CHICAGO, Illinois -- Elderly women who exercise regularly are less prone to suffer mental decline, another indication that physical activity helps stave off some of the frailties of aging, Reuters reported on Thursday. "This finding supports the hypothesis that physical activity prevents cognitive (mental) decline in older women," said study author Kristine Yaffe, a psychiatrist and neurologist at the University of California at San Francisco. The researchers tracked nearly 6,000 mostly white, healthy women aged 65 or older living in planned communities, such as nursing homes, whose mental faculties and levels of exercise were evaluated over a six- to eight-year span. Those in the highest quartile of exercise -- measured by calories expended walking, gardening, or more rigorous activities -- were 26 percent less likely to develop cognitive decline than those in the quartile who exercised the least. For example, for every mile (1.6 km) per day the women walked, they lowered their risk of mental decline by 13 percent, the researchers said. Walking speed was not a factor. A few previous studies have linked exercise with better mental health, but the latest study adjusted for confounding variables such as smoking and use of hormone replacement therapy. Yaffe cited several possible benefits of regular exercise among the elderly, including increasing cerebral blood flow, reducing the risk of cardiovascular disease in the body and brain, and stimulating nerve cell growth. The report, which was published in the journal ***Archives of Internal Medicine***, said at least one in 10 people older than 65 and half of those over 85 develop some form of cognitive impairment ranging from mild mental deficits to dementia. "Further research is needed to determine if physical activity programs could prevent clinically significant cognitive impairment and if our findings can be replicated in other populations," Yaffe wrote.