Study co-author Dr. Benjamin Willis, an epidemiologist at the Cooper Institute, added: "It's never too late to start exercising."
The new findings help solidify the "move-it-or-lose-it" message, according to experts not affiliated with the new study.
Dr. Richard Isaacson, director of the Alzheimer's division at the University of Miami Miller School of Medicine, said: "Based on most recent data, we know that the brain changes that lead to dementia occur 20 to 30 years before the onset of symptoms, so that is the time to make lifestyle changes. If you are worried about developing Alzheimer's or dementia, the time to make healthy lifestyle choices and changes is now."
This includes engaging in regular exercise, eating a healthy low-fat diet and making sure blood pressure and cholesterol levels are where they need to be, he explained.
"There is no magic bullet that will prevent Alzheimer's, but we have evidence that you can reduce your risk," Isaacson said. "This is an excellent study because it uses an objective measure of fitness: the treadmill test," he said.
These results show how fit a person is, not just how much they exercise, he noted.
Dr. Sam Gandy, the associate director of the Mount Sinai Alzheimer's Disease Research Center, in New York City, agreed that exercise can help keep a brain healthy.
"Three 30-minute sessions per week of either brisk walking or weight lifting is the standard recommendation for delaying or preventing dementia," Gandy said. "This is very, very important. The first thing I tell all my patients is to find an exercise they like and do it."
Although the study found an association between midlife fitness levels and later dementia risk, it did not establish a cause-and-effect relationship.
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