Introduction
In the United States, 1.7 million individuals are diagnosed with invasive cancer and > 600,000 people die as a result of malignant diseases annually,
1 which highlights the importance of cancer prevention. We have long known that physical activity is associated with a lower risk of colon and breast cancer,
2 but the 2018 US Physical Activity Guidelines Advisory Committee found strong evidence that physical activity is associated with a lower risk of several additional cancer sites, including endometrial, bladder, esophageal adenocarcinoma, kidney, and gastric.
3 Furthermore, our analysis of 1.44 million adults suggested that leisure-time physical activity is associated with a lower risk for as many as 13 cancer types.
4 Given this evidence, physical activity will play an increasingly important role in population-based cancer prevention efforts.
However, our understanding of the shape of the relation between physical activity and cancer risk and whether recommended amounts of physical activity (eg, 2.5-5 hours/week of moderate-intensity activity
5,6 or 7.5-15 metabolic equivalent task [MET] hours/week
5) are associated with lower risk is still limited. Recent comprehensive reviews of physical activity and cancer
3,7 have been unable to determine whether engagement in recommended amounts of physical activity is associated with significantly lower cancer risk because of heterogeneity in activity measurements and analytic methods used in available studies. Similar issues complicate our understanding of whether the intensity of physical activity is relevant to cancer risk. An understanding of the amount and intensity of physical activity associated with reduced risk is essential for application of evidence-based recommendations for cancer prevention. Therefore, the goal of this study was to quantify the leisure-time physical activity-cancer dose-response relationship. Specifically, we extend analyses from our pooled data to provide results applicable to clinical and public health practice by determining whether recommended amounts of leisure-time physical activity (7.5-15 MET hours/week) are associated with significantly lower cancer risk, describing the shapes of the physical activity-cancer dose-response relationships, and exploring whether the same amount of moderate- and vigorous-intensity leisure-time physical activity confers similar cancer risk reduction.
Discussion
This analysis of the dose-response relationship between leisure-time physical activity and cancer provides new insight into the amount of physical activity needed for a reduced risk of many cancer types—critical evidence for more precise cancer prevention messaging in the clinical and public health settings. In this pooled study of 750,000 adults, engagement in recommended amounts of leisure-time physical activity (7.5-15 MET hours/week), an equivalent of 2.5 to 5.0 hours/week of moderate-intensity activity (eg, brisk walking), was associated with significantly lower risk for breast, colon (men only), endometrial, kidney, myeloma, and liver cancer and non-Hodgkin lymphoma (women only). The strength of associations for recommended amounts of physical activity versus none ranged from a 6% to 10% lower risk for breast cancer to an 18% to 27% lower risk for liver cancer. The shape of the dose-response curves varied by cancer type. Both moderate- and vigorous-intensity activity seemed to be associated with lower risk for colon, breast, and kidney cancer, but the sample size for other cancer types was too limited to draw firm conclusions. Adjustment for BMI had a minimal impact on our results, except for endometrial cancer.
In the past decade, our knowledge of the number of cancers linked to physical activity has expanded substantially, but whether engagement in recommended amounts of leisure-time physical activity may lower risk for these cancers has remained unclear. In preparation for the second edition of the US Physical Activity Guidelines, an advisory committee conducted a comprehensive evaluation of > 40 systematic reviews and meta-analyses but concluded that “it was not possible to determine the exact levels of physical activity that provide a given level of effect”
3(pF4-56) because of methodological diversity in the investigations examined. Using harmonized leisure-time physical activity data from 9 cohorts, we found that engagement in recommended amounts of physical activity was associated with significantly lower risk for 7 cancer types and that additional benefits beyond 15 MET hours/week varied by cancer type. Our findings provide robust quantitative evidence that supports the new US Physical Activity Guidelines
5 for cancer prevention as well as recommendations that have been promoted by the American Cancer Society,
6 World Cancer Research Fund International,
31 the International Agency for Research on Cancer,
32,33 and the American College of Sports Medicine.
