Nutritional and Metabolic Classification of Amino Acids

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Older views of the nutritional classification of amino acids categorized them into two groups: indispensable (essential) and dispensable (nonessential). The nine indispensable amino acids (Table 10-1) are those that have carbon skeletons that cannot be synthesized to meet body needs from simpler molecules in animals, and therefore must be provided in the diet. Although the classification of the indispensable amino acids and their assignment into a single category has been maintained in this report, the definition of dispensable amino acids has become blurred as more information on the intermediary metabolism and nutritional characteristics of these compounds has accumulated. Laidlaw and Kopple (1987) divided dispensable amino acids into two classes: truly dispensable and conditionally indispensable. Five of the amino acids in Table 10-1 are termed dispensable as they can be synthesized in the body from either other amino

TABLE 10-1 Indispensable, Dispensable, and Conditionally Indispensable Amino Acids in the Human Diet

Indispensable

Dispensable

Conditionally Indispensablea

Precursors of Conditionally Indispensable

Histidine^

Alanine

Arginine

Glutamine/glutamate, asparate

Isoleucine

Aspartic acid

Cysteine

Methionine, serine

Leucine

Asparagine

Glutamine

Gl utamic acid/ammonia

Lysine

Glutamic acid

Glycine

Serine, choline

Methionine

Serine

Proline

Glutamate

Phenylalanine

Tyrosine

Phenylalanine

Threonine

Tryptophan

Valine

a Conditionally indispensable is defined as requiring a dietary source when endogenous synthesis cannot meet metabolic need.

b Although histidine is considered indispensable, unlike the other eight indispensable amino acids, it does not fulfill the criteria used in this report of reducing protein deposition and inducing negative nitrogen balance promptly upon removal from the diet. SOURCE: Laidlaw and Kopple (1987).

a Conditionally indispensable is defined as requiring a dietary source when endogenous synthesis cannot meet metabolic need.

b Although histidine is considered indispensable, unlike the other eight indispensable amino acids, it does not fulfill the criteria used in this report of reducing protein deposition and inducing negative nitrogen balance promptly upon removal from the diet. SOURCE: Laidlaw and Kopple (1987).

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acids or other complex nitrogenous metabolites. In addition, six other amino acids, including cysteine and tyrosine, are conditionally indispensable as they are synthesized from other amino acids or their synthesis is limited under special pathophysiological conditions (Chipponi et al., 1982; Harper, 1983; Laidlaw and Kopple, 1987). This is even more of an issue in the neonate where it has been suggested that only alanine, aspartate, glutamate, serine, and probably asparagine are truly dietarily dispensable (Pencharz et al., 1996).

The term conditionally indispensable recognizes the fact that under most normal conditions the body can synthesize these amino acids to meet metabolic needs. However, there may be certain physiological circumstances: prematurity in the young infant where there is an inadequate rate at which cysteine can be produced from methionine; the newborn, where enzymes that are involved in quite complex synthetic pathways may be present in inadequate amounts as in the case of arginine (Brunton et al., 1999), which results in a dietary requirement for this amino acid; or pathological states, such as severe catabolic stress in an adult, where the limited tissue capacity to produce glutamine to meet increased needs and to balance increased catabolic rates makes a dietary source of these amino acids required to achieve body nitrogen homeostasis. The cells of the small intestine become important sites of conditionally indispensable amino acid, synthesis, with some amino acids (e.g., glutamine and arginine) becoming nutritionally indispensable under circumstances of intestinal metabolic dysfunction (Stechmiller et al., 1997). However, the quantitative requirement levels for conditionally indispensable amino acids have not been determined and these, presumably, vary greatly according to the specific condition.

There now appears to be a requirement for preformed a-amino nitrogen in the form of glutamate, alanine, or aspartate, for example (Katagiri and Nakamura, 2002). It was previously thought that, in addition to the indispensable amino acids, simple sources of nitrogen such as urea and diammonium citrate together with carbon sources would be sufficient to maintain nitrogen homeostasis (FAO/WHO, 1965). However, there are now good theoretical reasons to conclude that this is not likely in the human (Katagiri and Nakamura, 2002). The mixture of dispensable and conditionally indispensable amino acids as supplied by food proteins at adequate intakes of total nitrogen will assure that both the nitrogen and specific amino acid needs are met.

PROTEIN AND AMINO ACIDS 595

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    What are the nutritional classification of amino acids?
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    What are the nutrutional classification of amino acid?
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