State of the epidemiological evidence on physical activity and cancer prevention
Introduction
The past 20 years of epidemiological research has generated a large body of evidence describing the benefits of physical activity in relation to cancer risk. Despite great progress in understanding this association, particularly with respect to colon and breast cancers, there is still uncertainty regarding the strength, consistency and dose–response of the associations between physical activity and most other cancer sites. It is also unclear what type, timing and dose of activity are required for significant benefit and also the population sub-groups that experience a reduced cancer risk with physical activity. To address the state of the knowledge surrounding these questions, we will provide an overview of the existing epidemiological evidence relating physical activity to cancer risk. Using the latest published literature, we also estimate the proportion of cancers that are attributable to inadequate physical activity levels in Europe and describe briefly some proposed biological mechanisms explaining physical activity–cancer relations. We finally highlight areas for future research.
Section snippets
Epidemiological evidence for an association between physical activity and cancer
The association between physical activity and all cancer sites has been systematically reviewed by national1 and international agencies2, 3 as well as by individual scientists.4, 5 The level of epidemiological evidence varies by cancer site. There is now convincing evidence for a beneficial effect of physical activity on the risk of colon cancer, probable evidence for an effect on breast and endometrial cancers, possible evidence for an effect on cancers of the prostate, lung and ovary, and
Population-attributable risks in Europe
From a public health perspective, a need exists to examine the burden of cancer that might be preventable if risks associated with a specific lifestyle factor could be removed or, at least minimised in a population. To estimate the cancer burden in Europe attributable to physical inactivity, we used physical activity estimates generated by Eurobarometer, Wave 58.2 conducted in 15 European Union countries in 200258 and estimates of gender-specific cancer incidence in 2008 reported by Ferlay and
Hypothesised biological mechanisms
For those cancers that are convincingly, probably or possibly related to physical inactivity, an understanding of the underlying biological mechanisms adds plausibility to the association, identifies specific targets for cancer prevention and helps guide future public health recommendations. Across the six cancer sites discussed earlier in this review, numerous biological pathways have been hypothesised and in some cases tested in randomised controlled trials (RCTs).2, 3, 66, 67, 68Table 3
Limitations of the existing evidence and future research directions
Several limitations exist in this field of cancer research that need to be considered, particularly when articulating new research directions. These include the difficulty in assessing physical activity, the limited number of controlled exercise intervention trials that have been specifically designed to examine the underlying biological mechanisms and the complete lack of randomised controlled trials of exercise for cancer prevention.
Inherent in observational epidemiological studies of
Conclusion
The quantity and quality of research on physical activity and cancer have increased exponentially in the past 20 years with a large body of evidence that is now becoming established and increasingly recognised worldwide. Physical activity is recognised as a modifiable lifestyle risk factor for most chronic diseases, including cancer, by international agencies.1, 2, 3 Evidence for the underlying biological mechanisms involved in the pathways between physical activity and cancer risk is also
Funding
Dr. Christine Friedenreich was supported by a Health Senior Scholar Award from the Alberta Heritage Foundation for Medical Research. Dr. Brigid Lynch was supported by a National Health and Medical Research Council Public Health Training Fellowship (#586727).
Conflict of interest statement
None declared.
Acknowledgement
The authors would like to thank Dr. Isabelle Soerjomataram for the Eurobarometer study data that were used in this manuscript in the estimation of the population-attributable risks.
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