Women in the intervention group exercised an average of 3.6 days per week for a total of 178 minutes per week. After one year, statistically significant declines in estradiol and free estradiol were found in the exercise group compared to the nonintervention group. In addition, slight increases in sex hormone-binding globulin were observed. These differences were all in the direction expected to reduce breast cancer risk for the exercise intervention group. There were no significant differences in estrone, testosterone, and androstenedione between exercisers and the nonintervention group after one year. The authors conclude that previously sedentary postmenopausal women can adhere to a moderate-intensity exercise program that results in hormonal changes that are consistent with lower risk of postmenopausal breast cancer.
Study provides an explanation for reduced risk of breast cancer and improved survival
The study results provide evidence of one possible mechanism whereby exercise reduces breast cancer risk and improves survival. The association between exercise and reduced risk of breast cancer has been observed among both premenopausal and postmenopausal women:
- A prospective study including 38,660 women aged 55 to 74 found that women with at least four hours per week of vigorous recreational physical activity had a significantly reduced risk of breast cancer compared with those who reported no such physical activity
- A study including 118,899 postmenopausal women in the prospective NIH-AARP Diet and Health Study found that those who engaged in more than seven hours per week of moderate-to-vigorous activity during the 10 years prior to baseline had a 16% reduction in risk of postmenopausal breast cancer compared to inactive women
- A study which compared rats whose energy balance was altered either by increasing physical activity (achieved by running on an activity wheel) or restricting calories, found that physical activity reduced mammary cancer incidence more than restricting calories. On the other hand, another study using cancer-prone mice found that mice who exercised survived less long than those who did not
- Another study found that physical activity may reduce the risk of breast cancer in BRCA1/2 mutation carriers
- A French-Canadian study of non-BRCA1/2 mutation carriers found that women who engaged in moderate physical activity had a significantly decreased risk of breast cancer, but that this reduction in risk was not evident for vigorous physical activity
- A study designed to evaluate the impact of physical activity on risk of breast cancer in premenopausal women found that women who engaged in leisure-time physical activity had a reduced risk. Women who had regularly engaged in high amounts of physical activity during both adolescence and adulthood appeared to derive the most benefit
- A 2010 study found that compared with no physical activity, any reasonably consistent recreational activity during the three years before breast cancer diagnosis was associated with a lower risk of death for women with estrogen receptor positive (ER+) tumors, but not those with ER- tumors. This association did not appear to differ by body mass index
- A prospective study that included 3,539 women in the California Teachers Study who were eventually diagnosed with invasive breast cancer found that women with intermediate or high levels of long-term physical activity before diagnosis had lower risk of death from breast cancer than women with low activity levels regardless of estrogen receptor status and breast cancer stage. However, the results were significant only for overweight women
- A prospective study including 1,483 breast cancer survivors reported that increased physical activity reduced mortality, especially among those who also consumed five or more daily servings of vegetables and/or fruits.
Moderate regular exercise might be more beneficial than extremely vigorous exercise such as training for and running in marathons. Some of the studies reported that the women with the very highest levels of physical activity did not benefit from it as much as those with moderate levels. This makes sense since extremely vigorous exercise increases free radical-induced oxidative stress.