Did you know that more than 200,000 women in the US are newly diagnosed with breast cancer each year? We have long known that physical activity decreases breast cancer risk and that excess weight increases the risk, but recent research has clarified those points with a few more specific details.
Cancer patients from Long Island were compared to women without cancer, and queried about exercise and body weight patterns over their lifetime. It turns out that the risk of pre-menopausal breast cancer is not significantly affected by exercise, but postmenopausal breast cancer occurrence risk is significantly reduced by 10-19 hours a week of recreational physical exercise occuring during the reproductive years (after birth of first child) all the way through the postmenopausal years. Exercise during reproductive years proved most helpful, resulting in an odds ratio of 0.67 (95% CI* = 0.48-0.94), or approximately a one-third reduction in breast cancer risk. Post menopausal exercise was almost as helpful, with an OR of 0.70 (95% CI = 0.52-0.95), meaning a 30% reduction in risk. The risk reductions were more substantial for hormone receptor positve tumors, the kind of breast cancer that is in general most amenable to all our ideas about preventing cancer.
When it comes to weight gain, a high amount of adult weight gain with no physical actiity confers a 28% increased risk of breast cancer. that falls to NO increase if those same weight-gaining premenopausal women engage in recreational physical activity, not bad, wouldn't you say? In postmenopausal years, even regular exercise is not sufficient to protect against the harmful effects of adult weight gain.
Bottom line, the greatest risk is for women who gain significant weight in their adult years and who do not participate in regular physical activity.
On the bright side, women can enjoy definite reduction in breast cancer risk when they are physically active 10-20 hours over a week - which might include arduous and gentle exercise - during reproductive and menopausal years. There is some indication that sustained participation in arduous exercise may reduce the benefit a tad. Best exercise plan, for many reasons, is probably two vigorous workouts a week, and gentler or more playful activity on a daily basis.
* 95% CI refers to the "confidence interval", or the range of values that is 95% likely to contain the single, ultimate and true value.