Background and Aim. Arsenic exposure is a global issue. Inorganic As (InAs) undergoes methylation to monomethyl (MMAs) and dimethyl-arsenical species (DMAs); full methylation to DMAs facilitates urinary excretion and can reduce risk of adverse health outcomes. Folate and creatine influence one-carbon metabolism which provides methyl groups for As methylation. We investigated the effects of folic acid (FA) and/or creatine supplementation on As metabolites and primary (PMI: MMAs/InAs) and secondary (SMI: DMAs/MMAs) methylation indices in blood in Bangladeshi adults. Methods. In this double-blind, placebo-controlled trial, 622 participants (20% folate deficient) were randomized to receive FA (400 or 800 μg/day), 3 g creatine/day, 3 g creatine+400 μg FA/day, or placebo for 12 weeks. For the following 12 weeks, half of the FA participants received placebo. All participants received As-removal water filters. Results. Blood As metabolite concentrations decreased in all groups due to filter use. After 1 week, the mean within-person increase in SMI for the creatine+400FA group exceeded the placebo group. The mean decreases in blood concentrations of MMAs (bMMAs) between baseline and week 12 was greater for all treatment groups compared to placebo, and the increase in bDMAs concentrations for the FA treated groups exceeded that of placebo. The decrease in PMI and increase in SMI in all FA groups at week 12 exceeded placebo. Data from week 24 showed evidence of rebound from week 12, with decreases in SMI in those who switched from 800FA to placebo, whereas for those who remained on 800FA, PMI and bMMAs concentrations continued to decline. Conclusions. FA supplementation lowered bMMAs and increased bDMAs in adults in Bangladesh. Evidence of rebound in As methylation capacity following FA cessation suggests short-term benefits of supplementation and underscores the importance of long-term interventions such as FA fortification and As mitigation in drinking water. Keywords: Arsenic, Nutritional Intervention
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