A new study confirms that for most women, 6 or more hours of strenuous recreational exercise each week can reduce the risk of invasive breast cancer, according to the findings of a new study.

While women with a family history of breast cancer didn’t have a reduced risk with exercise, all of the other women did, regardless of how old they were when they started exercising, Dr. Brian L. Sprague of the University of Wisconsin-Madison and colleagues found.

“A lot of women are particularly concerned about breast cancer, and our study just adds more evidence that physical activity is indeed related to breast cancer risk,” Sprague told Reuters Health.

His team interviewed 7,630 women who were free of breast cancer, 1,689 with very early-stage or in situ disease, and 6,391 with invasive breast cancer. All of the women ranged in age from 20 to 69.

While there was no link between physical activity and in-situ breast cancer risk, women who reported more than 6 hours of strenuous recreational exercise each week had a 23 percent reduced risk of developing invasive breast cancer compared to women who never exercised.

The reduction in risk was seen women who exercised early in life, after menopause, or in the recent past.

Sprague noted that there were relatively few women in the study with in situ disease, so it may be that the study was not powerful enough to identify a reduced risk with exercise, which other studies have reported. It’s possible, he added, that exercise may prevent invasive breast cancer by stopping in situ disease from progressing.

The researchers found no link between physical activity on the job and reduced breast cancer risk. However, Sprague noted, he and his colleagues had to use a fairly crude measurement to gauge occupational activity, based on job titles, so the current study does not necessarily mean on-the-job activity is not protective.

SOURCE: Cancer Epidemiology Biomarkers & Prevention, February 2007.