Effect of Exercise
Considerable scientific evidence suggests that physical activity reduces the risk of several cancer types with the evidence classified as convincing or probable for colon, breast, and endometrial cancers; possible for prostate, ovarian, and lung cancers; and null or insufficient for other cancers.There is also increasing evidence that physical activity improves some health indicators and quality of life after diagnosis although there have not yet been any reported clinical trials on the effect of postdiagnosis physical activity on the risk of cancer recurrence or survival.Challenges of these trials include the possibility of group differences in prognostic factors and treatments; the stresses of diagnosis, treatment, and recovery on a patient’s ability to exercise; and the need for a large trial to detect statistically significant differences between exercise and control groups.
The following sections review the scientific literature on cancer sites that have been studied most extensively in relation to physical activity. Epidemiologic research relating physical activity to ovarianand lung cancersare described elsewhere (see citations noted here). It is noteworthy that much of the epidemiologic evidence is based on studies that used questionnaires to estimate physical activity levels. Many factors must be considered when selecting a questionnaire,including its validity for the research question being asked.
The most consistent and strong evidence for a role of physical activity in cancer etiology exists for colon cancer. An average risk reduction of about 25 to 30% is observed in both men and women who undertake the highest level of assessed physical activity compared to the lowest level of activity in studies that have examined these associations(activity levels were not uniformly defined). These findings are likely to be independent of body weight changes. There is evidence for a dose–response effect with more benefit being observed for higher levels of activity, as defined in each study. These results have been observed in studies conducted in a variety of populations around the world, using varying methods for assessing physical activity and with various study designs.
Although 52 studies of physical activity and colon cancer have been identifiedsome aspects of this etiologic association remain unclear including whether the benefits of physical activity depend on menopausal hormone therapy use, dietary intake, or BMI. In addition, the time in life when physical activity is most beneficial for colon cancer prevention is unknown; greater risk reductions may result from higher activity levels over the lifetime as opposed to more recent activity.It is also unclear whether physical activity has a differential effect on various regions of the colon.64
Based on the overall evidence from studies of recreational activity, about 30 to 60 minutes per day of moderate- to vigorous-intensity physical activity may be needed to lower colon cancer risk significantly.An even greater benefit for colon cancer risk reduction may exist for vigorous-intensity activity,13 but the magnitude of this benefit is unclear.
Relatively recent research has been conducted on the role of leisure-time activity in improving colon cancer survival.Four cohort studies conducted by Meyerhardt and colleaguesall showed better survival among colorectal cancer survivors who were more physically active postdiagnosis. In addition, in the Melbourne Collaborative Cohort Study, prediagnosis exercise was associated with better disease-specific survival.
The largest prognostic study to date was conducted in 832 men and women with stage III colon cancer.In that study 18 to 26.9 MET-hours per week of postdiagnosis leisure-time activity lowered the risk of cancer recurrence or death by 49% compared with those who did less than 3 MET-hours per week. Furthermore, significant trends were found relating increasing activity levels to improved disease-free, recurrence-free, and overall survival. A minimum of 18 MET-hours per week of leisure activity improved disease-free survival rates regardless of sex, BMI, number of positive lymph nodes, chemotherapy type, age, or baseline performance status.
Extensive research has been conducted on the etiologic role of physical activity in relation to breast cancer risk, with the majority of studies concluding that women who are more physically active have a lower risk compared to sedentary women. Across 73 studies, the average risk reduction was about 25% for the highest versus the lowest activity categories compared,and there is consistent evidence of a dose–response effect, with greater risk decreases observed with higher levels of activity. All types of activity are beneficial, with somewhat stronger effects observed overall for recreational and household activity.As well, the effect appears to be significant more often in postmenopausal women and, on average, and is stronger in normal weight women, non-Caucasians, women without a family history of breast cancer, and women who are parous. Effects are also stronger for activity done over the lifetime or after menopause, activity of moderate or vigorous intensity, or activity of longer duration (hours per week).
Based on previous research, at least four hours per week of moderate- to vigorous-intensity activity may be necessary to reduce risk significantly. A few aspects of this association remain unclear, including whether the benefit of physical activity depends on the histologic type of the tumor, the hormone receptor status, and other molecular aspects.
