Chronic stress can speed up memory decline in older people who already have some impairment in their mental function, a new study in the American Journal of Psychiatry shows.

But being stressed doesn't appear to affect memory in older people without such impairment, Dr. Guerry M. Peavy of the University of California San Diego and colleagues found.

Research suggests a strong relationship between increased stress and memory loss, the researchers note, but few investigators have looked at stress and memory over time. Chronic stress may affect memory by causing prolonged release of so-called stress hormones, such as cortisol, resulting in damage to the brain.

To investigate, the researchers followed 52 people 65 to 97 years old for up to three years. Twenty-five had no loss of mental function at the beginning of the study, while the remaining 27 showed evidence of mild mental impairment.

To measure stress, the researchers asked study participants about whether they had experienced stressful life events in the previous year or six months, such as being hospitalized or having a death in the family. A person was considered to have high stress if they reported at least one such event in a given period.

Among the individuals who were already somewhat impaired, those with high levels of stress showed faster decline in mental function, especially degree of dementia and memory function. But stress didn't influence mental function over time in people who had no impairment at the study's outset.

Already-impaired people with high cortisol levels showed a slower rate of mental decline than those with lower levels, which was an unexpected finding, the researchers note. It's possible, they say, that this potentially neurotoxic hormone may actually have protective effects in people who already have some loss of mental function.

SOURCE: American Journal of Psychiatry, December 2009.