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We all know that working out is good for us. But precisely how much or how little exercise is needed to gain various health benefits, and whether the same dose of exercise that bolsters heart health, for instance, is also ideal for the brain has remained unclear.

For the new study, which was published last month in PLOS One, scientists with the University of Kansas Alzheimer’s Disease Center in Fairway, Kan., and other institutions recently decided to see if they could determine just how much exercise is needed to improve the ability to think.

They began by recruiting 101 sedentary older adults, at least 65 years of age, who were generally healthy, with no symptoms of dementia or other cognitive impairments.

They chose these volunteers, said Dr. Jeffrey Burns, the co-director of the University of Kansas Alzheimer’s Disease Center and the study’s senior author, because the men and women had reached the age at which many of us begin to develop the first worrisome declines in our memory and thinking skills.

The scientists brought these volunteers to the lab and had them complete a series of tests, including measurements of their aerobic capacity and how well they could remember and think.

Then the volunteers were randomly assigned to one of four groups. People in the control group continued their normal lives.

People in the other three groups were assigned to walk briskly. One group began exercising for 75 minutes per week, which is half of the current recommendation of 150 minutes per week of moderate exercise. Another group was assigned to exercise for the recommended 150 minutes per week. And the third group was directed to exercise for 225 minutes per week, or 150 percent of the recommended amount.

The exercising volunteers reported to a local YMCA for their workouts, which consisted of supervised brisk walks on a treadmill for anywhere from 25 minutes to an hour. (Some volunteers used an elliptical machine for some of their workouts.)

After 26 weeks, all of the participants returned to the lab to repeat the original tests.

At this point, the groups displayed notable differences, especially in the physical realm. The more someone had exercised, the more his or her endurance capacity had increased, which was hardly a surprise. The volunteers in the control group were no more fit than they had been; those in the group exercising for 75 minutes per week were somewhat more fit; those exercising for 150 minutes per week were fitter still; and those walking for 225 minutes per week were the most fit of all.

“There was a very clear dose-response relationship” between walking and fitness, Dr. Burns said.

That relationship was murkier when the scientists looked at thinking, however.

In general, the researchers found, most of the exercisers showed improvements in their thinking skills, especially in their ability to control their attention and to create visual maps of spaces in their heads, two aspects of cognition that are known to decline with age.

But these gains were about the same whether people had exercised for 75 minutes a week or 225 minutes. Those volunteers who had exercised the most scored slightly better on some cognitive tests at the end of the study period than those exercising less, but the difference was barely significant.

Over all, “a small dose of exercise” may be sufficient to improve many aspects of thinking and more sweat may not provide noticeably more cognitive benefit, Dr. Burns said.

On the other hand, more exercise will likely make you more aerobically fit, Dr. Burns said, which has other health effects.

This dichotomy underscores the importance for scientists to do a better job of nailing down how different amounts of exercise differently affect us, Dr. Burns said.

“We need to be able to say, this much exercise will provide these benefits,” he said.

Such precision might inspire more people to take up and continue exercising, he said.

To that end, he and his colleagues are working on an ambitious study to determine just how much and what types of exercise might help to delay the onset of Alzheimer’s disease or other forms of dementia, though it is unclear whether exercise can actually help prevent the disease. The results won’t be available for some time.

In the meantime, though, the encouraging takeaway from the new study, he pointed out, is that briskly walking for 20 or 25 minutes several times a week — a dose of exercise achievable by almost all of us — may help to keep our brains sharp as the years pass.

(Source: well.blogs.nytimes.com)

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