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Higher levels of physical activity are associated with decreased risk for breast cancer, although importance of the timing of physical activity is less clear. Also unclear is whether exercise is protective through hormonal modulation. Further analysis of data from the Nurses' Health Study addresses both of these questions.
More than 120,000 women (age range at baseline, 30–55) participated in the study; during 20 years of follow-up, about 4800 cases of invasive breast cancer (most diagnosed postmenopausally) were documented. No association was found between physical activity at study entry and breast cancer risk, but women who increased their weekly physical activity levels, especially after menopause, from 1 hour to ≥3 hours of brisk walkin…