The essential nutrients are those materials which must be supplied by the diet to produce and maintain optimum health. Several species of animals have now been raised to maturity upon diets of essentially known composition and there is reason to believe that practically all of the essential nutrients have been identified. Nearly all of these have been synthesized and the gross effects of the corresponding deficiency diseases have been documented. On the other hand, the absolute amounts required under varied conditions and by different species, the long term effects of slight deficiencies or excesses and the biochemical mechanisms involved, are still being actively investigated. Few statements in this field can be accepted as definite and the reader should be duly cautions. The original expectation that once the various factor were identified and requirements determined it would be relatively simple to determine which diets were adequate or inadequate, is still far in the future. We now recognize numerous interrelationships between nutrients, variation between individuals of the same species, great differences among species, intestinal synthesis of variable amount depending upon the diet and sanitary conditions, adaptations to diets, and there are undoubtedly other factors influencing requirements. When one considers the permutations possible among the forty odd nutrients, the possibilities and difficulties become clear.
Nutritional requirements must be considered as approximations based upon the average individual and perhaps under average conditions as we understand them today. They are much less applicable to the individual. Recommended allowances as distinct from requirements are always set somewhat above estimated requirements to provide a margin of safety.