Postnatal fall and rise of circulating 25-hydroxyvitamin D in the rat*.

Plasma 25-hydroxyvitamin D levels (5 to 6 ng. per millilter at birth) decreased significantly at age 12 days (1to 2 ng. per millilter) in rat neonates. At 3 to 4 weeksof age, the plasma concentrations of 25-hydroxyvitamin D had risen to birth levels or higher. Chromatography of lipid extracts of neonatal tissues following in vivo administration of 'H-25-hydroxyvitamin D' from the milk of mothers who had been given 'H vitamin D' intravenously. Following intraperitoneal injection of equal (or 3-fold larger doses to older pups) doses of 'H-25-hydroxycholecalciferol, the half-lives of 'H in plasma were 2.9 to 3.0 days and 2.8 to 3.05 days in young and oldersuckling pups, respectively. These data collectively suggest that young suckling rats,unlike weaning rats, do not receive and/or produce sufficient 25-hydroxyvitamin D to maintain the blood concentration observed at birth.