The finding is based on more than 1,700 adults aged 65 and older who didn't have dementia at the study's start. Those who reported exercising at least three times per week were nearly a third less likely to develop dementia -- primarily Alzheimer's -- over six years.
The brain benefits didn't require being in tip-top shape or doing grueling daily workouts.
"Even those elderly people who did modest amounts of gentle exercise, such as walking for 15 minutes three times a week, appeared to benefit," researcher Eric Larson, MD, MPH, says in a news release.
If confirmed, the findings could be yet another reason for elders to exercise, even if they're not in great physical shape, write Larson and colleagues.
Their study appears in the Annals of Internal Medicine. Larson directs the Center for Health Studies at Group Health Cooperative, a Seattle HMO.
Exercise and Dementia
Dementia isn't a normal part of aging, but it becomes more common with age. Alzheimer's is the most common form of dementia in older adults.
Exercise's general health benefits are well known. But studies have been mixed on whether exercise thwarts dementia, note Larson and colleagues. So they conducted their own study.
Participants were followed for six years. None scored poorly on initial tests of mental skills including memory, judgment, and language ability.
The researchers were picky about that for a reason. They wanted to make sure that participants weren't already headed for dementia.
Participants were screened for dementia every other year during the six-year study. All were members of the HMO where Larson works.
Active or Idle?
At the study's start, participants were asked about their exercise habits. The key questions: Do you exercise? If so, how many days per week?
More than three-quarters reported exercising at least three times weekly.
All types of exercise counted, including walking, bicycling, hiking, swimming, aerobics, and weight training. Participants were also asked about their smoking, alcohol use, education, medical history, and other factors that might affect their chances of developing dementia.
The participants were not told to work out or change anything else about their lives.
Holding Dementia at Bay
Most participants didn't develop dementia, but 158 did. Alzheimer's was their most common type of dementia.
People who reported exercising at least three times weekly were 32% less likely to develop dementia than those who exercised less often, the study shows. That figure takes other risk factors into account, like age, sex, diabetes, high blood pressure, and heart disease.
Larson's team stops short of saying that exercise prevented dementia. Instead, they write that exercise appeared to delay dementia.
"We believe these findings may be useful if they are confirmed because Alzheimer's disease is one of the most feared illnesses of aging and is frequently cited as a reason for not wanting to 'get old,'" the researchers write.
"People do not want to lose their independence and quality of life as a consequence of aging," they continue.
Not Superfit? No Problem
Participants' physical skills were also checked. They were timed while walking 10 feet and standing up from a chair. They also took tests of balance and grip strength.
Those who weren't great at those tasks but exercised anyway cut their dementia risk most. Fitter seniors also benefited, but not as much.
Larson adds, "Even if you're 75 and have never exercised before, you can still benefit by starting to exercise now." Of course, you should get a doctor's approval before doing so.
How Might Exercise Slow Dementia?
If exercise delays dementia, science hasn't figured out exactly how it happens.
A journal editorial states, "It is enticing to think that engaging in regular exercise can delay or prevent the development of Alzheimer's disease," since the disease's established risk factors (like age and genetics) won't budge.
That is, you can't change your age or genes, but you can decide whether or not to exercise. "We are edging closer to placing prevention of cognitive deterioration and of dementia on the long list of health benefits induced by physical activity," write the editorialists. They included Laura Podewils, PhD, MS, of the CDC.