7Our detailed examination of the physical activity-cancer dose-response relationship revealed 2 unique observations. First, in contrast to the archetypal curvilinear dose-response observed between physical activity and all-cause and cardiovascular mortality,
18,34 approximately half of the cancers associated with physical activity were found to have linear dose-response curves (ie, colon, breast, endometrial and head and neck cancer, esophageal adenocarcinoma), with the lowest risk at levels well above the recommended minimum level of activity. This finding may explain in part previous observations that higher levels of activity were needed to achieve significantly lower risk of colon and breast cancer.
35 Given these linear associations, substantially lower relative risk estimates may have only been observable at higher activity levels in previous studies, particularly given smaller participant numbers and more limited statistical power in individual studies. Of note, for several cancers with a curvilinear association in our data (eg, kidney, gastric, and liver cancer), most of the risk reduction observed was associated with recommended amounts of physical activity. Second, we observed nearly a 3-fold difference in the strength of associations for engaging in 7.5-15 MET hours/week when comparing results for breast cancer (6%-10% lower risk) with liver cancer (18%-27% lower risk). This large difference in the shape and strength of association for the same dose of physical activity may reflect important differences in the underlying biologic mechanisms for distinct cancer types. For example, the primary mechanisms proposed to explain associations with breast cancer are circulating factors (eg, sex steroid hormones, insulin, inflammatory biomarkers) that may exert less-direct effects on breast tumorigenesis in response to exercise.
36 In contrast, in addition to these systemic effects, exercise exerts a direct effect on glucose, glycogen, and lipid metabolism in the liver
37 and may reduce risk of or reverse nonalcoholic fatty liver disease, an emergent risk factor for liver cancer.
38-40 Together, these findings suggest that there are fundamentally different physical activity dose-response relationships for all-cause and cardiovascular mortality and some cancers and substantial variation in the underlying biologic mechanisms that link physical activity to different types of cancer. Additional research is needed to better understand these differences.
Limitations of this report should be considered. First, even with > 750,000 participants, our patient numbers were limited for some types of cancer and our participants were primarily white. Second, our analyses of moderate- versus vigorous-intensity were also limited by the number of cohorts with detailed physical activity measures. Third, we relied on self-reported physical activity measures. Although there is adequate evidence for validity of leisure-time physical activity questionnaires in comparison with objective and physiologic criterion measures,
41,42 we anticipate measurement error and probable attenuation of associations observed. Future research using self-report could be strengthened by incorporating quantitative bias analyses to explore attenuation of risk estimates as a result of measurement error. Fourth, by design, we examined only leisure-time physical activity at one point in time and did not evaluate associations with total activity levels or different domains of behavior. Finally, this was an observational study that cannot demonstrate causality, and we cannot rule out effects of unmeasured or residual confounding.
Our study also had a number of strengths. A unique strength was our focus on a single domain of physical activity (leisure-time), and pooling of original MET hours/week data for 9 prospective cohorts from the United States, Europe, and Australia. This feature overcomes many of the limitations inherent in available meta-analyses related to wide variation in the types of activity examined, differences in summary measures assessed, and dissimilarities in how activity was classified in individual studies (eg, tertiles, quintiles).
3 The pooling of original data also allowed us to quantify dose-response relationships using restricted cubic splines, which afforded greater statistical power and avoided arbitrary categorization of activity levels.
28 Our results provide the best available description of the dose-response for leisure-time physical activity and cancer risks of which we are aware.
In this pooled analysis of prospective studies, we found that engagement in recommended amounts of leisure-time physical activity (7.5-15 MET hours/week) was associated with lower risk for 7 cancers (colon, breast, endometrial, kidney, myeloma, liver, non-Hodgkin lymphoma). Additional benefits for engaging in still higher levels of physical activity were cancer type dependent owing to heterogeneity in the dose-response curves observed. These findings provide direct quantitative support for the levels of activity recommended for cancer prevention and provide actionable evidence for ongoing and future cancer prevention efforts.