Depression can affect anyone at any stage of their life. According to the Centers for Disease Control and Prevention (CDC), about 1 in 6 people in the United States will experience depression at some point in their life.
Scientists do not know the exact cause of depression, but many believe it is a combination of psychological, genetic, biological, and environmental factors.
Certain risk factors are already known; for instance, having relatives who have had depression, experiencing traumatic events, going through a major life change, and using alcohol or drugs.
Depression and dementia
Previously, scientists have identified an association between depression and an increased risk of dementia later in life.
A 2015 study published in JAMA Psychiatry, for example, found that people with depression had an 83 percent increased risk of acquiring dementia compared with people who did not have depression.
The same study also found that people who had depression and type 2 diabetes were at even higher risk of developing dementia, with a 117 percent increased risk compared with people who had neither condition.
A linked commentary to the 2015 study said that, while scientists at this stage do not yet know whether treatment of depression may offer protection against cognitive decline and onset of dementia, the "hypothesis is plausible."
The new study — conducted by psychologists at the University of Sussex in the United Kingdom — is the first to provide substantial evidence of the relationship between depression and impairment of overall cognitive function in the general population.
The researchers conducted a systematic review of 34 longitudinal studies (long-term observational studies) that had investigated links between depression or anxiety and cognitive decline. This included assessing data from 71,000 participants.
To get a clearer picture of how depression might influence the aging brain, the authors excluded any participants who were diagnosed with dementia at the start of the study.
The team concluded that people who had experienced depression had more extensive cognitive decline later in life than people who had not experienced depression.
What does this mean for patients?
The researchers believe that these findings could have implications for dementia research, and that they may help provide clues to potential early interventions.
"This study is of great importance — our populations are aging at a rapid rate, and the number of people living with decreasing cognitive abilities and dementia is expected to grow substantially over the next 30 years."
Co-lead study author Darya Gaysina
Gaysina continues, "We need to protect the mental well-being of our older adults and to provide robust support services to those experiencing depression and anxiety in order to safeguard brain function in later life."
Gaysina's colleague Amber John, meanwhile, warns against interpreting the study's results as suggesting that everyone with depression will experience cognitive decline.
"It's not inevitable that you will see a greater decline in cognitive abilities," John notes, "and taking preventative measures such as exercising, practicing mindfulness, and undertaking recommended therapeutic treatments, such as cognitive behavioral therapy, have all been shown to be helpful in supporting well-being, which in turn may help to protect cognitive health in older age."