2.6. Total Circulating 25(OH)D Concentrations
Infant blood samples were collected at delivery and again at the 3-to-5-year-old visits. Serum samples were stored at −20 °C and were then assayed in duplicate using the Diasorin commercial radioimmunoassay (Diasorin, Stillwater, MN) as previously described to measure the total 25(OH)D concentration [