Dietary Methionine Intake and Risk of Incident Colorectal Cancer: A Meta-Analysis of 8 Prospective Studies Involving 431,029 Participants

BACKGROUND: Methionine is one of the key components of one carbon metabolism. Experimental studies indicate that methionine may reduce inflammation-induced colon cancer. However, epidemiologic findings as to whether dietary methionine intake influences colorectal cancer incidence in humans are inconsistent. OBJECTIVE: To investigate the relationship between dietary methionine intake and risk of colorectal cancer by performing a meta-analysis of prospective studies. METHODS: Eligible studies were identified by searching PubMed and Embase and by reviewing the bibliographies of the retrieved publications. The summary risk estimates were computed using both a random- effects and a fixed-effects model. RESULTS: Eight eligible prospective cohort studies involving 431,029 participants and 6,331 colorectal cancer cases were identified. According to the random-effects model, the summary relative risks (RRs) for the highest compared with the lowest intake of methionine were 0.89 (95% confidence interval [CI] = 0.77-1.03) for colorectal cancer, 0.77 (95% CI = 0.64 - 0.92) for colon cancer, and 0.88 (95% CI = 0.55-1.42) for rectal cancer. In the stratified analysis, a significant inverse association between dietary methionine intake and risk of colorectal cancer was observed in studies with longer follow-up time (RR=0.81, 95% CI= 0.70- 0.95), in Western studies (RR= 0.83, 95% CI = 0.73 - 0.95) and in men (RR = 0.75, 95% CI= 0.57-0.99). We found no indication of publication bias. CONCLUSION: This meta-analysis indicates that dietary methionine intake may be associated with decreased risk of colorectal cancer, especially colon cancer. More prospective studies with long follow-up time are needed to confirm these findings.