A recent study from Washington University School of Medicine in St. Louis shows that exercise starting during adolescence can protect girls from breast cancer ( Journal of the National Cancer Institute , May 2008). Researchers tracked 65,000 nurses who answered questionnaires about exercise. Within six years, 550 were diagnosed with breast cancer before menopause. Those who exercised regularly as teens were 23 percent less likely to develop premenopausal breast cancer than their sedentary peers. Ages 12 to 22 appear to be the most important time to use exercise to help prevent breast cancer.
Higher estrogen levels increase risk of breast cancer. Exercise lowers estrogen by reducing body fat. Fat cells make estrogen, and after the menopause fat cells are the principal source of estrogen. Other factors that increase breast cancer risk are also related to higher estrogen levels: starting to menstruate at an early age, late onset of menopause, taking estrogen after menopause, and being overweight.