There's plenty of evidence that mental exercise, such as crossword puzzles and reading, may reduce Alzheimer's risks, but previous studies on brain benefits from physical exercise had conflicting results.
The new findings, contained in two studies, clarify how much exercise might be beneficial and are good news for older people who want to avoid mental decline but "don't like doing all that awful, sweaty stuff," said Bill Thies, vice president for medical and scientific affairs of the Alzheimer's Association.
"This just says, 'Go for a walk"' and bolsters evidence that what's good for the heart may be good for the brain, said Thies, who was not involved in the research.
"Keep eating your veggies, too" could be another mantra, according to a Dutch study, showing that Europeans ages 70 to 90 who ate a Mediterranean-style diet rich in fruits, vegetables, fish and olive oil 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.
The studies appear in Wednesday's Journal of the American Medical Association.
"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, who led the diet study.
While the studies involved older patients, they don't answer whether adopting healthy habits late in life has the same benefits as a lifetime of healthy behavior, since participants weren't asked how long they'd engaged in the activities.
One of the exercise studies involved 2,257 retired men in Hawaii ages 71 to 93. 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.
Walking was "probably representative of a lifetime behavior" for most of the men, given Hawaii's exercise-friendly climate, said University of Virginia biotstatistician Robert Abbott, the lead author.
A similar study, involving 16,466 female nurses ages 70 to 81, found that even women who walked a leisurely 1 1/2 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," said study author Jennifer Weuve, a Harvard School of Public Health researcher.
Thies 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.