Arsenic methylation efficiency increases during the first trimester of pregnancy independent of folate status

Exposure to inorganic arsenic during pregnancy may negatively influence the offspring, though efficient metabolism of arsenic to dimethylarsinic acid (DMA) likely reduces the health risks. This study aimed to evaluate methylation of arsenic over the entire pregnancy and the influence of nutritional status. We studied longitudinally the arsenic metabolite pattern in the urine of 324 pregnant women exposed to arsenic via drinking water and food in rural Bangladesh. Metabolism of arsenic to DMA increased markedly over the course of pregnancy, with the greatest improvement occurring in the first trimester, along with a marked decrease in the most risk-associated monomethylated metabolite. This improvement in methylation was not associated with nutritional status, including vitamin B12 and folate. Efficient methylation to DMA was associated with improved urinary excretion of arsenic, relative to blood arsenic concentrations, indicating that micronutrient-independent up-regulation of arsenic metabolism already in early pregnancy may provide protection for the fetus.