CHICAGO, April 8 (UPI) -- Depression is a risk factor for Alzheimer's disease rather than a subtle early sign of its underlying pathology, a U.S. study suggests.

Researchers analyzed data from the Rush Religious Orders Study, a cohort of 917 older Catholic clergy without dementia at study onset. Study participants were tracked for 13 years and 190 developed Alzheimer's disease.

The study, published in the Archives of General Psychiatry, found, consistent with earlier findings, having more depressive symptoms at baseline was associated with increased incidence of Alzheimer's disease and mild cognitive impairment.

Robert S. Wilson, a neuropsychologist at the Rush University Medical Center's Alzheimer's Disease Center and colleagues, say those who developed Alzheimer's disease showed no increase in depressive symptoms before clinical diagnosis.

"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," Wilson said in a statement.