The concept of nutritional essentiality was firmly established less than 100 years ago. It arose from observations that certain diseases observed in human populations consuming poor diets could be prevented by including other foods in the diet and that failure of animals fed on diets composed of purified components or restricted to one or a few foodstuffs to grow and survive could similarly be corrected by including another food or an extract of the food in the diet. The food constituents that were found to prevent these problems were classified as indispensable (or essential) nutrients. Nutrients that could be deleted from the diet without causing growth failure or specific signs of disease were classified as dispensable (or nonessential).
This classification of nutrients served well through the 1950s as the basis of recommendations for treating dietary deficiency diseases, offering dietary advice to the public, and establishing food regulations and policy. As information about nutrients accrued, however, some essential nutrients were found to be synthesized from precursors, interactions among some nutrients in the diet were found to influence the need for others, and later, in some conditions, such as prematurity, certain pathologic states, and genetic defects, the ability of the body to synthesize several nutrients not ordinarily required was found to be so impaired that a dietary source was needed. As a result, the system of classifying nutrients simply as indispensable or dispensable has been modified to include a category of conditional essentiality (1).
Recently, associations observed between the risk of developing certain chronic and degenerative diseases and the consumption of some dispensable nutrients and nonnutrient components of foods, as well as the beneficial effects sometimes observed with high intakes of some essential nutrients, have raised questions about the adequacy of the present system of nutritional classification of food constituents ( 2, 3, 4, 5 and 6). In this chapter evolution of the concept of nutritional essentiality is outlined and problems encountered in classifying food constituents on the basis of their effects on health and disease are identified.
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