Breast milk selenocystine as a biomarker for selenium intake in lactating women at differential geographical deficiency risk in China

Background and Objectives: A reliable biomarker for optimal selenium (Se) intake in lactating women is not currently available. Methods and Study Design: Daily dietary Se intake in lactating women was calculated from a 24-hour meal record survey for over 3 days. Se levels in plasma and breast milk were measured through inductively coupled plasma mass spectrometry. Plasma selenoprotein P 1 levels and glutathione peroxidase 3 activity were measured using an enzyme-linked immunosorbent assay. Ultra-performance liquid chromatography-tandem mass spectrometry was used to analyze proteinaceous Se species in enzymatically digested breast milk. Results: Dietary Se intakes of lactating women from Liangshan, Beijing, and Enshi were 41.6 +/- 21.2 ng/d, 51.1 +/- 22.6 ng/d, and 615 +/- 178 ng/d, respectively (p<0.05). The Se levels in the blood and breast milk were significantly associated with the dietary Se intake (p<0.05). The proteinaceous Se species in breast milk were SeMet and SeCys2. The levels of SeMet in the lactating women from Liangshan, Beijing, and Enshi were 3.31 +/- 2.44 ng Se/mL, 7.34 +/- 3.70 ng Se/mL, and 8.99 +/- 9.64 ng Se/mL, while that of SeCys2 were 13.7 +/- 12.0 ng Se/mL, 35.6 +/- 20.9 ng Se/mL, and 57.4 +/- 13.2 ng Se/mL, respectively. Notably, the concentration of SeCys2, the metabolite of unstable SeCys, reached a saturation platform, whereas no similar phenomenon were found for the total Se SeMet from Se-containing proteins. Conclusions: SeCys2 in breast milk is a potential biomarker for determining the optimal Se intake in lactating women.