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Why Exercise Can Lower Breast Cancer Risk

A girl who's physically active into her preteen years might be less likely to develop breast cancer when she grows up, suggests a new study.

The reason: She may have her first period later, delaying her exposure to peak levels of estrogen, says Leslie Bernstein, Ph.D., a cancer researcher at the University of Southern California, in Los Angeles. She studied girls ages 9 to 13 over four years. Those who were moderately active for at least four hours a week  -- not just sports, but informal time running around the playground  -- began menstruating latest.

While delaying puberty by a year lowers lifetime breast-cancer risk by only a modest 6 to 8 percent, there are other benefits to keeping little girls moving. "Physical activity also reduces the risk of childhood obesity and, in adulthood, heart disease, depression, and Type 2 diabetes, as well as other cancers," says Dr. Bernstein.