Folic acid supplementation enhances arsenic methylation: results from a folic acid and creatine supplementation randomized controlled trial in Bangladesh

Abstract

Background: Arsenic exposure through drinking water persists in many regions. Inorganic As (InAs) is methylated to monomethyl-arsenical species (MMAs) and dimethyl-arsenical species (DMAs), facilitating urinary excretion. Arsenic methylation is dependent on one-carbon metabolism, which is influenced by nutritional factors such as folate and creatine.

Objective: This study investigated the effects of folic acid (FA) and/or creatine supplementation on the proportion of As metabolites in urine.

Design: In a 24-wk randomized, double-blinded, placebo-controlled trial, 622 participants were assigned to receive FA (400 or 800 μg per day), 3 g creatine per day, 400 μg FA + 3 g creatine per day, or placebo. The majority of participants were folate sufficient; all received As-removal water filters. From wk 12-24, half of the participants receiving FA received placebo.

Results: Among groups receiving FA, the mean decrease in ln(%InAs) and %MMAs and increase in %DMAs exceeded those of the placebo group at wk 6 and 12 (P < 0.05). In the creatine group, the mean decrease in %MMAs exceeded that of the placebo group at wk 6 and 12 (P < 0.05); creatine supplementation did not affect change in %InAs or %DMAs. The decrease in %MMAs at wk 6 and 12 was larger in the 800 µg FA than in the 400 µg FA group (P = 0.034). There were no differences in treatment effects between the 400 µg FA and creatine + FA groups. Data suggest a rebound in As metabolite proportions after FA cessation; at wk 24, log(%InAs) and %DMAs were not significantly different than baseline levels among participants who discontinued FA supplementation.

Conclusions: The results of this study confirm that FA supplementation rapidly and significantly increases methylation of InAs to DMAs. Further research is needed to understand the strong cross-sectional associations between urinary creatinine and As methylation in previous studies. This trial was registered at https://clinicaltrials.gov as NCT01050556.

FIGURE 1

Arsenic methylation. Ingested inorganic As (InAs) undergoes a series of stepwise biotransformation reactions. Arsenite (As^III) is actively methylated by arsenic methyltransferase (AS3MT) using the methyl donor S-adenosylmethionine (SAM) to form monomethylarsonic acid (MMAs^V). MMAs_V is reduced to monomethylarsonous acid (MMAs^III), and MMAs^III is methylated to form dimethylarsinic acid (DMAs^V). GSH, reduced glutathione; GSSG, oxidized glutathione.

FIGURE 2

Study design. 400FA, 400 μg FA per day treatment group; 800FA, 800 μg FA per day treatment group; creatine, 3 g creatine per day treatment group; creatine + 400FA, 3 g creatine and 400 μg FA per day treatment group; FA, folic acid.

FIGURE 3

Total urinary As, SG, and total urinary As adjusted for SG. Placebo (n = 90); 400FA (n = 133); 800FA (n = 129); creatine (n = 94); creatine + 400FA (n = 96). 400FA, 400 μg FA per day treatment group; 800FA, 800 μg FA per day treatment group; creatine, 3 g creatine per day treatment group; creatine + 400FA, 3 g creatine and 400 μg FA per day treatment group; FA, folic acid; SG, specific gravity.

FIGURE 4

Percentage urinary As metabolites at wk 0–12. Symbols represent raw means, and lines represent the predicted means from a linear model with repeated measures of urinary As metabolites. As shown in Table 2, ln(%InAs) was significantly different between treatment groups at wk 1, 6, and 12; %MMAs and %DMAs were significantly different between treatment groups at wk 6 and 12 (ANOVA P < 0.001). The 800FA group had significantly lower ln(%InAs) at wk 1 than the placebo group (Dunnett's t test P < 0.05). The 3 groups receiving FA supplements had significantly lower %InAs and %MMAs, and higher %DMAs than the placebo group at wk 6 and 12 (Dunnett's t test P < 0.05). The proportions of urinary As metabolites were not significantly different between the creatine and placebo groups at any follow-up (Dunnett's t test P < 0.05). Placebo (n = 90); 400FA (n = 133); 800FA (n = 129); creatine (n = 94); creatine + 400FA (n = 96). 400FA, 400 μg FA per day treatment group; 800FA, 800 μg FA per day treatment group; creatine, 3 g creatine per day treatment group; creatine + 400FA, 3 g creatine and 400 μg FA per day treatment group; DMAs, dimethyl-arsenical species; FA, folic acid; InAs, inorganic arsenic; MMAs, monomethyl-arsenical species.