Epidemiological evidence supports the role of physical activity in cancer prevention (
31–
33). A National Institutes of Health (NIH)–AARP study found an 18% reduction in colon cancer risk (relative risk 0.82; 95% confidence interval [CI], 0.73–0.82) among people ages 50–71 years who exercised at least five times per week compared with like-aged people who never or rarely exercised (
34). Other studies in European and Asian populations found similar associations (
35–
39). Convincing evidence of a preventive effect exists for colon cancer, where meta-analyses of over 60 studies suggest a reduction of 20%–25% in colon cancer risk in people with the highest level compared with people with the lowest level of physical activity. The source of physical activity does not matter, as occupational and leisure-time activities confer similar reductions in risk, and a dose-response effect exists (
31,
33). Despite these convincing data, public awareness of the role of physical activity in the prevention of colon cancer is low (
40). Similar effects are not seen in the epidemiology of rectal cancer risk, where physical activity does not appear to have a protective effect (
41).
Modeling the effect of physical activity suggests that population compliance with recommended levels of activity would reduce the population incidence of colon cancer by up to 21% (
33,
42). Associations between physical activity and risk of cancer are also probable for breast and endometrial cancer and possible for lung, ovarian, and prostate cancer. Between 9% and 19% of breast, lung, colon, endometrial, prostate, and ovarian cancers in Europe were estimated to have been preventable in 2008 if the population had maintained sufficient levels of activity. Sufficient levels in this analysis were defined as 30 minutes of moderate-intensity activity for 5 days or 20 minutes of vigorous-intensity activity for 3 days above a base level of 60 minutes of moderate-intensity activity every day. No association for rectal cancer or other cancer sites and physical activity definitively exist (
33). Therefore, one could speculate that physical activity may be a non-pharmacologic intervention not only to decrease an initial colon cancer, but perhaps also could decrease the risk of second colon and non-colorectal cancers by modifying underlying risk factors that predisposed a survivor to the initial colon cancer.
Physical activity also appears to affect disease outcome and recurrence after diagnosis. Although epidemiological evidence suggests physical activity has the greatest effect on colon cancer incidence, cohort studies in both colon and rectal cancer survivors suggest that physical activity also may affect outcomes after diagnosis and treatment although the literature does not always distinguish between these two populations (). Haydon et al. evaluated general physical activity and walking in a cohort of 526 colon and rectal cancer survivors (all stages) prospectively enrolled in the Melbourne Collaborative Cohort Study. In the overall study population, regular physical activity, even as little as once per week, was associated with an absolute improvement of 14% in overall survival and 12% in disease-specific survival at five years compared with no regular activity. This effect was greatest in survivors with right colon tumors and in survivors with stage II or III disease, who had a 39% reduction in all-cause mortality and 51% reduction in disease-specific mortality. No effect on survival was noted, however, in the subgroup with rectal cancer or when regular walking was evaluated alone, suggesting that this benefit may be confined to colon cancer and requires a more vigorous physical activity routine than walking alone (
43).
| Table 1Studies evaluating impact of physical activity on outcomes after a diagnosis of colon or rectal cancer |
Meyerhardt et al. also examined the role of physical activity, as measured in metabolic equivalent task (MET)-hours per week (described in ), after colon or rectal cancer diagnosis and treatment in three cohorts of patients. Physical activity of at least 18 MET-hours per week was associated with a lower rate of colorectal cancer–specific death and overall mortality, compared with less than 18 MET-hours per week in 573 women diagnosed with stages I to III colon and rectal cancers enrolled on the Nurses Health Study. An increase in physical activity after diagnosis also improved disease-specific and overall survival in this population, regardless of prediagnosis activity level (
44). Similar findings were seen in 668 men with stages-I-to-III colon and rectal cancers enrolled in the Health Professionals Follow-up Study. The amount of activity required to achieve survival benefits, however, was higher in the Health Professionals Follow-up Study (≥ 27 MET-hours per week) (
45). This difference may be due to a larger number of men (50.4%) engaging in high levels of physical activity at study entry than did women (25.9%) in the Nurses Health Study (
44,
45). In both studies, the benefit of physical activity remained after adjusting for stage, age, body mass index, year of diagnosis, tumor location including rectum, and exclusion of people who died within two years of questionnaire response. Pre-diagnosis physical activity levels, however, did not have an effect on post-diagnosis outcomes (
44,
45).
| Table 2MET-hours* per week equivalents in walking (46) |
The benefit of physical activity may be independent of other treatment, as suggested in 832 patients with stage III colon cancer receiving fluoropyrimidine-based therapy as part of a randomized adjuvant therapy trial. Patients engaging in at least 18 MET-hours per week of activity enjoyed a statistically significant 47% improvement in disease-free survival compared with inactive patients. Relapse-free and overall survival were also improved with increased physical activity, even after adjusting for other predictors of survival or excluding patients who died within six months of assessment. This benefit was independent of sex, body mass index, age, number of positive nodes, performance status, or type of chemotherapy received (
46). The protective effect of physical activity can be seen with as few as 6 MET-hours per week, levels off above 30 MET-hours per week, and may differ based on gender (
44–
46). Physical activity may therefore provide additional benefit in recurrence and survival outcomes above the benefit seen with adjuvant chemotherapy, although the exact amount and mode of activity needs further investigation.