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. The intestine is a source of Biotin, Pantothenic Acid and Vitamin K; these are supplied by "friendly"' intestinal bacteria, though the exact amounts supplied are difficult to assess.

The term "vitamin" dates from 1912, and the first vitamin to be isolated was vitamin A in 1913. Thiamin was discovered in 1926, vitamin K in 1929, and vitamin C in 1932. Vitamin B₁₂ was the most recent vitamin to be discovered (1948). 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. B vitamins are universally distributed; fat-soluble vitamins may be absent from some types of organisms. Each of the vitamins plays a specific role in the body; a deficiency of one vitamin cannot be eliminated by consuming an excess of another.

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. As an example, vitamin C in cells is defined chemically as L-ascorbic acid, identical to synthetic L-ascorbic acid. 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. Synthetic vitamin E, called d,l-alpha-tocopherol, is a mixture of both left- and right-handed molecules, while the natural alpha-tocopherol is a single form called d-tocopherol. The synthetic product is adjusted to provide the same biological activity as the natural form. Certain vitamins like vitamin B₁₂ possess structures that are too complex for a convenient lab synthesis. Microbial sources have been selected to produce large amounts.

LINKS: Vitamins Here some sample abstract titles to give you some general info on the value of supplementing your diet with daily intakes of absorbable vitamins and just one disease condition --- CANCER.

• Chemoprevention of Animal Tumors by Retinoids. Richard C. Moon, David L. McCormick, Rajendra G. Mehta. Laboratory of Pathophysiology, Life Sciences Division, IIT Research Institute, Chicago, Ill., USA

• In vitro Regression of Naturally Occurring Alveolar Lesions of Mouse Mammary Glands Induced by Vitamin A. N. T. Telang, N.H. Sarkar'. Laboratory of Molecular Virology, Memorial Sloan-Kettering Cancer Center and Department of Genetics and Molecular Biology, Sloan-Kettering Division, Graduate School of Medical Sciences, Cornell University, New York, N.Y., USA

• Chemoprevention of Azo-Dye-Induced Liver Carcinogenesis in the Rat by a Natural Carotenoid. R. W. Shearer. Issaquah Health Research Institute, Issaquah, Wash., USA

• Radioprotection by Vitamin E: Effects on Hepatic Enzymes, Delayed Type Hypersensitivity, and Postirradiation Survival of Mice. V Srinivasan 3, A.J. Jacobs, S.A. Simpson, IF Weiss. Biochemistry Department, Armed Forces Radiobiology Research Institute, Bethesda, Md., USA

• Influence of Vitamin C on Estrogen-Induced Renal Carcinogenesis in Syrian Hamster. Joachim G. Liehr, William J. Wheeler, Annie M. Ballatore. Analytical Chemistry Center and Department of Biochemistry and Molecular Biology, University of Texas Medical School at Houston, Houston, Tex., USA