Being Breastfed in Infancy and Risk of Colorectal Cancer and Precursor Lesions

Background & Aims: Emerging evidence implicates the importance of perinatal and early-life exposures in colorectal cancer (CRC) development. However, it remains unclear whether being breastfed in infancy is associated with CRC risk in adult life, particularly early adulthood. Methods: We prospectively investigated the association between history of being breastfed and risk of CRC and its precursor lesions among 66,634 women 46–93 years of age from the Nurses’ Health Study and 92,062 women 27–68 years of age from the Nurses’ Health Study II. Cox regression and logistic regression for clustered data were used to estimate hazard ratios for CRC and odds ratios for CRC precursors, respectively. Results: During 3.5 million person-years of follow-up, we identified 1490 incident cases of CRC in 2 cohorts. Having been breastfed was associated with a 23% (95% confidence interval [CI], 10% to 38%) increased risk of CRC. The risk of CRC increased with duration of being breastfed (P_trend < .001). These findings were validated using breastfeeding information from the mothers of a subset of participants. Among younger participants from the Nurses’ Health Study II, a significant association was observed between being breastfed and increased risk of high-risk adenomas under 50 years of age (odds ratio, 1.46; 95% CI, 1.16 to 1.83). Consistently, having been breastfed was associated with increased risk of CRC among participants ≤55 years of age (hazard ratio, 1.38; 95% CI, 1.06 to 1.80). Conclusions: Being breastfed in infancy was associated with increased risk of CRC in adulthood, including among younger adults. However, further research is needed to understand the underlying biological mechanisms, as this association does not establish causation.