Two new studies published on Monday found that people who have a history of depression are twice as likely to develop Alzheimer’s disease.

How exactly depression influences the brain is not known, but the theory has been that depression shrinks specific areas leaving the brain vulnerable to the development of Alzheimer’s.

“We don’t know yet whether depression contributes to the development of Alzheimer’s disease or whether another unknown factor causes both depression and dementia. We’ll need to do more studies to understand the relationship between depression and dementia,” lead researcher of one of the studies, Dr. Monique M.B. Breteler, from the Erasmus University Medical Center in Rotterdam said in a statement, according to the Washington Post.

Dr. Breteler’s study followed 486 people, aged 60 to 90 who did not have dementia, of which 134 had had at least one episode of depression. After 6 years, 33 people developed Alzheimer’s disease.

The study, published in Neurology, concluded that those who had had an episode of depression were 2.5 times more likely to develop Alzheimer’s compared with people who have never had depression. Moreover, for people whose depression occurred before they were 60, the risk for developing Alzheimer’s was fourfold greater than those who had never had depression.

The conclusions of Dr. Breteler’s study were reinforced in the April issue of the Archives of General Psychiatry, which also suggests that depression may be a risk factor for Alzheimer’s.

The paper, belonging to Robert S. Wilson, of Rush University Medical center in Chicago, found that among 917 older men and women, those who had symptoms of depression at the beginning of the study were more likely to develop Alzheimer’s.

“Depressive symptoms may be associated with distinctive changes in the brain that somehow reduce neural reserve, which is the brain's ability to tolerate the pathology associated with Alzheimer's disease. Understanding the mechanisms linking depressive symptoms with dementia could suggest novel approaches to delaying dementia onset, because animal research suggests diverse means by which the adverse effects of Chronic stress may be modified,” Wilson and his colleagues wrote.

Although the studies found a connection between depression and Alzheimer’s, experts question whether this connection really exists.

“There are quite a few papers about the association between depression and Alzheimer’s with conflicting results. No one really knows if there is a connection between depression and Alzheimer’s,” said Yaakov Stern, a professor of clinical neuropsychology at Columbia University in New York City.

Overall, Rebecca Wood of the Alzheimer’s Research Trust welcomed the studies, saying, “Identifying people at higher risk could lead to ways to reduce the number of people who develop dementia, help researchers to understand more about dementia and create new avenues of research."