Meta-analyses of Observational and Genetic Association Studies of Folate Intakes or Levels and Breast Cancer Risk

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Abstract
Background: Evidence from case–control studies suggests that increasing dietary folate intake is associated with a reduced risk of breast cancer. However, large cohort studies have found no such association, and animal studies suggest that folate supplementation may promote tumorigenesis. We conducted a meta-analysis to summarize the available evidence from observational studies on this issue and a meta-analysis of the association between a common polymorphism in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene, a key enzyme in folate metabolism, and breast cancer risk. Methods: We searched Medline and ISI Web of Knowledge databases for relevant studies that were published through May 31, 2006. We used random-effects analysis to calculate odds ratios (ORs) for case–control studies or relative risks (RRs) for cohort studies for a 100-μg/d increase in folate intake. Unadjusted odds ratios were calculated for the studies of MTHFR genotype based on published genotype frequencies. Results: A total of 13 case–control studies and nine cohort studies were included in the meta-analysis of folate intake and breast cancer risk. We found a summary OR of 0.91 (95% confidence interval [CI] = 0.87 to 0.96) from the case–control studies and a summary RR of 0.99 (95% CI = 0.98 to 1.01) from the cohort studies for a 100-μg/d increase in folate intake. We found evidence that the case–control studies may have suffered from substantial publication bias. The case–control and cohort studies may have been subject to measurement error, confounding, and possibly spurious associations arising from subgroup analyses; in addition, the case–control studies were potentially subject to recall bias and publication bias. Seventeen studies were included in the meta-analysis of MTHFR C677T genotype and breast cancer risk. We found no difference in breast cancer risk between MTHFR 677 TT homozygotes and CC homozygotes (OR = 1.05, 95% CI = 0.88 to 1.25), and there was no evidence of an interaction between folate intake and MTHFR genotype on breast cancer risk. Conclusion: A lack of dietary folate intake is not associated with the risk of breast cancer.