Use of Free Amino Acid Concentrations in Blood Plasma in Evaluating the Amino Acid Adequacy of Intact Proteins for Chick Growth

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Abstract
Concentrations of free amino acids in the plasma of chicks were studied to develop a method which will enable the prediction of the amino acid adequacy of intact proteins for chick growth. Plasma free amino acid patterns of chicks fed a standard crystalline amino acid diet were compared with those resulting from feeding the same diet in which the crystalline amino acid component was replaced by an equivalent amount of protein (N × 6.25). Various fishmeals, some overheated, were used as the source of intact protein. Feeding procedures were used that assured an equal amount and rate of intake of all nutrients among experimental groups prior to time of blood sampling. A comparison of the plasma amino acid levels of the chicks fed fishmeal with those fed the control diet showed that, in general, the concentrations of the essential amino acids were lower in the fishmeal group, with the exception of lysine. This amino acid occurs in relatively high concentrations in good quality fishmeal. The amino acids which appeared to be most deficient in the fishmeals were histidine, valine, methionine and threonine. Chick growth studies, however, have established that these fishmeals respond first to methionine then to single additions of histidine, phenylalanine and possibly threonine. When the intact fishmeals were mildly overheated prior to feeding, lower plasma concentrations of lysine and threonine resulted indicating a loss of availability of these amino acids. More drastic heating resulted in a decrease of all amino acids. The technique described offers a method of comparing the relative availability of amino acids in proteins subjected to various processing alterations, but appears to be ineffective in its present form for predicting the order and degree of amino acid limitation in the fishmeals studied.