Middle-aged women have long been advised to get active to lower their risk of breast cancer after menopause. The payoff from starting young is new.

"This really points to the benefit of sustained physical activity from adolescence through the adult years, to get the maximum benefit," said Graham Colditz of Washington University School of Medicine in St. Louis, the study's lead author.

Researchers tracked nearly 65,000 nurses ages 24 to 42 who enrolled in a major health study. They answered detailed questionnaires about their physical activity 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, when it's typically more aggressive.)

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

The biggest impact: regular exercise from ages 12 to 22.

"This is not the extreme athlete," Colditz said.

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; during adulthood, walking was most common.

Why would it help? A point of exercise in middle age and beyond is to keep off the pounds. After menopause, fat tissue is a chief source of estrogen.

In youth, however, physical activity itself is believed to lower 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 Alpa Patel, a cancer-prevention specialist at the American Cancer Society, who praised the research.

And while the study examined only premenopausal breast cancer, "it's certainly likely and possible," Colditz added, that the protection will last long enough to affect more common postmenopausal breast cancer as well.

If you were a bookworm as a teen, it's not too late, Patel said. Other research on the benefits of exercise shows mothers should join their daughters for bike rides or at least a walk around the block.