Breastfeeding and breast cancer risk: an evaluation based on a systematic review of epidemiologic evidence among the Japanese population.

OBJECTIVE We reviewed epidemiological studies on breastfeeding and breast cancer among Japanese women. This report is part of a series of articles written by our research group, whose aim was to evaluate the existing evidence concerning the association between health-related lifestyles and cancer. METHODS Original data were obtained from MEDLINE searches using PubMed or from searches of the Ichushi database, complemented by manual searches. Evaluation of associations was based on the strength of evidence and the magnitude of association, together with biological plausibility. RESULTS Three cohort studies and five case-control studies were identified. Cohort studies failed to find a significant inverse association between breastfeeding and the risk of breast cancer. Most of the case-control studies observed a statistically significant or non-significant risk reduction for women who ever had breastfed or for women with a longer duration of breastfeeding. Experimental studies have supported the biological plausibility of a protective effect of breastfeeding on breast cancer risk. CONCLUSIONS We conclude that breastfeeding possibly decreases the risk of breast cancer among Japanese women.