Vitamin; types, sources and functions

What is Vitamin?

 Among the nutrients required for the many physiologic functions essential to life are the vitamins. Unlike other classes of nutrients, the vitamins do not serve structural functions, nor does their catabolism provide significant energy. Instead, their various uses each tend to be highly specific, and, for that reason, the vitamins are required in only small amounts in the diet. The common food forms of most vitamins require some metabolic activation to their functional forms.

 Vitamin is an organic compound distinct from fats, carbohydrates, and proteins l is a natural component of foods in which it is usually present in minute amounts l is essential, also usually in minute amounts, for normal physiological function (i.e., maintenance, growth, development, and/or production) l causes, by its absence or underutilization, a specific deficiency syndrome l is not synthesized by the host in amounts adequate to meet normal physiological needs. It will quickly become clear, however, that, despite its usefulness, this operating definition has serious limitations, notably with respect to the last clause, for many species can indeed synthesize at least some of the vitamins. Four examples illustrate this point:

Vitamin; types, sources and functions


Vitamin C: Most animal species have the ability to synthesize ascorbic acid. Only those few that lack the enzyme l-gulonolactone oxidase (e.g., the guinea pig, humans) cannot; only for them can ascorbic acid properly be called vitamin C

Vitamin D: Individuals exposed to modest amounts of sunlight can produce cholecalciferol, which functions as a hormone. Only individuals without sufficient exposure to ultraviolet light (e.g., livestock raised in indoor confinement, people spending most of their days indoors) require dietary sources of vitamin D.

Choline: Most animal species have the metabolic capacity to synthesize choline; however, some (e.g., the chick, the rat) may not be able to employ that capacity if they are fed insufficient amounts of methyl-donor compounds. In addition, some (e.g., the chick) do not develop that capacity fully until they are several weeks of age. Thus, for the young chick and for individuals of other species fed diets providing limited methyl groups, choline is a vitamin.

Niacin: All animal species can synthesize nicotinic acid mononucleotide (NMN) from the amino acid tryptophan. Only those for which this metabolic conversion is particularly inefficient (e.g., the cat, fishes) and others fed low dietary levels of tryptophan require a dietary source of niacin. With these counterexamples in mind, the definition of a vitamin can be understood as having specific connotations for animal species, stage of development, diet or nutritional status, and physical environmental conditions.

The “vitamin caveat:”

Some compounds are vitamins for one species and not another. Some compounds are vitamins only under specific dietary or environmental conditions. Thirteen substances or groups of substances are now generally recognized as vitamins; others have been proposed.5 In some cases, the familiar name is actually the generic descriptor for a family of chemically related compounds having qualitatively comparable metabolic activities. For example, the term vitamin E refers to those analogs of tocol or tocotrienol6 that are active in preventing such syndromes as fetal resorption in the rat and myopathies in the chick. In these cases, the members of the same vitamin family are called vitamers. Some carotenoids can be metabolized to yield the metabolically active form of vitamin A; such a precursor of an actual vitamin is called a provitamin.



The Vitamins

Fourth Edition

Gerald F. Combs, Jr

Professor Emeritus

Cornell University

Ithaca, NY

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