BackgroundEvidence from observational studies suggests that inadequate folate status enhances colorectal carcinogenesis, but results from some randomized trials do not support this hypothesis. ObjectiveTo assess the effect of folic acid supplementation on recurrent colorectal adenoma, we conducted a cost-efficient, double-blind, randomized trial among participants of 2 large prospective cohorts, the Health Professionals Follow-Up Study and the Nurses’ Health Study. DesignParticipants were randomly assigned to receive folic acid (1 mg/d) (n = 338) or placebo (n = 334) for 3–6.5 y. The primary endpoint was any new diagnosis of adenoma during the study period (May 1996–March 2004). Secondary outcomes were adenoma by site and stage and number of recurrent adenomas. Associations were also examined by plasma folate concentrations at baseline. ResultsIncidence of at least one recurrent adenoma was not significantly associated with folic acid supplementation [relative risk (RR): 0.82; 95% CI: 0.59,1.13; P = 0.22]. Among participants with low plasma folate concentrations at baseline (≤7.5 ng/mL), those randomly assigned to receive folic acid experienced a significant decrease in adenoma recurrence (RR: 0.61; 95% CI: 0.42, 0.90; P = 0.01), whereas for subjects with high folate concentrations at baseline (>7.5 ng/mL), supplemental folic acid had no significant effect (RR: 1.28; 95% CI: 0.82, 1.99; P = 0.27, Pinteraction = 0.01). Contrary to findings from another clinical trial, there was no evidence for an increased risk of advanced or multiple adenomas. ConclusionsOur results do not support an overall protective effect of folic acid supplementation on adenoma recurrence. Folic acid supplementation may be beneficial among those with lower folate concentrations at baseline. This trial was registered at clinical trials.gov as NCT00512850.

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