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
Plasma free amino acid patterns of chicks fed a standard crystalline amino acid diet were compared with those resulting from feeding (a) sesame meal with and without lysine supplementation, and (b) raw, heated and overheated soybean meal, with and without methionine supplementation. A low level of lysine in the plasma of sesame-fed chicks showed that lysine is the first limiting amino acid in this protein. Similarly, methionine was observed to be the first limiting amino acid in all the soybean meals studied. Upon supplementation of these meals with their respective limiting amino acids, deficiencies of histidine, threonine and methionine were still indicated. Comparisons of meals with and without supplementation of their first limiting amino acid showed an increased utilization of all amino acids for protein synthesis. This was demonstrated by a marked reduction of all other essential amino acids in the plasma. An overall apparent loss of digestibility results from severe heat treatment of proteins. In addition, the amino acids of raw soybean meal were shown to be less available to the chick than those of properly heated soybean meal. It appears that the plasma amino acid assay described can be of considerable value when a qualitative comparison of the amino acid availability of similar proteins is desired. The use of the technique to predict the amino acid adequacy of intact proteins for chick growth, however, was shown to be dependent upon the extent to which the standard reference diet provides the amino acid requirements for chick growth. Such a reference diet must contain neither a deficient nor an excess quantity of any of these amino acids.