Vitamin D in Milk

Published: January 14, 2003

Q. I have been told that the vitamin D added to skim milk (because removing fat removes the D) remains entirely on the side of the container, so that it is effectively not available. Is this true?

A. ''The amount of vitamin D3 in the milk, and the amount that one gets from its consumption, is the amount that's stated on the label,'' said Dr. Sheldon S. Hendler, co-editor of the PDR for Nutritional Supplements.

Vitamin D3, or cholecalciferol, the form of D found in milk, is practically insoluble in water, he said, so one would think there would be none or very little in fat-free milk. But, he said, vitamin D in any type of milk, fat-free or not, is mainly bound to proteins (which are the same in all types of milk) and is not found free in solution or bound to the wall of the milk carton.

''This is also how vitamin D is transported in the blood,'' he said, ''bound to vitamin D-binding proteins in the blood.''

Vitamin D is a conditional vitamin, Dr. Hendler explained; that is, it is only a vitamin, a substance required in the diet, when the body does not make it in the skin through the action of ultraviolet rays from the sun. The vitamin D in milk comes either from the cow's milk itself (the cow also makes cholecalciferol in its skin through the UV activation of 7-dehydrocholesterol) or from the fortification of the milk with cholecalciferol. Most of the vitamin D3 in milk comes from fortification, Dr. Hendler said. C. CLAIBORNE RAY