Those who reported an average of 3.5 hours per week of sweat-producing physical activity during this time halved their chances of developing breast cancer later in life, said the researchers.
Study lead author Dr. Joan Dorn, an assistant professor of social and preventive medicine, and colleagues, re-analysed data from a diet study of U.S. women between the ages of 40 and 85, conducted from 1986 to 1991.
The study involved 740 menopausal and postmenopausal women with breast cancer, and 810 postmenopausal controls. They were asked how regularly they had spent time exercising to the point of breaking into a sweat two, 10 and 20 years prior to the interview and when they were 16 years old.
The results showed that exercise produced modest protective effects for both premenopausal and postmenopausal women who were active at any one time period. Dorn said postmenopausal women who were active at all four time periods had a 50 percent lower risk than their peers.
However, particular significance seemed to apply to being active at a point 20 years in the past, because all women exercising at this point halved their risk of developing breast cancer. In addition, those active at the age of 16 reduced their risk by 35 to 45 per cent.
Dorn suggested that these particular time points might apply to significant physiological events in women's lives, such as a first pregnancy.
Writing in the journal Medicine and Science in Sports and Exercise, she said, "It's possible that strenuous exercise during this period of a woman's life has an impact on breast cancer by favorably affecting a hormonal milieu in the process of change.
"These results, combined with what we know about the benefits of physical activity in protecting against other chronic diseases, are enough to tell women to get out there and get some exercise."
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