The role of physical activity in breast cancer survival has been examined in 15 observational studies conducted thus far.Eight of these studies suggested that higher physical activity levels were associated with a significantly decreased risk of breast cancer mortalityor overall mortality,implying that physically active people with breast cancer may have improved prognosis with fewer recurrences and deaths compared with sedentary survivors. The largest prognostic studies to date were conducted in the Breast Cancer Family Registryand the Collaborative Women’s Longevity Studywith each study enrolling more than 4,000 breast cancer survivors. The latter study found a 51% decrease in breast cancer mortality among the most physically active as well as evidence for a dose–response effect of decreasing the risk of breast cancer death with increasing levels of total recreational activity postdiagnosis.In the Breast Cancer Family Registry study, all-cause mortality was decreased by 23 to 29% in women who were recreationally active three years prediagnosis compared to inactive women, whereas no association was found with lifetime physical activity.
Twenty of the 25 published epidemiologic studiessuggest a protective effect from physical activity in endometrial cancer risk; no association was reported in five studies.Overall, evidence suggests about a 20 to 30% decreased risk for the most active versus the least active study participants; also, activity of light to moderate intensity may lower risk, whereas sitting time may increase risk.Despite these findings, recent reviews of this literaturehave emphasized the need for further research studies that have more detailed assessments of lifetime physical activity and that consider all types and parameters of activity. Furthermore, it remains somewhat unclear how independent this association is from BMI or whether this effect depends on menopausal hormone therapy use.
No observational studies have been published on the role of exercise in endometrial cancer survival, but one randomized controlled trial examined how a six-month intervention of lifestyle counseling could influence physical activity levels, dietary habits, weight loss, and quality of life in endometrial cancer survivors.This study was able to achieve more weight loss and increased exercise levels in the intervention group than in the control group and demonstrated that this type of lifestyle intervention is feasible and could result in sustained behavior change over a yearlong period.
There is inconsistent evidence regarding the association between physical activity and prostate cancer, with about one third (16 out of 42) of the studies conducted thus far indicating a protective effect.The magnitude of the risk reduction is modest, on average around 9%,and there remains a lack of clarity on whether the benefit from physical activity varies according to other factors such as age, race, family history, and BMI. The effect of physical activity may also be more restricted to advanced prostate cancers. Some evidence is emerging that higher levels of lifetime physical activity may decrease prostate cancer risk.Both occupational and recreational activities have been associated with decreased prostate cancer risk.
The inconsistency across prostate cancer studies may be attributed to several factors. First, prostate cancer is a slow-growing tumor with a long latency period, and a large percentage of men die with evidence of undiagnosed prostate cancer. Therefore, some studies may have been unable to detect a difference in physical activity levels between the cancer patients and the “healthy” control populations because of latent, nonclinical prostate cancer among the controls. Second, healthier, physically active men may be more likely to be screened for prostate cancer, and hence more likely to be diagnosed, than less active men. As a result, some study populations might not have accurately reflected the general population of cancer patients, and true risk reductions were attenuated. Finally, it has been hypothesizedthat studies including a greater proportion of screen-detected, early-stage prostate cancer cases might reveal weaker associations between physical activity and prostate cancer risk than studies of advanced prostate cancer. A study by Littman and colleaguesfound a strong inverse association between physical activity and prostate cancer risk in men with no history of recent PSA testing, but no association was found in men with a history of recent PSA testing. Another studyshowed no difference in risk based on PSA screening history, casting doubt on this hypothesis.
Only one observational study has reported on physical activity and prostate cancer survival.In that study of 2,705 nonmetastatic prostate cancer survivors from the Health Professionals Follow-Up Study, men who engaged in leisure-time physical activity postdiagnosis had significantly lower risks of all-cause and prostate cancer mortality; significant trends were noted, with increasing MET-hours per week corresponding with greater reductions in risk. Men reporting at least three hours per week of vigorous activity (versus less than one hour per week) had a 61% lower risk of death from prostate cancer.