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(Health.com) -- People who have symptoms of depression in middle age may be at increased risk of dementia decades later, a new study suggests.

Using medical records, researchers tracked more than 13,000 people in a large northern California health plan from roughly their 40s and 50s into their 80s. Compared to people who had never been depressed, those who experienced symptoms of depression in middle age -- but not later in life -- were about 20% more likely to go on to develop dementia.

Those who received a depression diagnosis later in life only were at even greater risk. That group had about a 70% increased risk of dementia compared to their depression-free peers, according to the study, which was published this week in the Archives of General Psychiatry.

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In a first, the researchers also found that the timing of the depression seemed to predict which type of dementia an individual would develop. Late-life depression was linked with Alzheimer's disease, while mid-life depression was mostly connected with a related condition known as vascular dementia.

Although Alzheimer's disease and vascular dementia share many of the same outward symptoms, they're associated with different processes in the brain. In Alzheimer's, memory loss and other symptoms are believed to be caused by protein deposits that interfere with brain function. Vascular dementia, on the other hand, appears to occur when blood flow to certain areas of the brain is interrupted, such as during strokes and so-called mini-strokes.

The study participants were 3.5 times more likely to develop vascular dementia if they'd experienced depression symptoms in both middle age and later in life, which suggests that "recurring depression over the life course seems to be triggering vascular changes that puts [people] at risk for vascular dementia," says lead author Deborah E. Barnes, Ph.D., an associate professor of psychiatry at the University of California, San Francisco.

By contrast, depression that appears for the first time in old age may simply be an early symptom of Alzheimer's rather than a stand-alone condition, Barnes says.

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Depression in middle age linked to dementia