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New research reveals that people with depression are at a higher risk of developing dementia, suggesting that proper treatment of depressive symptoms could lower a person's risk for cognitive decline later in life.
"We've known for a long time that people with some depression are more likely to develop cognitive decline and dementia in old age than people without depression," the study's lead author Robert S. Wilson, PhD, neuropsychiatrist at the Rush Alzheimer's Disease Center, told Yahoo Health. "But dementia takes a long time to develop, more than a decade, and there's been school of thought that depression was perhaps an early sign of the development of dementia and not a true risk factor. Here we show that is definitely not the case."
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Another theory has suggested that depression and dementia were caused by the same abnormality in the brain, which would mean that higher levels of dementia would lead to more severe depression. The current study revealed the opposite. "We found that people did not become more depressed and some even became less depressed after they developed dementia," said Wilson. "Furthermore, depression was not related to the common brain [abnormalities] that really drive dementia in old age. So depression appears to be a genuine risk factor for cognitive decline."
According to the Centers for Disease Control and Prevention, 9.1 percent —approximately 28 million — Americans suffer from depression, with people ages 45 to 64 are at the highest risk. Wilson said that most of the participants in his study did not suffer from major depression and showed only mild to moderate symptoms, yet they still were more likely to suffer cognitive decline. "These were not necessarily people who were going to see a psychiatrist for their condition," he added. "The message is that mild to moderate depressive symptoms make a difference by the time you reach old age, so we should think about more aggressively treating these less severe cases."
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The study involved 1,764 people with an average age of 77 who had no thinking or memory problems at the start of the study. Participants were screened every year for symptoms of depression, such as loneliness and lack of appetite, and took tests on their thinking and memory skills for an average of eight years. A total of 680 people died during the study, and autopsies were performed on 582 of them to look for the plaques and tangles in the brain that are related to dementia as well as other signs of damage in the brain.
During the study, 922 people (52 percent) of the participants developed mild cognitive impairment (MCI), or mild problems with memory and thinking abilities that is often a precursor to Alzheimer's disease. A total of 315 people (18 percent) developed dementia. Those who developed both MCI and dementia were more likely to have a higher level of symptoms of depression before they were diagnosed.
Although the current research supports the theory that depression is a risk factor for dementia, the causal link between the two conditions is still being investigated. The most popular theory has to do with stress hormones in the brain. "Research has shown for years that if you place animals under chronic stressful conditions, a series of changes in the brain occur that are linked to elevated levels of stress hormones," said Wilson. "Most people think that it's the higher levels of stress hormones that are at least partially responsible the connection between depression and cognitive decline."
So can treating depression really lower a person's risk of developing dementia? While that link would be game-changing, right now there is no supporting evidence. "Our study, we hope, will lead to some change in that," said Wilson. "We need to understand what depression is doing to the brain that is causing cognitive decline. But regardless of why it's happening, our results suggest that treating depression should reduce dementia risk, and we think research into that is warranted."