Intra-day and inter-day variations of methylmercury and inorganic mercury concentrations in breast milk

There is little information regarding the concentrations and variability of methylmercury (MeHg) in breast milk. This study examines the variations in the concentrations of total mercury (THg), MeHg, and inorganic mercury (IHg) in breast milk and suggests a method for collecting breast milk samples for MeHg research.

Breast milk samples were obtained from 11 lactating women, 3-5 months after birth. Mercury species in breast milk were measured by high-performance liquid chromatography with inductively coupled plasma mass spectrometry. Breast milk was examined for foremilk and hindmilk, intra-day (morning, afternoon, evening), and inter-day (morning for three consecutive days) variations. The variation in concentration among individuals was measured using the intraclass correlation coefficient (ICC).

The mean maternal age was 33.1 years, 45.5 % of children were first children, and 63.6 % were male. The median MeHg in foremilk was 0.18 (0.05-0.55) ng/g and in hindmilk was 0.58 (0.18-1.18) ng/g. The variation between foremilk and hindmilk was significant for MeHg and THg but minor for IHg. Intra- and inter-day variations in mercury were analyzed using ICC, and all concentrations were >0.9, indicating high reliability. MeHg to THg ratios were 66.7 % and 75.9 % in foremilk and hindmilk.

This study found variations in THg and MeHg concentrations between foremilk and hindmilk samples. To assess MeHg levels in breast milk, collect both foremilk and hindmilk for analysis. No intra- or inter-day variations were observed for MeHg or IHg. More infant exposure info is useful for precise analysis.