TUESDAY, July 19 (HealthDay News) -- Older adults who keep active may be helping to reduce their odds of losing their mental abilities, two new studies suggest.

Both reports were published online July 19 in the Archives of Internal Medicine, to coincide with presentations scheduled to be presented Tuesday at the International Conference on Alzheimer's Disease in Paris.

"We looked at an objective measure of physical activity -- most previous studies looked at self-reported levels of physical activity, which always has some inherent error," said the lead researcher of the first study, Laura E. Middleton, from the Heart and Stroke Foundation Center for Stroke Recovery at the Sunnybrook Research Institute in Toronto.

Using this measure, the researchers found that those who were the most physically active had a 90 percent lower risk of developing significant cognitive decline, compared with those who had the least physical activity, she said.

"This suggests, because this method is able to capture all types of physical activity, that low-intensity physical activity may be important," Middleton said. "So not just jogging, swimming or biking, but maybe just moving around the house, doing chores, walking outside, may also be important for protection against cognitive impairment."

"We shouldn't just be encouraging people to exercise, we should discourage them from being sedentary," she added.

For the study, Middleton's team collected data on 197 men and women who took part in the ongoing Health, Aging and Body Composition study. The participants had an average age of 74 when they started the study and none had any cognitive difficulties, the researchers noted.

To determine the effects of activity on mental ability, the researchers measured the total amount of energy the participants used. To do this, they used a method called "doubly labeled water," which shows how much water a person loses, which is an objective measure of a persons metabolic activity.

Over two to five years of follow-up, Middleton's group found that those with the highest levels of physical activity had the lowest odds of developing any cognitive impairment, compared with those who had the least amount of physical activity.

These findings were confirmed by having participants take the Modified Mini-Mental State Examination. The researchers also took factors such as Modified Mini-Mental State Examination scores at the start of the study, demographic factors, body mass, sleep, self-reported health and diabetes.

Middleton noted that while these findings cannot be said to be causal, but it "is an association between physical activity and cognitive change."

In the second study, a team led by Marie-Noel Vercambre, from the Foundation of Public Health, Mutuelle Generale de l'Education Nationale in Paris, looked at the effect of physical activity among women who were part of the Women's Antioxidant Cardiovascular Study, which included women with vascular disease or three or more risk factors for heart disease.

Vercambre's group determined the level of physical activity among 2,809 women at the start of the study and every two years thereafter. In addition they conducted phone interviews with the women that included tests of mental ability and memory. These tests were given at the start of the study and three or more times over the next 5.4 years.

The researchers found that women who were most physically active had the lowest rate of developing cognitive decline. In addition, women who took a brisk 30-minute walk every day, or its equivalent, had the lower risk of cognitive impairment.

Dr. Eric B. Larson, from the Group Health Research Institute in Seattle and author of an accompanying journal editorial, commented that the association between physical activity and mental ability probably had to do with overall vascular health.

"As we get older, our brains are probable less able to withstand stress," he said. But exercise improves vascular health, he added.

Larson thinks the benefits of exercise on mental ability can accrue even if one starts exercising later in life. "There may be even more benefit, because your state is more risky," he said. "Just keeping up walking for an older person is a huge benefit."

Even after dementia has started, exercise can be a benefit, Larson said. "Walking once, twice or four times a week with a caregiver leads to a better outcome and a happier person," he said.

More information

For more on dementia, visit the U.S. National Library of Medicine.