All-trans retinol is the main form of vitamin A present in milk, the cis isomer of retinol being sometimes observed in a very low amount in cow’s milk.
The amount of vitamin A in milk varies with the carotene content of the feed. Normally, vitamin A potency is highest when the cow is on succulent pastures in the spring and lowest when the cow is hay-fed during the winter season.
The vitamin A potency of milk can be increased to a level that approaches summer milk by feeding rations high in carotene content.
Concentrations of vitamin A and carotenoids in milk are also dependent on animal species. Indeed caprine milk is richer (30%) in retinol and conversely beta-carotene is 30% higher in cow’s milk than goat’s milk.
Guernsey milk contained more carotene than preformed vitamin A, Jersey approximately equal proportions whereas the Holstein milk the carotene constituted approximately 30 percent and the vitamin A, 70 percent of the total biological active vitamin A.
Vitamin A in cow’s milk
