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Diabetes and depression are common among older people and up to 20 percent of adults with type 2 diabetes have major depression, according to background information in the study. In addition, both of these disorders appear to be associated with an increased risk of dementia.
"Both depression and diabetes have been identified as risk factors for dementia in general and Alzheimer's disease in particular," noted Dr. Marc Gordon, an expert not connected to the new study.
Researchers led by Dr. Mark Sullivan of the University of Washington, Seattle, tracked outcomes for nearly 3,000 people who had type 2 diabetes and were at high risk for heart disease. The patients' thinking and memory (or "cognitive") abilities and levels of depression were assessed at the start of the study and the participants were followed for 40 months.
Patients with depression showed greater mental declines over the course of the study. The effect of depression on risk of mental decline was not affected by factors such as heart disease; age; treatments to control blood pressure, cholesterol or insulin levels; or intensive versus standard treatments to lower blood sugar, the investigators wrote.
The study was published online Aug. 14 in the journal JAMA Psychiatry.
Although the study found that depression in type 2 diabetes patients was associated with dementia risk, it could not prove a cause-and-effect relationship. Nevertheless, "since dementia is one of the fastest growing and most dreaded complications of diabetes, our findings may be important for public health," Sullivan and colleagues concluded.
Gordon, who is chief of neurology at Zucker Hillside Hospital in Glen Oaks, N.Y., agreed that the link between depression and mental decline in diabetic patients "remains unclear."
"It is possible that depression is an early manifestation of an underlying disease process that may eventually result in cognitive decline," he said. "It remains to be seen whether this effect is any different from what would be seen in a nondiabetic population, or whether antidepressant treatment would alter the risk of cognitive decline."
Another expert said the study has potentially wide implications.
"Previous studies have suggested that one in five older diabetic [patients] may suffer from major depression," noted Dr. Gisele Wolf-Klein, director of geriatric education at the North Shore-LIJ Health System in New Hyde Park, N.Y. She said that the "striking finding" from Sullivan's group "will need further study to see if depression treatment can decrease the risk of cognitive decline in patients with diabetes."
Dr. Gayatri Devi is a neurologist at Lenox Hill Hospital in New York City who specializes in the early diagnosis and treatment of memory disorders related to aging. She said it's always been difficult to sort out cause-and-effect when it comes to links between depression and mental decline.
"The question often is whether the depression is the result of the patient's self-awareness that something is cognitively 'off,' or whether the depression independently causes cognitive decline," Devi said.
-- Robert Preidt
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SOURCES: Marc L. Gordon, M.D., chief of neurology, Zucker Hillside Hospital, Glen Oaks, N.Y. and researcher, The Feinstein Institute for Medical Research, Manhasset, N.Y.; Gisele Wolf-Klein, M.D., director of geriatric education, North Shore-LIJ Health System, New Hyde Park, N.Y.; Gayatri Devi, M.D., neurologist, Lenox Hill Hospital, New York City; JAMA Psychiatry, news release, Aug. 14, 2013