November 22 (Health.com) — Diabetes and depression often occur together, but it's always been a chicken-or-egg scenario. Does diabetes make people depressed or are depressed people more likely to develop diabetes? Now a large new study suggests it's both.
People with diabetes have a higher risk for developing depression than those without the chronic condition and those who are depressed are more likely to develop type 2 diabetes, according to a report in the November 22 issue of Archives of Internal Medicine.
What's more, the study found that the more severe the depression or diabetes, the higher the risk for the other condition.
For example, women who took antidepressants—often a sign of more serious and longer-lasting depression—had a higher risk of developing diabetes than women who weren't taking mood-elevating drugs.
Similarly, diabetics in the study who were already taking insulin to control their condition—a signal that the disease is progressing—were much more likely to suffer from depression than people not taking insulin.
"We can say that the two conditions are linked to each other and are both the causes and the consequences of each other," says the study's senior author, Frank Hu, M.D., Ph.D., professor of nutrition and epidemiology at Harvard School of Public Health. The authors followed 65,381 women aged 50 to 75 who were participating in the landmark Nurses’ Health Study, and recorded depression and new cases of type 2 diabetes over a 10-year period.
One in every 10 adults in the U.S. has diabetes, including nearly 1 in 4 people 60 or older. About 90% of people with diabetes have type 2 diabetes, which is more likely to occur with aging, excess weight, and a sedentary lifestyle. (People with type 1, an autoimmune disease unrelated to aging or lifestyle, were not included in the study.)
Dominique Musselman, M.D., associate professor of psychiatry and behavioral sciences at the University of Miami Miller School of Medicine, says one important implication of the study is that lifestyle changes—namely exercise—may be useful in preventing both conditions.
"There's a biological element to both these conditions that may be contributing to the other. One way to potentially manage your stress and decrease risk of developing diabetes is a lifestyle factor of exercise, or at least treating milder cases with exercise," she adds. "These are both seriously disabling chronic illnesses that are hard to exorcise."
Researchers have long suspected that type 2 diabetes and depression were a two-way street, but had a hard time nailing it down.
In the new study, women answered questions about their physical and mental health every two years from 1996 through 2006, and provided information on lifestyle factors, including exercise and body mass index (BMI).
Body mass index and lifestyle, especially physical activity (or the lack thereof), were linked to the risk of both diabetes and depression.
But even after taking these into account, people who were depressed turned out to be 17% more likely to develop diabetes than people who weren't depressed. Depression was measured either by a doctor's diagnosis, by the fact that the person was taking antidepressants, or by participants' own description of their symptoms such as how often they felt nervous or "down."
Those taking antidepressants were at a 25% increased risk of diabetes compared to women who weren't depressed.
The trend worked in the other direction as well. Even after controlling for mood disorders and related risk factors, people with diabetes had a 29% higher risk of developing depression compared with nondiabetics. Those taking insulin therapy had a 53% increased risk.
Any number of factors could be driving the relationship, in addition to weight and being sedentary, the authors say.
Next: Why the two are linked
There's the sheer drag of having diabetes. "It's a real downer. Diabetes is a lifelong illness. It's not like getting pneumonia, where after two weeks you're better," says Jacob Warman, M.D., chief of endocrinology at the Brooklyn Hospital in New York City. "You have to watch your diet, take medication, check sugars, be on insulin."
Also, Dr. Hu says, "people who are depressed are more likely to be overweight and obese and we know that being overweight and obesity are the most important risk factors for diabetes."
Many common antidepressants, such as selective serotonin reuptake inhibitors (SSRIs) are associated with weight gain, according to Dr. Hu.
Also, Dr. Hu says, "those medications may have deleterious effects on insulin resistance and other metabolic risk factors [such as glucose metabolism] but this is an open question."
There's likely also a psychosocial component. "People who have diabetes are likely to have chronic stress which is associated with the management of diabetes and diabetes complications and decreased quality of life and impaired mental health," Dr. Hu says. "In the long-run, this can contribute to an increased risk of depression."
Stress hormones such as cortisol rise after long struggles with depression and this has been shown to increase insulin resistance, a hallmark of diabetes, Dr. Hu says.
Higher cortisol levels also contribute to abdominal obesity, a major risk factor for type 2 diabetes.
"These two conditions are becoming more and more common. Clinicians and also individuals should pay attention to the coexistence of those two problems," Dr. Hu says. "I think it's very important for clinicians to provide psychosocial counseling to diabetes patients and those diagnosed with depression should get more aggressive monitoring of their glucose markers and their diabetes status."