New York, Aug 20 (IANS): Your saliva can tell how strong your memory and analytical skills are, a new study shows.

Saliva of healthy individuals contains stress hormone cortisol. People with higher levels of cortisol in the evening were more likely to have a smaller total brain volume and to perform worse on tests of thinking and memory skills, the study found.

"Studies have shown that depression increases the risk for dementia, but we don't know much about how this relationship occurs," said study author Lenore J. Launer from the National Institute on Ageing in Bethesda.

"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," Launer explained.

The study involved 4,244 people with an average age of 76 who did not have dementia.

Participants had a brain scan to look at brain volume and took tests of their thinking and memory skills.

Saliva samples were taken from the participants once in the morning and in the evening to determine cortisol levels. Participants were divided into three groups based on cortisol levels of high, medium and low.

People with the highest level of cortisol were more likely to have a smaller overall brain volume than those with lower levels of cortisol, with a difference of 16 millilitres between the two groups.

Those with the highest level of cortisol also performed worse on the memory and thinking tests than those with low levels of the hormone.

"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.

The study was published online in Neurology, the medical journal of the American Academy of Neurology.