Vitamin D is absent from many multi-vitamins because it is fat-soluble rather than water-soluble, more expensive and the recommended doses are open to debate. What's more, it's one of the few vitamins with significant negative effects if overdosed. People don't know how to dose milligrams and give absurd quantities to newborn babies with terrible consequences.

Oral absorption is not very good. It is generally more effective to spend an afternoon outdoors.