A study of more than 1.4 million Danish adults found that those diagnosed with depression were more than twice as likely to subsequently develop dementia, a link that suggests depression may increase the risk of dementia, researchers said.
A study published in American Medical Association Journal of Neurology Men and women diagnosed with depression reported more than double the risk of dementia, even when diagnosed as young or middle-aged adults.
The study looked at registries of about 250,000 citizens diagnosed with depression and about 1.2 million Danish citizens without depression. People with depression were 2.4 times more likely to develop dementia later in life than those without depression. The study reported that the association between depression and dementia was related to whether a person was diagnosed with depression early, middle, or late in life.
The researchers cautioned that while the study looked at a link between depression and dementia, it didn’t explain why such a risk exists.
“Our study simply showed that a relationship exists, it didn’t investigate the mechanism,” said Holly C. Elser, a neurologist and epidemiologist at the University of Pennsylvania who led the study.
Elzer said future research may examine whether childhood experiences and genetic factors are common causes of depression and dementia. Another open question is whether the brain chemistry changes found in depressed patients increase the risk of later dementia.
Elzer said future research may also examine whether depression causes behavioral changes that increase the risk of dementia. Examples include poor diet, reduced physical activity, tobacco and alcohol use, and social isolation.
Other studies have linked dementia with later-diagnosed depression. The JAMA Neurology study said depression was being studied as a possible “reactive or early symptom of cognitive decline.”
However, according to the JAMA Neurology study, previous studies have reported mixed results about the association between early-life depression and middle-aged depression. Previous studies did not track individuals as long as the Danish registry, which tracked individuals from 1977 to 2018.
A JAMA Neurology study found that men and women diagnosed with depression had more than double the risk of dementia. Men are at a slightly higher risk, but studies show that men are less likely to seek medical attention and may experience more severe symptoms when diagnosed.
The study also examined whether patients treated with antidepressants 6 months before and 6 months after diagnosis of depression differed in the subsequent incidence of dementia. However, the study found no significant difference in the incidence of dementia among patients taking antidepressants.
Elser said the researchers didn’t know whether people were receiving cognitive-behavioral therapy, and they didn’t have data on how severe people’s depression was. She said she wants to see data to see if cognitive therapy early in life affects the risk of dementia later in life.
“Our paper further emphasizes the importance of managing depression clinically when it develops, as it can have lifelong effects,” said Elser.