Depression has been linked to loss of brain cells
People who have had depression may be more prone to Alzheimer's disease, two studies suggest.
Dutch researchers found Alzheimer's was 2.5 times more likely in people with a history of depression.
Similarly, US researchers, examining Catholic clergy, found those with signs of depression were more likely to go on to develop Alzheimer's.
The Dutch appears in the journal Neurology and the US study in Archives of General Psychiatry.
The Dutch study was small - 486 people over an average of six years, with just 33 people developing Alzheimer's.
But it found that people who showed signs of depression before the age of 60 were four times more likely to develop Alzheimer's.
The researchers, from the Erasmus University Medical Center in Rotterdam, said more work was needed to fully understand the link between Alzheimer's and depression.
Lead researcher Dr Monique Breteler said: "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."
One theory is that depression leads to the loss of cells in two areas of the brain, the hippocampus and the amygdala, which then contributes to Alzheimer's disease.
However, the latest study found no difference in the size of these two brain areas in people with depression and people who had never developed the condition.
The findings were echoed in a second study by Rush University in the US published in Archives of General Psychiatry.
The researchers followed more than 900 members of the Catholic clergy for up to 13 years during which time 190 developed Alzheimer's.
They found that those with more signs of depression at the start of the study were more likely to develop Alzheimer's.
But there was little evidence of an increase in depressive symptoms during the early stages of disease.
Even after the diagnosis of Alzheimer's was made there was no general increase in depression, but rather an increase that was confined to individuals with certain personality traits.
The researchers said their findings suggested that depression was a risk factor for Alzheimer's disease - rather than a subtle early sign of its underlying pathology.
Researcher Dr Robert Wilson said: "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."
Rebecca Wood, of the Alzheimer's Research Trust, said the research was interesting, and potentially useful.
She said: "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."
Dr Susanne Sorensen, head of research at the Alzheimer's Society, said: "More research is needed to clarify the relationship between dementia and depression and determine whether depression causes changes in the brain that make dementia more likely."