Physical activity and colorectal cancer risk: an evaluation based on a systematic review of epidemiologic evidence among the Japanese population.

OBJECTIVE: Higher levels of physical activity have been consistently associated with a decreased risk of colon cancer, but not rectal cancer, in Western populations. The present study systematically evaluated epidemiologic evidence on the association between physical activity and colorectal cancer risk among the Japanese population.
METHODS: Original data were obtained from MEDLINE searched using PubMed or from searches of the Ichushi database, complemented by manual searches. The associations were evaluated based on the strength of evidence, the magnitude of association and biologic plausibility.
RESULTS: Two cohort studies and six case-control studies were identified. A weak to strong protective association between physical activity and colon cancer risk was observed in both cohort studies, showing a graded relationship, and among the majority of case-control studies, with some showing a dose-response relationship. The association observed in cohort studies was more consistent and stronger in men than in women and for proximal colon cancer than for distal colon cancer. A protective association with rectal cancer was found only in case-control studies, but the evidence was less consistent and weaker than that observed for colon cancer.
CONCLUSIONS: Physical activity probably decreases the risk of colorectal cancer among the Japanese population. More specifically, the evidence for the colon is probable, whereas that for the rectum is insufficient.