Total blood mercury, plasma homocysteine, methylmalonic acid and folate in US children aged 3-5Â years, NHANES 1999-2004

► A subset of US boys aged 3-5 years, with both plasma methylmalonic acid (MMA) above and serum folate levels below the sample medians, showed a significant inverse association between total blood mercury and plasma homocysteine. This relationship was not evident in all other US boys and girls aged 3-5 years. ► Boys with higher MMA and lower serum folate levels had significantly higher mean plasma homocysteine levels compared to all other boys. ► Boys with higher MMA and lower serum folate levels had significantly lower mean total blood mercury levels compared to girls. ► Among boys with both higher MMA and lower serum folate levels, total blood mercury was significantly inversely associated with homocysteine at levels of mercury exposure well below the 5.8 μg/L reference dose for maternal cord blood methylmercury exposure associated with adverse neurobehavioral endpoints.