"What we found was a strong relationship between activity energy expenditure and the risk of incident cognitive impairment, she says, "with those of higher activity energy expenditure had 90 percent reduced risk of incident cognitive impairment over the follow-up period compared to those with very low energy activity expenditure."

Laura Middleton and her colleagues describe their findings online in the Archives of Internal Medicine, published by the American Medical Association.

In the same issue, another paper - this one from French researchers led by Marie-Noël Vercambre of the Foundation of Public Health in Paris - studied the exercise-dementia link in a large group of women with cardiac risk factors such as obesity or diabetes. In this study, the women who got the equivalent of a brisk, half-hour walk every day had a lower risk of cognitive impairment.

Dr. Eric Larson of the Group Health Research Institute in Seattle, Washington, says the studies add to the evidence that physical activity can reduce the risk of cognitive decline.

"It's not obvious to people that exercise would make your brain healthier," he says. "And as each study does more detailed analyses of special groups or a different way of making the measurements, it just makes the scientific basis for this relationship a lot more convincing."

Larson writes that, with accumulating evidence of the link, research should now focus on how best to encourage people to be active, especially in later life.