Methylmercury Exposure While Breast Feeding?

I was asked recently about the potential for mercury exposure to an infant during breast feeding. Not much is known about it, but a Swedish study was published late last year in Environmental Health Perspectives. This study of 20 mothers and their infants showed that a small amount of methylmercury was transferred to an infant from breast milk. Infant exposures declined throughout the first 13 weeks of life, which was not entirely expected because infants do not readily detoxify methylmercury through demethylation. The decline is thought to be due to rapid infant growth. Median blood-methylmercury levels in the infant declined from 1.1 ug/L four days after birth to 0.38 ug/L after 13 weeks. The highest blood-methylmercury in an infant four days after birth was 4.4 ug/L and 1.1 ug/L at 13 weeks. These levels fall below the 5.8 ug/L level used to develop the Reference Dose for methylmercury. The most interesting observation by the investigators was that the women apparently complied well with mercury advisories in fish, and limited their consumption of freshwater fish that might contain high mercury concentrations (marine fish consumption was not recorded, a bit of an oversight for our uses, given the popularity of tuna as a source for fish in the diet). Maternal blood-methylmercury levels generally were less than 2 ug/L (in the NHANES survey, the 75th percentile blood-methylmercury level for all women ages 16 to 49 was 1.7 ug/L). The NHANES survey reported that half of the women who reported eating fish at least twice a week had a blood-methylmercury of 3 ug/L or greater.

The lack of data on seafood consumption limits the use of this study, but overall it appears that if you are concerned about limiting methylmercury exposure to your infant, you’ll continue to adhere to fish consumption advisories while breast feeding. Don’t forget to take your fish oil capsules.