Transfer of Vitamin A from Bovine Liver to Milk

The contributions of liver stores and the diet to the vitamin A contents of colostrum and milk were studied in eight mature Angus cows with previously established stores of tritium-labeled vitamin A. Samples of liver, serum and milk were taken within 2 hr. after parturition and at intervals during the first 12 weeks of lactation. Total vitamin A and alcohol and ester forms of vitamin A were determined on these samples as well as the radioactivity associated with each fraction. Average results indicated that liver stores furnished 36% of the vitamin A in colostrum and 55% of the vitamin A in milk. Most of the vitamin A activity and radioactivity in liver, colostrum and milk was present in the ester fraction. In serum, about 70% of the vitamin A activity and all of the radioactivity appeared in the alcohol fraction. Specific activities for vitamin A in colostrum and milk were consistently higher than for serum. The results indicate that vitamin A is removed from the liver as the alcohol and that the mammary gland is capable of transferring vitamin A alcohol from blood to milk in preference to vitamin A ester.