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Saliva could predict an individual's risk of developing dementia, according to a new study.

"Studies have shown that depression increases the risk for dementia, but we don't know much about how this relationship occurs," lead researcher Lenore J. Launer, PhD, of the National Institute on Aging in Bethesda, and a member of the American Academy of Neurology, said in a news release. "High levels of the stress hormone cortisol have been found in people with depression, and the theory is that cortisol has a toxic effect on the hippocampus area of the brain, which plays an important role in memory."

The latest study involved 4,244 people with an average age of 76 who were asked to give saliva samples and undergo brain scans.

Researchers found that participants with higher levels of cortisol were significantly more likely to have smaller brain volume than those with lower levels of cortisol. Participants with higher levels of cortisol also scored significantly lower on memory and thinking tests compared to their lower level counterparts.

"Since this study just looked at a snapshot in time, we don't know which came first: the high levels of cortisol or the loss of brain volume," Launer said. "It's possible that the loss of brain volume that can occur with aging leads to a lesser ability of the brain to stop the effects of cortisol, which in turn leads to further loss of brain cells. Understanding these relationships may help us develop strategies to reduce the effects of cortisol on the brain and thinking skills."

The latest study was published in the online issue of the journal Neurology.