The study of nearly 65,000 women, published in the Journal of the National Cancer Institute, found that those who were physically active had a 23 percent lower risk of breast cancer before menopause. High levels of physical activity from ages 12 to 22 contributed most strongly to the lower breast cancer risk.

Researchers at Washington University School of Medicine in St. Louis and Harvard University in Boston said that one-quarter of all breast cancers are diagnosed in women before menopause.

"We don't have a lot of prevention strategies for premenopausal breast cancer, but our findings clearly show that physical activity during adolescence and young adulthood can pay off in the long run," lead investigator Dr. Graham Colditz of the Washington University School of Medicine in St. Louis said in a statement.

The researchers found the age-adjusted incidence rates for invasive breast cancer dropped from 194 cases per 100,000 person-years in the least active women to 136 cases in the most active. The most active women reported the equivalent of running 3.25 hours a week or walking 13 hours a week.