Three hours of moderate exercise per week significantly reduced circulating estrogens in postmenopausal women, according to a new study published in the current issue of Cancer Research
. The finding may explain why women who exercise regularly lower their risk for breast cancer.
"Exercise is an effective way for postmenopausal women to increase their chances of avoiding breast cancer," said Anne McTiernan, M.D., Ph.D., a member of the Public Health Sciences Division, Fred Hutchinson Cancer Research Center, Seattle.
Dr. McTiernan led a year-long study that examined differences between women who exercised regularly compared with women who limited their activity to stretching. The study targeted postmenopausal women who were sedentary, and overweight or obese at the beginning of the trial.
Within three months of undertaking the five-day per week exercise program, serum levels of estrogens dipped significantly in the more active postmenopausal women. After 12 months of routine exercise, women who decreased body fat by more than 2 percent also had a 16.7 percent reduction in free serum estradiol, a 13.7 percent reduction in serum estradiol, and an 11.9 percent reduction in serum estrone, a less estrogenic form of estrogen. Estradiol is a female sex steroid with a more potent estrogenic effect than estrone, a different form of estrogen. Estrone concentrations are equivalent to estradiol levels in the blood prior to menopause, but normally increase in postmenopausal women.
The moderately intensive exercise regiment initially aimed for aerobic activity resulting in the women reaching 40 percent of maximal heart rate for 16 minutes per session. The exercise workload increased gradually to the point where the women reached 60-75 percent maximal heart rate for 45 minutes per session. The women trained primarily on treadmills, stationary bicycles, or by walking outdoors. The women who trained averaged 171 minutes of exercise per week in five workout sessions performed either in their homes or in athletic gymnasiums.
"This study indicates that exercise can lower levels of circulating estrogens and increase levels of a protein called sex hormone binding globulin (SHBG--a blood-borne protein) that binds up free-estrogen in the serum," Dr. McTiernan said. Increased levels of SHBG act to regulate the circulating levels of free, biologically available estrogens.
Dr. McTiernan noted studies showing that women on low-fat diets reduced serum levels of estradiol by 6.2 percent. The active women in the exercise study reduced estradiol level another ten percent with no dietary limitations during the study. The women who persisted with the exercise-training schedule lost an average of only 3 pounds of body fat during the 12-month study, but reduced estradiol blood levels by about one-sixth from pre-trial levels.
Although blood estrogen levels decreased, no drop in whole-body bone density was evident in the women who exercised compared with their pre-trial levels, or compared with the control group of women who merely stretched for exercise.