Survival Time and Biochemical Changes in Chicks Fed Diets Lacking Different Essential Amino Acids

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Abstract
Preceding death, three stages were observed in young chicks fed purified amino acid diets, lacking one or more essential amino acids: (1) a rapid loss of weight; (2) a relatively long period of essentially stable weight; and (3) another period of rapid weight loss, followed by death. The loss of weight in the first period and the length of the second period varied with the missing amino acid. Median survival times in days for chicks fed diets lacking the following amino acids are: isoleucine, 18; valine, 19; both phenylalanine and tryrosine, 23; methionine, 26; arginine, 27; threonine, 27; leucine, 28; both methionine and cystine, 28; all essential amino acids, 28; phenylalanine, 34; the combination of valine, isoleucine and leucine, 34; all amino acids, 35; tryptophan, 36; lysine, 53; and histidine, 60. Explanations for these survival times can be categorized as follows: (1) chicks fed diets lacking either isoleucine or valine suffered a severe amino acid imbalance, rapidly affecting the free amino acid levels in tissues and causing early death; (2) chicks fed diets lacking either lysine or histidine survived longer, possibly because of drawing on body stores of peptides (carnosine) and proteins (hemoglobin) especially rich in these amino acids for synthesis of necessary proteins; and (3) chicks fed diets lacking the other essential amino acids, either singly or in combination, survived approximately the same length of time as chicks fed an amino acid-free diet, although the effect on body composition differed. Chicks fed the essential amino acid-deficient diets lost body fat much faster than those fed the amino acid-free diet. All chicks had near-normal feather development and considerable skeletal growth at the expense of muscle tissue. Except for some chicks fed threonine-free diets, all chicks surviving past the median death time which were re-fed normal, complete diets had a rapid weight gain. The only lesion observed was a varied incidence of folded tongue.