Vitamin D compounds in cows' milk.

The milk from cows fed normal levels of vitamin D has been found to contain approximately 40 IU per liter of vitamin D activity. A 14-fold increase in dietary vitamin D intake causes only a doubling of the amount of vitamin D in milk. This was determined by measuring stimulation of intestinal calcium transport in the vitamin D-deficient rat. Four vitamin D compounds were then isolated from cow's milk using a combination of conventional chromatography on Sephadex LH-20 and Lipidex 5000 followed by high-performance liquid chromatography. 24,25-Dihydroxycholecalciferol and 1,25-dihydroxycholecalciferol were measured using binding protein assays. One liter of milk contained 27 ng and 4.9 ng, respectively, of these two metabolites. Together these account for about 15% of the vitamin D activity. Cholecalciferol was found to be present at a concentration of 281 ng/liter or 11 IU/liter of biological activity. The milk contained 145 ng/liter 25-hydroxycholecalciferol or 29 IU/liter of activity. Therefore the known vitamin D compounds fully account for the biological activity observed in milk. It is therefore clear that no evidence could be found for the existence of a highly active water-soluble form of vitamin D in milk.