Amino Acid Imbalance

T h e term 'amino acid imbalance' can be used in different ways.It can be used to describe any fairly drastic change in the pattern of amino acids in a diet, particularly one that leads to adverse effects.Used in this way it is non-specific and its historical significance is lost.We use the term in a more restricted way to describe dietary amino acid patterns resembling those originally referred to as amino acid imbalances by Elvehjem & Krehl (1955).The observations they made were similar to those of Morrison, Reynolds & Harper (1960) presented in Table I .They used a basal diet that was low in protein (9 7' of casein supplemented with sulphur-containing amino acids) and devoid of nicotinic acid.If this diet is supplemented with nicotinic acid, it is primarily deficient in threonine, but when nicotinic acid is omitted, tryptophan becomes the most limiting amino acid.T h e growth of rats fed on the basal diet supplemented with gelatin or threonine is less than that of a control group receiving no supplement.The growth retardation is prevented by further supplements of either nicotinic acid or tryptophan (Morrison et al. 1960).Addition of 6 % of the tryptophandeficient protein, gelatin, results in a quite drastic change in the dietary pattern of amino acids, but addition of threonine (in some experiments as little as 0 . 1 % of L-threonine) changes the pattern very little; yet, both of these treatments cause retardation of growth.Originally amino acid imbalance was thought to be a highly specific condition that involved tryptophan as a precursor of nicotinic acid.Salmon (1954) later showed that the growth of rats receiving nicotinic acid was depressed if the amount of gelatin in the diet was increased.Similar changes in the amino acid pattern of a variety of diets were subsequently shown to retard the growth of several species of animals (Harper, 1958).It therefore appeared that the phenomenon of amino acid imbalance was a general one involving primarily amino acid interrelationships.