Precursor-Product Relationships. Many substances that are physiologically, but not nutritionally, essential are synthesized from specific essential nutrients. If the products of the synthetic reactions are present in the diet, they may exert sparing effects that reduce the need for the precursor nutrients. Less phenylalanine and methionine are required, particularly by adults, when the diet includes tyrosine and cystine, for which they are, respectively, specific precursors. Birds, which do not synthesize arginine, have a high requirement for this amino acid. Inclusion in the diet of creatine, for which arginine is a precursor, reduces the need for arginine. Effects of this type, however, have not been explored extensively (23).
Precursors of Essential Nutrients. Tryptophan is a precursor of niacin. The need for niacin is therefore reduced by dietary tryptophan, but the efficiency of conversion differs for different species. The cat has an absolute requirement for niacin, but the rat converts tryptophan to niacin very efficiently. Human requirements for niacin are expressed as niacin-equivalents: 60 mg of dietary tryptophan equals 1 mg of niacin. b-Carotene, and to a lesser extent other carotenoids, are precursors of retinol (vitamin A). Human requirements for vitamin A are expressed as retinol-equivalents: 1 pg retinol-equivalent equals 1 pg of retinol or 6 pg of b-carotene. These are examples of interactions that alter the dietary need for essential nutrients ( 24). They are not examples of conditional essentiality.
Imbalances and Disproportions of Nutrients. High proportions of some nutrients in the diet can influence the need for others. This phenomenon was first recognized when additions of amino acids that stimulated growth of young rats fed on diets low in tryptophan and niacin were found to precipitate niacin deficiency—an example of a vitamin deficiency induced by an amino acid imbalance. With diets that contain adequate niacin but are low in tryptophan, amino acid disproportions increased the need for tryptophan and depressed growth (25). Many examples of this type of imbalance, involving a variety of amino acids, have been observed in young animals. The growth-depressing effects result from depressed food intake mediated through alterations in brain neurotransmitter concentrations (26).
Dietary imbalances can also increase needs for some mineral elements (23, 27). Disproportionate amounts of molybdenum and sulfate in the diet increase the dietary need for copper and precipitate copper deficiency in animals consuming an otherwise adequate amount of copper. Extra manganese in the diets of sheep or pigs increases the need for iron to prevent anemia, and excess iron reduces the absorption of manganese. The presence in the diet of phytic acid, which binds zinc as well as other multivalent cations, impairs zinc absorption and increases the need for zinc. Thus, phytic acid can precipitate zinc deficiency in both humans and animals.
Dietary needs for some essential nutrients are influenced by the proportions of macronutrients in the diet. The need for vitamin E in the diet increases as the amount of fat rich in polyunsaturated fatty acids is increased ( 28). Thiamin functions mainly as part of the cofactor for decarboxylation of the a-ketoacids arising from metabolism of carbohydrates and branched-chain amino acids; hence, the need for thiamin depends upon the relative proportions of fat, carbohydrate, and protein in the diet. Fat has long been known to exert a "thiamin-sparing" effect (29).
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