A new study has found a strong association between increased levels of the stress hormone cortisol and impaired memory in middle-aged adults. The research suggests that extended periods of stress while a person is in their 40s and 50s can result in reduced cognitive skills, and even brain shrinkage.

The study followed over 2000 subjects with an average age of around 48 years. At the beginning of the study subjects underwent cognitive and memory testing and then around eight years later took the same tests alongside MRI brain scans and blood samples to identify cortisol levels. Cortisol is a well-known stress hormone, produced by the body to help regulate metabolism, blood sugar and immune responses.

"Cortisol affects many different functions, so it is important to fully investigate how high levels of the hormone may affect the brain," says Justin B. Echouffo-Tcheugui, lead author on the study. "While other studies have examined cortisol and memory, we believe our large, community-based study is the first to explore, in middle-aged people, fasting blood cortisol levels and brain volume, as well as memory and thinking skills."

The results suggested that high cortisol levels could be associated with reduced memory and visual perception, when compared to subjects that displayed lower cortisol levels. Looking at the correlation between cortisol levels and brain volume surprised the researchers, with a statistically significant association between high cortisol levels and a reduced cerebral brain volume. Even stranger, this cortisol/brain volume correlation was primarily seen in women and not men.

The research, while undeniably compelling, should be taken with caution. This is a classic case of an observational study trying to conclude causation from correlation. There is no evidence at this stage to suggest that high levels of cortisol can actively cause these cognitive deficits.

Another significant problem with the study is that despite undertaking the cognitive testing on two occasions, up to on average eight years apart, only one single blood sample was collected from each participant. So it is unknown whether the cortisol levels for each subject were consistent with long-term high levels. Cortisol is a hormone that is known for particular fluctuations in volume across a single day, so without several tests in individuals across a stretch of time it is difficult to suggest these individuals suffered from consistent high cortisol levels.

Nevertheless, the study does fit in with a decent body of research affirming the negative effects of sustained high cortisol levels on a person's well-being and health. We do know that persistent stress can increase a person's risk of heart disease, weight gain and gastrointestinal problems, so it isn't a stretch to hypothesize a connection between cortisol and cognitive impairment.

The researchers suggest that physicians should take this study as another push to counsel patients with high cortisol levels. "This study adds to the prevailing wisdom that it's never too early to be mindful of reducing stress," notes Sudha Seshadri, senior author and professor of neurology at UT Health San Antonio.

The study was published in the journal Neurology.