What is the precise mechanism by which exercise reduces chance of cancer?
Vigorous exercise (even for five minutes) causes increased DNA damage http://www.ncbi.nlm.nih.gov/pubmed/20839226. The increased DNA damage would stimulate the defenses including the immune system (e.g. natural killer cells) http://www.ncbi.nlm.nih.gov/pubmed/16618710, and the boosted immune system would eliminate cancer cells, reducing cancers based on the immune suppression model of cancer http://www.ncbi.nlm.nih.gov/pubmed/27928220. The reduction of cancers with increased exercise (physical activity) has indeed been observed http://www.ncbi.nlm.nih.gov/pubmed/18506190.