Except for vitamin D and vitamin C, vitamins are present in animal tissue only if the animal consumes foods containing them or harbors microorganisms capable of synthesizing them. Natural vs. Synthetic Vitamins Natural vitamins are those occurring in food. All substances classified as vitamins have been isolated from animal or plant sources, and most have been chemically synthesized in the lab to establish their structures. In other words, synthetic vitamins are usually identical to the product in cells. Most vitamins found in supplements are chemically synthesized, because there simply is not enough of most vitamins extracted from plant materials to meet world demand.
Most vitamin C comes from a few major commercial sources worldwide.
A few synthetic vitamins differ from the natural forms. The synthetic product is adjusted to provide the same biological activity as the natural form. Certain vitamins like vitamin B 12 possess structures that are too complex for a convenient lab synthesis. Microbial sources have been selected to produce large amounts. Chemoprevention of Animal Tumors by Retinoids.