By Amy Norton

NEW YORK (Reuters Health) - Offering yet more reason to get regular exercise, a new study shows that running and other aerobic activities improve nervous-system control of the heart in young men.

The study, of 149 healthy young adults, found that 12 weeks of aerobic exercise improved the autonomic nervous system's regulation of the heart -- at least in men. In general, the training lowered men's resting heart rate and improved their results on a measure of heart-rate variability -- heart's ability to speed up or slow down in response to demands. The benefit was not seen in women, however, the researchers report in the American Journal of Public Health.

Still, the finding does not mean that exercise is no help to women's hearts, according to lead researcher Dr. Richard P. Sloan, a professor of behavioral medicine at Columbia University Medical Center in New York.

"There are many benefits of exercise, and this study looked at only one of those," Sloan told Reuters Health.

What the findings imply, he said, is that women may need to work harder than men to improve autonomic control of the heart.

The autonomic nervous system controls involuntary activities of the body, heart function being one. Regular aerobic exercise is one of the keys to a healthy heart, and one reason, the thinking has been, is that exercise improves autonomic regulation of the heart.

To study that theory, Sloan's team randomly assigned 149 healthy, sedentary adults in their 20s and 30s to 12 weeks of strength training or aerobic conditioning -- activities such as running, pedaling on a stationary bike or using a stair-climbing machine.

The volunteers then returned to their sedentary ways for 4 weeks, for a period of "deconditioning."

Overall, the researchers found, aerobic exercise, but not strength training, improved exercisers' fitness levels. Similarly, only aerobic conditioning improved autonomic regulation of the heart -- an effect that disappeared during the deconditioning period.

It's not clear why aerobic exercise improved autonomic control of the heart only in men, Sloan said. It's possible that sex hormones -- either estrogen or testosterone -- play a role, but that is unproven, he noted.

Whatever the reason, Sloan stressed, the findings do not negate the importance of exercise for women's hearts. "This in no way gives anyone an excuse not to exercise," he said.

SOURCE: American Journal of Public Health, May 2009.

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