The link was found for both young and middle-aged adults, but the risk was higher for men.
The study was a collaboration between researchers at the Perelman School of Medicine at the University of Pennsylvania and Aarhus University in Denmark. It followed 1.4 million Danish citizens over nearly four decades, according to a press release announcing the findings.
Overall, those who were diagnosed previously with depression were 2.41 times more likely to receive a dementia diagnosis eventually, the release stated.
"While depression diagnosed later in life is generally thought to be an early symptom of dementia, our results suggest that a depression diagnosis at any point in adulthood increases the risk of dementia later on," first author Holly Elser, M.D., PhD, a neurology resident at Penn, said in the release.
"Previous studies with smaller sample sizes and shorter follow-up times have consistently illustrated the link between dementia and depression diagnosed later in life, but with our long-term analysis, we were able to precisely estimate the association between dementia and depression over an individual's life span," she explained.
The correlation existed regardless of the age at which the depression was diagnosed.
Those hospitalized for depression multiple times had a stronger risk of dementia later in life.
There didn't seem to be any increased risk among those who were prescribed antidepressants within six months of being diagnosed with depression, the study found.
Dr. Rehan Aziz, geriatric psychiatrist at Jersey Shore University Medical Center and associate professor of psychiatry and neurology at the Hackensack Meridian School of Medicine in New Jersey, was not part of the study but shared his thoughts on the findings.
"The study links depression and dementia - but it does not explain why the risk exists," he told Fox News Digital.
The doctor said he has several theories, however.
"One is that depression and dementia may share similar genetics and cause similar brain changes," he said.
"For example, we know that depression can cause shrinkage of the hippocampus, an area of the brain that is important for recent memories."
"We know that depression can cause shrinkage of the hippocampus, an area of the brain that is important for recent memories."
"While the shrinkage is reversible with [the] treatment of depression, it is probable that it has later-life effects," he hypothesized.
Depression can also create a state of inflammation in the brain and impact other brain chemicals, Aziz noted, which could also trigger later-life memory problems.
Although the study findings stated that the prescription of antidepressants didn't seem to impact the risk of dementia, Aziz said more research is needed to make a definitive call.
"For example, the study did not comment on which antidepressants were used, the dosages ordered, duration of treatment or the effectiveness of the treatment," he said.
"It's also not clear if other depression treatments were used."
Mental health conditions should "treat depression aggressively," Aziz said, with a combination of medications, psychotherapy and lifestyle interventions such as diet and exercise.
"Sadness and depression are not the same thing."
"Clinicians should also discuss the possible linkage between depression and dementia and emphasize to patients the importance of adhering to depression treatment," he added.
The doctor also pointed out that "sadness and depression are not the same thing."
"Everybody experiences sadness from time to time, and that should not increase the risk of dementia," he said.
"Clinical depression stops one's life. It can impair functioning and prevent people from working, maintaining relationships, experiencing joy and taking care of themselves."
In extreme situations, people with depression may even have thoughts about not wanting to live anymore, he warned.
"It is important that anybody experiencing depression seek prompt medical evaluation so they can start treatment," Aziz said.
Some 17.3 million people age 18 and older in the U.S. - about 7.1% of the adult population - suffer from major depressive disorder, according to the Depression and Bipolar Support Alliance.
It is estimated that around 6.2 million Americans age 65 and older are living with Alzheimer's disease, the most common type of dementia, per the Alzheimer's Association.
The researchers acknowledged the limitations of the paper and the need for further research of the link between the two mental disorders.
"It is still unclear what about depression increases the risk of a dementia diagnosis," said Elser in the press release.
"And I hope to see further research that evaluates whether the link between depression and dementia may be biological, a result of behaviors associated with depression, like social isolation and other changes in key health behaviors or some combination of these mechanisms," he said.
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