Being highly fit in middle age may keep the mind sharp, lowering the risk of dementia and Alzheimer’s disease later in life, a new study suggests.

Using patient records, researchers found that people who had the greatest levels of fitness in midlife had a 36 percent lower incidence of developing dementia than those who were the least fit.

In the study, the most in-shape people had fitness levels that averaged 13.1 metabolic equivalents (METs), while those who were the least fit had levels averaging 8.1 METs. One metabolic equivalent is the amount of oxygen you use when you are inactive. An average healthy, but nonathletic adult has a peak exercise capacity of about 8 to 10 METs. Walking briskly at a pace of 3 miles per hour is the equivalent of 3.3 METs; cycling at a 10 to 16 mph pace is the equivalent of 6 to 10 METs; and running at an 8 mph pace is the equivalent of 13.5 METs.

The researchers looked at records of fitness levels for nearly 20,000 adults, most of whom were in their 40s and 50s. The men and women had been patients at the Cooper Clinic in Dallas, part of The Cooper Institute, which studies the role exercise plays in preserving health. The patients’ baseline fitness levels, collected between 1971 and 2009, had been determined using a treadmill exercise test. Everyone who participated in the study was free from conditions known to increase dementia risk, such as a previous heart attack or stroke.

The researchers then examined the patients’ Medicare records from 1999 to 2009 and found that nearly 9 percent of the study participants, or 1,659 people, had developed dementia within an average of 24 years. (Ninety-five percent of the study participants had completed their baseline fitness assessment at least nine years prior to enrolling in Medicare.)

The results of research on exercise and dementia risk have been mixed. Previous studies have shown that moderate exercise, such as walking, swimming and yoga, may reduce the risk of memory problems. But in 2010, the National Institutes of Health said there wasn’t sufficient evidence to suggest that increasing physical activity would stave off dementia.

Research has linked high fitness levels to a lower risk of cardiovascular disease and stroke, which increase the chances of developing dementia, including Alzheimer’s disease. However, in the current study, the association between fitness levels and dementia risk held even after accounting for patients who had suffered a stroke. This suggests that fitness may have benefits for cognition independent of lowering cardiovascular risk, explained lead study author Dr. Laura DeFina, medical director of research at The Cooper Institute. and lead study author.

While the new study suggests that higher levels of fitness during midlife may keep the mind sharp, the researchers can’t say for sure whether other factors, such as an overall healthy lifestyle, diet or a higher level of education could have partially accounted for the association. Nor can they recommend a level of physical activity that may cut dementia risk based solely on this study.

Still, the researchers urge people to follow the physical activity guidelines recommended by the Centers for Disease Control and Prevention. Those call for 150 minutes per week — about 30 minutes a day — of moderate-intensity exercise, such as brisk walking, or 75 minutes a week of vigorous-intensity aerobic activity, or an equivalent combination of the two.

Even people who are a little past midlife can still make changes. It’s never too late to reap the health and cognitive benefits of exercise, said study co-author Dr. Benjamin Willis, an epidemiologist at The Cooper Institute. “Sure, the best time to plant a tree may have been 20 years ago, but the second-best time is now,”” he said.

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