In a large cohort of postmenopausal women, those participating in moderate- to vigorous-intensity recreational physical activity, such as brisk walking, bicycling, and swimming, before or after a diagnosis of breast cancer experienced a lower risk of all-cause and breast cancer–specific mortality. Furthermore, women who increased or maintained physical activity at recommended levels of approximately 3 h/wk after diagnosis, including those who were inactive prior to diagnosis, experienced lower risk of all-cause mortality compared with women who remained inactive.
To our review, this study represents the largest single cohort analysis to prospectively evaluate relationships among pre- and postdiagnosis physical activity, change in physical activity, and all-cause mortality and breast-cancer specific mortality in postmenopausal women diagnosed breast cancer. After a diagnosis of breast cancer, women who reported participating in moderate- to vigorous-intensity physical activity for 9 or more MET-h/wk (~3 h/wk of brisk walking) experienced a 46% less risk of all-cause mortality and 39% less risk of breast cancer–specific mortality. Although severity of disease could influence whether a woman is able to be physically active after diagnosis, our analyses excluded women who rarely walked outside the house for 10 minutes or more without stopping and those who died within 1 year of the physical activity assessment. The former excludes women with severe comorbidities and the latter effectively excludes women receiving chemotherapy during their physical activity assessment. Moreover, a benefit for physical activity was seen regardless of the stage of disease at diagnosis.
Our results of an inverse association between physical activity and mortality are similar to other reports examining this relationship in breast cancer survivors (4
). However, our study expands on previous research by examining change in physical activity from before to after a breast cancer diagnosis. Following diagnosis, women who increased their physical activity level to 9 ore more MET-h/wk experienced a 33% less risk of all-cause mortality compared with women who were inactive or insufficiently active both before and after diagnosis. This finding suggests that adopting an active lifestyle after a cancer diagnosis can improve prognosis.
We examined associations between physical activity and mortality stratified by 4 variables including BMI, stage, ER, and HER2 status. Subgroup analyses did reveal an association between physical activity and all-cause mortality among women diagnosed with ER-positive cancers; however, tests for interaction were nonsignificant and therefore our findings may be due to chance. However, the finding does provide some evidence that physical activity may modulate estrogen or other factors that influence hormone receptor function (15
). Other potential mediating factors include change in body fat, insulin, insulin-like growth factors, adipocytokines, or inflammation (17
). Randomized trials of exercise have shown a benefit of exercise on decreasing insulin and insulin-like growth factors in women treated for breast cancer (18
Study strengths include the large sample size and the prospective assessment of physical activity before and after diagnosis. Additional strengths include the use of a validated physical activity assessment tool, geographic diversity of the cohort, centralized adjudicated outcomes, and comprehensive adjustment for relevant prognostic and lifestyle factors.
Study limitations include a focus on recreational physical activity rather than total physical activity that includes household and occupational activities; however, previous studies examining all types and intensities of physical activity and survival after a breast cancer diagnosis have observed the strongest associations for recreational physical activity and improved survival (7
). For our postdiagnosis analyses, our sample size is smaller than the sample size for the prediagnosis analyses. Because enrollment into WHI occurred over many years, some women were diagnosed with breast cancer 6+ years after enrollment, thus not allowing us to capture enough follow-up time to examine survival analyses.
Other study limitations include an absence of information on systemic adjuvant therapy. However, we did adjust for stage and grade, and these variables have been shown to be correlated with adjuvant therapy. In the Health, Eating, Activity, and Lifestyle (HEAL) Study (7
), the correlation between treatment and stage was r
= 0.41, P
< 0.0001. Also, 25% and 78% of HEAL women diagnosed with stages I and II/III disease, respectively, received chemotherapy, further supporting the use of stage as a proxy of treatment. In the HEAL analyses, after adjusting for stage, further adjustment for treatment did not change the hazard ratio (7
). However, there is the possibility that women who are more active are better able to withstand and complete treatment. In fact, a post hoc finding from an exercise trial conducted by Courneya and colleagues showed that breast cancer survivors randomized to exercise versus control had a higher chemotherapy completion rate (89% vs. 84%, P
= 0.03; ref. 19
). Thus, one mechanism mediating the observed association between physical activity and improved survival may be improved adherence to adjuvant treatment. Thus, maintaining or increasing exercise during treatment is likely beneficial for completing therapy, and in turn improving prognosis. Randomized controlled trials of exercise on disease-free survival should also be conducted to confirm these findings.
In conclusion, moderate- to vigorous-intensity physical activity for 9 or more MET-h/wk before or after a breast cancer diagnosis may improve survival, even among women reporting low physical activity prior to diagnosis. Large-scale, randomized trials are needed to confirm these findings. In the meantime, women diagnosed with breast cancer should be made aware of the current evidence associating higher physical activity levels to lower risk of death due to breast cancer and all causes.