Older adults with high cortisol levels in their saliva had smaller brain volume and poorer memory and thinking skills than those with low levels of the hormone.
Published in the journal Neurology, the study reveals older individuals with high levels of the "stress hormone" cortisol in their saliva had smaller brain volumes, which was associated with poorer performance on tests of memory and thinking.
Study co-author Dr. Lenore J. Launer, of the National Institute on Aging - a part of the National Institutes of Health - and colleagues believe their findings could lead to a saliva test that helps determine which individuals may be at risk of Alzheimer's disease and other dementias, as well as strategies that may reduce cortisol's potential negative impact on cognitive function.
Previous studies have suggested that depression raises the risk of dementia for older adults. But Dr. Launer and colleagues say it is unclear why.
"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," says Dr. Launer.
The researchers set out to investigate this theory further, analyzing the cortisol levels, brain volumes and cognitive skills of 4,244 adults of an average age of 76 who were free of dementia.
Brain volume around 16 millimeters smaller in subjects with high cortisol levels
Participants' cortisol levels were measured from saliva samples that were taken on a single day - 45 minutes after awakening in the morning and again in the evening. Depending on their cortisol levels, participants were allocated to one of three groups: high, medium or low.
Subjects' brains were scanned using magnetic resonance imaging (MRI), allowing researchers to assess brain volume.
In addition, participants were required to take part in tests that assessed memory and thinking skills.
Compared with subjects in the low cortisol group, those with high cortisol levels were found to have overall brain volumes around 16 millimeters smaller. The difference in brain volume size was particularly noticeable in gray matter regions rather than white matter regions, according to the team.
What is more, participants with high cortisol levels performed worse on memory and thinking tests than those with low levels of the hormone.
The researchers admit that because their study only looked at a short time period, they are unable to determine whether high levels of cortisol or loss of brain volume occurs first, but Dr. Launer has a theory:
"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 fact that saliva samples were only taken on a single day is a limitation of the study, say the authors, as it increases risk for cortisol measurement errors. However, they say their large sample size counteracts this limitation.
This study follows another published last month that suggests an individual's risk of Alzheimer's and mild cognitive impairment (MCI) could be predicted through a saliva test that measures specific compounds more pronounced in people with such conditions.