The most fundamental question in nutrition concerning protein and amino acids is simply, what amount of protein is required in the diets of humans to maintain health? This question has several parts. First, we must evaluate the intake of both protein and the amounts of the individual amino acids in that protein. Second, this question must be answered in humans (a) over the complete range of life and development, (b) in sickness and in health, and (c) under different conditions of work and environment. These questions and the methods used to answer them are discussed below in terms of their individual components.
When we discuss the amino acid composition of a specific protein source, we generally focus upon the amount of essential acids contained in it because these are the amino acids that are indispensable to our diet. Which amino acids are dispensable and which are indispensable was originally determined by testing whether a diet deficient in a particular amino acid would support growth in a rat. However, there are important species differences between rats and humans that limit the comparison of rat to man. Furthermore, the growth retardation model, which is effective with the rat, is not applicable to humans. An alternative for studying amino acid requirements in humans is the N balance technique. A diet that is adequate in total N but deficient in an essential (indispensable) amino acid cannot produce a positive N balance because protein can be synthesized only if each amino acid is present in adequate amounts. The nonessential amino acids can be synthesized if protein intake is adequate, but limited intake of an essential amino acid limits the amount of protein that can be synthesized. The body is then faced with a dietary excess of the other nonlimiting essential and nonessential amino acids that it cannot put into protein. Therefore, these amino acids must be oxidized to urea, and a negative N balance results.
The classic studies of Rose and colleagues measured N balance in humans fed diets deficient in individual amino acids. They determined that eight amino acids produced a negative N balance when they were deficient in the diet of adult humans (152, 153 and 154). Although the enzymatic pathways are missing for synthesis of these essential amino acids, several of these amino acids (such as the BCAAs) have a catabolic pathway whose first step is reversible (i.e., transamination to form branched-chain keto acids). The rat growth model was used to show that growth can be supported by supplying the keto acids of these essential amino acids. Various formulations have been proposed for supplying the carbon skeleton of several essential amino acids (e.g., the BCAAs) without adding N, which is detrimental in disease states such as renal disease (155).
Another related question is whether dispensable, nonessential amino acids ever become indispensable. If a nonessential amino acid is used in the body faster than it is made, it becomes essential for that condition (6). Tyrosine and cysteine are made from phenylalanine and methionine, respectively, but if insufficient phenylalanine or methionine is consumed, tyrosine and cysteine also become deficient and essential. This question must be answered across the range of life from infancy to the elderly as well as in sickness and health. For example, enzymes for amino acid metabolism mature at different rates in the growing fetus and newborn infant. Histidine is essential in infants but not necessarily in healthy children or adults. Therefore, the classification of "essential" or "nonessential" depends upon species, maturation (i.e., infant, growing child, or adult), diet, nutritional status, and pathophysiologic condition. Also to be considered is whether a particular amino acid given in excess of requirement has properties that may ameliorate or improve a clinical condition. These considerations must somehow be evaluated for each population group in which they are important.
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