Classification

From a nutritional standpoint, the most important classification of amino acids is the division between those that are essential (or indispensable) and those that are nonessential (or dispensable). Essential amino acids may be defined as those that the body cannot synthesize in sufficient quantities.

This classification is based on work carried out by W. C. Rose in the 1930s. Young, rapidly growing rats were fed purified diets from which one amino acid was removed. For some of the amino acids, this made

Table 2 Essential amino acids for the rat

Valine

Isoleucine

Leucine

Tryptophan

Phenylalanine

Threonine

Methionine

Histidine

Lysine

(Arginine)

no difference to the rats' growth rate—these are the nonessential amino acids shown in Table 2. For the essential amino acids removal from the diet resulted in immediate cessation of growth, followed by loss of weight, decline in food intake, and eventual death of the rats. The response to the removal of arginine was less dramatic because the rats continued to grow, but at a reduced rate. Thus, it appeared that the rat can synthesize arginine, but not at a high enough rate to support maximal growth.

It has subsequently been shown that the reason why certain amino acids are essential is that their carbon skeletons cannot be synthesized in mammalian cells. As long as the carbon skeletons are present, all amino acids except threonine and lysine can be formed by transamination. It should be noted, however, that tyrosine can only be synthesized from phenylalanine, and cysteine can only be synthesized from methionine.

Rose also determined which amino acids are essential for man by carrying out nitrogen balance experiments on healthy young adult volunteers. He showed that nitrogen balance could be maintained on a diet in which the only source of nitrogen was a mixture of the 10 amino acids that are essential for the rat. He then found that histidine and arginine could also be removed without affecting nitrogen balance. Thus, the 8 amino acids that are essential for adult man are shown in Table 3.

More recent work has identified certain circumstances, usually associated with disease or recovery from malnutrition, in which the addition of particular nonessential amino acids to an otherwise

Table 3 Essential amino acids for man

Valine

Isoleucine

Leucine

Phenylalanine

Tryptophan

Threonine

Methionine

Lysine adequate diet appears to cause an unexpected improvement in either nitrogen balance or growth rate. It is hypothesized that the rate at which the body can synthesize these particular amino acids is limited, and that in extreme circumstances the requirement for them becomes greater than the rate at which they can be synthesized. These amino acids are thus sometimes called conditionally essential amino acids, and these include glycine, arginine, histidine, and glutamine.

See also: Amino Acids: Metabolism; Specific Functions. Protein: Synthesis and Turnover; Requirements and Role in Diet; Digestion and Bioavailability; Quality and Sources; Deficiency.

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