As knowledge of nutritional needs expanded, nutrients were classified according to their essentiality. This type of classification was applied initially to amino acids. In the early 1920s, Mendel used the term indispensable for amino acids that are not synthesized in the body. The term nonessential was used widely for those that are not required in the diet. This term was not considered satisfactory because these amino acids, although not required in the diet, are physiologically essential. Block and Bolling used the term indispensable for organic nutrients with carbon skeletons that are not synthesized in the body, and dispensable, which does not carry the broad implication of the term nonessential, for those with carbon skeletons that can be synthesized (15, 16).
Nutritional essentiality is characteristic of the species, not the nutrient. Arginine is required by cats and birds but not by humans. Also, it is not synthesized by the young of most species in amounts sufficient for rapid growth. It may, therefore, be either dispensable or indispensable depending on the species and stage of growth. Ascorbic acid (vitamin C), which is required by humans and guinea pigs, is not required by most species.
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