Exercise in teen years cuts risk of breast cancer

WASHINGTON — Get your daughters off the couch: New research shows exercise during the teen years, starting as young as 12, can help protect girls from breast cancer when they’re grown.

Middle-aged women have long been advised to get active to lower their risk of breast cancer after menopause.

What’s new is that starting young pays off, too.

Researchers tracked nearly 65,000 nurses ages 24 to 42. They answered detailed questionnaires about their physical activity dating back to age 12. Within six years of enrolling, 550 were diagnosed with breast cancer before menopause. A quarter of all breast cancer is diagnosed at these younger ages.

Women who were physically active as teens and young adults were 23 percent less likely to develop premenopausal breast cancer than women who were sedentary, researchers reported Wednesday in the Journal of the National Cancer Institute.

The biggest effect was regular exercise from ages 12 to 22.

The women at lowest risk reported doing 3 hours and 15 minutes of running or other vigorous activity a week — or, for the less athletic, 13 hours a week of walking. Typically, the teens reported more strenuous exercise while, during adulthood, walking was most common.

After menopause, fat tissue is a chief source of estrogen. In youth, however, the theory is that physical activity itself lowers estrogen levels. Studies of teen athletes show that very intense exercise can delay onset of menstrual cycles and cause irregular periods.

The moderate exercise reported in this study was nowhere near enough for those big changes. But it probably was enough to cause slight yet still helpful hormone changes, said Dr. Alpa Patel, a cancer prevention specialist at the American Cancer Society.

And while the study examined only premenopausal breast cancer, “it’s certainly likely and possible” that the protection from youthful exercise will last long enough to affect postmenopausal breast cancer, too, Colditz added.