By Kathleen Doheny
"What this argues for is, we need to do a better job of both identifying diabetes and depression and then really treating them once identified," said study researcher Dr. Dimitry Davydow, an associate professor of psychiatry and behavioral sciences at the University of Washington School of Medicine in Seattle.
The researchers also took into account pre-existing medical conditions, such as cerebral vascular problems, complications such as kidney problems and other ailments.
"Even after taking those into account, diabetes itself raised the risk of dementia by 15 percent, depression by 83 percent and the two together by 107 percent," Davydow said.
The association was especially strong in people younger than 65. In that age group, "a quarter of the cases [of dementia] were attributed to depression and diabetes," he said.
In Western populations, type 2 diabetes and major depression are increasingly common. And as many as 20 percent of people with diabetes, which is rapidly increasing in younger groups, also have depression, the researchers said in background notes with the study.
"To our knowledge, this is the first study to look at this issue in this way," Davydow said. The findings were published online April 15 in JAMA Psychiatry.
The study points out a complicated link between depression, diabetes and dementia, but does not establish a direct cause-and-effect relationship.
"They are less likely to take medications if they are depressed. Those who have diabetes are more likely to suffer from depression," he added.
Moreover, diabetes makes it more likely that plaque will develop in blood vessels, which can lead to strokes and dementia, Davydow said.
Diabetes and depression each threaten brain health, said Dr. Charles Reynolds III, of the University of Pittsburgh Medical Center, and author of a commentary accompanying the study.
Both ''pose threats to vascular health, thereby impeding maintenance of healthy brain aging and functioning, and posing a risk for cognitive decline," he said.
Reynolds urged those who have a combination of diabetes and depression to get treatment for both in order to protect their brain.
"Lifestyle choices, such as increasing physical activity, will also benefit the management of both conditions," he added.
The research team -- led by Davydow and the study's recently deceased first author, Dr. Wayne Katon -- followed the study participants from 2007 through 2013. All patients were dementia-free at the start.
The researchers said nearly 20 percent of participants had a diagnosis of depression, about 9 percent had diabetes, and nearly 4 percent had both.
Over the study period, more than 59,600 men and women (2.4 percent) got dementia -- at age 81, on average. Of those, 26 percent had only depression, 11 percent had only type 2 diabetes and nearly 7 percent had both.