[Can physical activity prevent cancer?].
BACKGROUND
Physical activity has been an important influence on the evolution of our gene pool and the optimal functioning of our body. Physical activity has recently been discussed as important in relation to cancer-risk.


METHODS
A total of 182 studies related to the association between physical activity and risk of cancer are included in the present study. We have used international accepted criteria in the validation of the strength of the association between a potential risk factor and cancer-risk.


RESULTS
We conclude that there is convincing evidence that physical activity reduces the risk of colon cancer: the evidence is probable for breast cancer and possible for prostate, endometrial and lung cancer. Physical activity does not have any influence on rectal cancer. The evidence for all the remaining cancer sites reviewed remains insufficient to make any conclusions at this time. No increased risk due to high levels of physical activity has been observed for any cancer type. Physical activity has an independent protective effect on site-specific cancer; this effect cannot be explained by potentially confounding factors such as body mass or diet.


CONCLUSIONS
We recommend including physical activity as a modifiable risk factor in order to reduce cancer risk throughout life. More studies focusing on biological mechanisms are needed. Furthermore, improvements in the physical activity assessments used associated with site-specific cancer risk is needed. Finally, there is a need for intervention studies designed to study the carcinogenic process and the specific cancer type with biological markers and intermediate steps in the development of cancer.
Thune
I
I
Institutt for samfunnsmedisin, Universitetet i Tromsø 9037 Tromsø. Inger.Thune@ism.uit.no
Smeland
S
S
D000818:Animals; D019317:Evidence-Based Medicine; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D009369:Neoplasms / Q000517:prevention & control*; D012307:Risk Factors