How Exercise Works to Protect Against Breast Cancer

We know that prevention of cancer is one benefit of exercise, but how is this protection achieved?  What’s the supposed biological mechanism?  It’s probably related to estrogen exposure, but how about postmenopausal women?   Canadian researchers, writing in the Journal of Clinical Oncology, have investigated this.

A year-long randomized trial of exercise was done in 320 postmenopausal women aged 50 to 74.  The women, who were sedentary, either undertook aerobic exercise for 45 minutes 5 days a week, or maintained their usual level of activity.  Baseline, 6-month, and 12-month levels of sex hormones (estrone, estradiol, androstenedione, and testosterone) and sex-hormone-binding globulin (SHBG) were measured.  (Estradiol is the most important form of estrogen found in the body; estradiol and estrone are other estrogens.)

The women in the exercise group fell a little short of their goal – they averaged only 3½ exercise sessions a week.  But after 12 months, there were significant reductions in estradiol and free estradiol compared to the controls, and significant increases in SHBG.  There were no differences in the other hormones measured.

In fact, this is the first study to actually show a significant effect of exercise on estrogen levels – previously it was just one of several theories of the possible mechanisms of protection against breast cancer. The next step will be to find exactly how exercise produces the increases in SHBG and estradiol, especially in postmenopausal women. . .