A diagnosis of depression in adulthood can double the risk of developing dementia at older ages, according to a new study.

The study, published today Monday in the journal "JAMA Neurology," relied on data from over 1.4 million Danish citizens who were followed from 1977 to 2018, as stated by the study's lead author Dr. Holly Elser, an epidemiologist and resident physician in neurology at the University of Pennsylvania.

The study indicated that individuals were identified as having depression or not and were followed over the years to observe who developed dementia later in life, and the researchers adjusted for factors like education, income, cardiovascular diseases, diabetes, substance use disorders, and bipolar disorder.

The large dataset and the numerous analyses used by the researchers made their findings robust and reliable, but the study is limited due to the lack of information such as genetic data, Dr. Natalie Marchant, an assistant professor in the Department of Psychiatry at University College London, stated.

Depression in late adulthood is often thought to be an early symptom of dementia, and many previous studies have linked the two, Elser noted, but the latest study shows a connection between the risk of dementia and a diagnosis of depression in early and mid-life as well.

She added, "Therefore, our findings provide strong evidence that depression is not only an early symptom of dementia, but also that depression increases the risk of dementia."

Although the relationship between depression and dementia has proven strong, there are still questions the study does not answer, for example, there may be shared risk factors for both depression and dementia that occur early in life, depression may increase the risk of dementia through changes in levels of key neurotransmitters, or depression may lead to changes in health behaviors that in turn increase the risk of dementia.

Elser added in an email, "There is a clear need for future research to explore the potential mechanisms linking early adulthood depression to subsequent onset of dementia."

Marchant said another part of the findings that may need further exploration is the stronger association present in men more than women, adding, "This is an interesting finding, and I hope it will be further investigated, and it supports the idea that we need to routinely consider dementia risk factors in men and women separately as there may be different mechanisms involved."