Vitamins either cannot be synthesized by the body or they cannot be made in adequate amounts, so they must be supplied by the diet. As examples of the latter, vitamin D can be made in the skin when exposed to sunlight, while some Niacin can be made from the amino acid tryptophan. Thiamin was discovered in 1926, vitamin K in 1929, and vitamin C in 1932. Before a compound can be classified as a vitamin, it must be proven that animals must obtain the compound from their diet. Typically, scientists test lab animals such as mice with a diet free of the test substance, together with a dose of antibiotics to eliminate intestinal bacteria.
Vitamins originate chiefly from plant sources.
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.