Some previous studies found that physical activity might stave off mental decline. But the new findings, contained in two studies, show that the activity does not have to be super strenuous.
In more good news for older people, another study suggests that the benefits of a Mediterranean diet rich in fish, olive oil and fruits and vegetables extend into old age, increasing longevity even in men and women in their 70s, 80s and 90s.
"This study is important because it is often thought that diet, alcohol, physical activity and smoking doesn't matter anymore in old age," said nutrition researcher Kim Knoops of The Netherlands' Wageningen University, the lead author.
The studies appear in Wednesday's Journal of the American Medical Association.
One study, involving 2,257 retired men ages 71 to 93, found that those who walked less than a quarter-mile a day were almost twice as likely to develop Alzheimer's or other forms of dementia as men who walked more than two miles daily.
A study of 16,466 female nurses ages 70 to 81 found that even women who walked a leisurely 1½ hours a week did better on tests of mental function than less active women.
"We were a bit surprised that something so modest as walking would be associated with apparent cognitive benefits. That was really the surprise," said Jennifer Weuve, a Harvard School of Public Health researcher who led the nurses study.
Previous studies have linked mental exercise, such as crossword puzzles and reading, with a reduced risk of Alzheimer's. The new research shows physical exercise helps, too.
Bill Thies, vice president for medical and scientific affairs of the Alzheimer's Association, offered some possible theories for how exercise might boost brain function.
He said research in mice has suggested that exercise might reduce brain levels of amyloid, a sticky protein that clogs the brain in Alzheimer's patients. Also, Thies said, studies have shown that exercise boosts levels of hormones necessary for nerve cell production, and increases blood flow to the brain.
The study results are good news for older people who want to avoid mental decline but "don't like doing all that awful, sweaty stuff," Thies said. "This just says, `Go for a walk."'
"Keep eating your veggies, too" could be another mantra, according to the Dutch study, showing that Europeans ages 70 to 90 who ate a Mediterranean-style diet had a 23 percent lower risk of death during a 10-year follow-up than those with less healthy eating habits.
A 65 percent lower mortality risk was found in those who combined the Mediterranean-style diet with three other healthy habits - moderate alcohol use, no smoking and a half-hour or more per day of physical activity, including walking.
Previous research has linked the diet with a lower risk of heart disease.
The new study does not say how long any of the participants were on the heart-healthy diet.
By Lindsey Tanner