Betaine and choline status modify the effects of folic acid and creatine supplementation on arsenic methylation in a randomized controlled trial of Bangladeshi adults

Abstract

Purpose: Methylation of ingested inorganic arsenic (InAs) to monomethyl- (MMAs) and dimethyl-arsenical species (DMAs) facilitates urinary arsenic elimination. Folate and creatine supplementation influenced arsenic methylation in a randomized controlled trial. Here, we examine if baseline status of one-carbon metabolism nutrients (folate, choline, betaine, and vitamin B12) modified the effects of FA and creatine supplementation on changes in homocysteine, guanidinoacetate (GAA), total blood arsenic, and urinary arsenic metabolite proportions and indices.

Methods: Study participants (N = 622) received 400 or 800 μg FA, 3 g creatine, 400 μg FA + 3 g creatine, or placebo daily for 12 weeks.

Results: Relative to placebo, FA supplementation was associated with greater mean increases in %DMAs among participants with betaine concentrations below the median than those with levels above the median (FDR < 0.05). 400 μg FA/day was associated with a greater decrease in homocysteine among participants with plasma folate concentrations below, compared with those above, the median (FDR < 0.03). Creatine treatment was associated with a significant decrease in %MMAs among participants with choline concentrations below the median (P = 0.04), but not among participants above the median (P = 0.94); this effect did not significantly differ between strata (P = 0.10).

Conclusions: Effects of FA and creatine supplementation on arsenic methylation capacity were greater among individuals with low betaine and choline status, respectively. The efficacy of FA and creatine interventions to facilitate arsenic methylation may be modified by choline and betaine nutritional status.

Clinical trial registration: Clinical Trial Registry Identifier: NCT01050556, U.S. National Library of Medicine, https://clinicaltrials.gov ; registered January 15, 2010.

Keywords: Arsenic methylation; Betaine; Choline; Creatine; Folic acid; One-carbon metabolism.

Conflict of interest statement

Conflicts of interest/Competing interests: The authors declare that they have no conflicts of interest or competing interests.

Figures

Figure 1

Arsenic methylation. Arsenite (AsIII) is methylated to form monomethylarsonic acid (MMAsV) by arsenic methyltransferase (AS3MT) using the methyl donor S-adenosylmethionine (SAM). MMAsV is subsequently reduced to monomethylarsonous acid (MMAsIII) and methylated to form dimethylarsinic acid (DMAsV).

Figure 2

(a) One-carbon metabolism. Folic acid (FA) is reduced to dihydrofolate and tetrahydrofolate (THF) by dihydrofolate reductase. 5,10-methylene-THF is formed by serine hydroxymethyl-transferase through the transfer of one-carbon units from serine to THF, which is for thymidylate synthesis or reduced to 5-methyl-THF. Folate obtained through the diet can enter one-carbon metabolism as 5-methyl-THF. A one-carbon unit is transferred from 5-methyl-THF to homocysteine by methionine synthase using vitamin B12 as a cofactor to form methionine and THF. Homocysteine can also be remethylated in the liver by betaine homocysteine methyltransferase using betaine as the methyl donor. Methionine is activated to from S-adenosylmethionine (SAM) by methionine adenosyltransferase enzymes. SAM serves as the methyl donor for numerous reactions including arsenic methylation and biosynthesis of creatine, generating the methylated products and S-adenosylhomocysteine (SAH). SAH, which serves as a product inhibitor for most methyltransferase enzymes, hydrolyzed to homocysteine, and can either be remethylated to methionine or be directed towards the transsulfuration pathway. Adapted with permission from [15]. (b) Major consumers of SAM. An estimated 50% of SAM is consumed by the final step of endogenous creatine synthesis by GAMT, and 40% of SAM is consumed by phosphatidylcholine biosynthesis. (c) Creatine metabolism and the methionine cycle. In the kidney, arginine:glycine amidinotransferase (AGAT) produces guanidinoacetate (GAA). Dietary and/or supplemental creatine reduces GAA biosynthesis through the pretranslational inhibition of AGAT. GAA is released from the kidney and taken up by the liver where it is methylated using the methyl donor SAM to form creatine and SAH. SAH is hydrolyzed to homocysteine. 5-mTHF can regulate SAM and SAH levels through potent inhibition of GNMT. Creatine is transported to tissues and phosphorylated to phosphocreatine. Creatine and phosphocreatine are converted to creatinine and excreted in urine.

Figure 3

Differences in mean change in urinary As metabolite proportions (week 12 – week 0) between treatment and placebo groups stratified by baseline choline and betaine below and above median. P-values are from Wald test for differences between strata in treatment effects based on linear models for within-person change in As metabolite proportions adjusting for baseline levels of As metabolite proportions, Baseline choline median = 11.42 nmol/mL; baseline betaine median = 43.63 nmol/mL. * Wald test for differences between strata P < 0.10; ** P < 0.05; *** FDR < 0.05.