Researchers say girls can help protect themselves from breast cancer later in life just by exercising.

Researchers at Washington University in St. Louis say a study of 65,000 nurses has found that those who began a lifetime of exercise when they were as young as 12 were 23 percent less likely to develop postmenopausal breast cancer.

The study finds that three hours a week of running or other vigorous exercise, or simply walking 13 hours a week have provided the best results.

Writing in the Journal of the National Cancer Institute, lead author Doctor Graham Colditz said the study indicates that "sustained physical activity from adolescence through the adult years" lowers a woman's chances of developing breast cancer.