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While that's not a new discovery, the studies plug critical gaps in the scientific literature and corroborate previous reports linking exercise to reduced rates of mental impairment in older adults. 

The message is now clearer than ever: "If you stay physically active, you're buying protection for your brain," says Eric B. Larson, M.D., the vice president for research at Group Health Cooperative, a nonprofit health-care system based in Seattle.

The studies appear in the July 25 print edition of the Archives of Internal Medicine and were published online today to coincide with the International Conference on Alzheimer's Disease, taking place this week in Paris.

One of the studies included 2,809 women over the age of 65 who had a history of heart disease or stroke, or at least three risk factors for those conditions. That's noteworthy because most previous studies on exercise and dementia have focused on healthy people, according to Dr. Larson, who wrote a commentary accompanying the new research.

Exercise may be particularly important for these women, since unhealthy cholesterol levels, high blood pressure and other conditions that affect blood-vessel health have been linked to the memory and language problems known as cognitive decline, which often precedes Alzheimer's disease and other forms of dementia.

Researchers in Paris and at the Harvard School of Public Health, in Boston, reanalyzed data from a study originally designed to examine the role of antioxidant vitamins in heart health.

Beginning in 1995, the women answered biennial surveys on how often they engaged in various types of exercise (such as jogging, swimming, walking, and climbing stairs). Several years later, the researchers then gave them a series of telephone-based cognitive and memory tests on four separate occasions spread out over a four- to six-year period.

The more active the women were, the better their performance on the test. And they didn't have to be marathoners. The most active women, who were getting the equivalent of 30 minutes or more of brisk walking every day, experienced much slower cognitive decline than those who got little or no exercise. According to the researchers, the difference amounted to being five to seven years younger, cognitively speaking.