Determination of Amino Acid Needs of the Young Pig by Nitrogen Balance and Plasma-Free Amino Acids

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Abstract
The relation between graded dietary levels of a single amino acid, at levels from deficient to superoptimal and the level of the same free amino acid in the plasma and to nitrogen balance was studied for lysine, isoleucine, leucine, and histidine. Addition of an amino acid to a diet deficient in that amino acid does not cause a significant increase in its concentration in the plasma at dietary levels less than the dietary need. When superoptimal levels of an amino acid are present in the diet, plasma-free levels of this amino acid increase. The requirements, as a percent of the diet, determined by plasma-free amino acid and nitrogen balance studies, respectively, were: lysine−0.86, 0.81; isoleucine−0.46, 0.53; leucine−0.78, 0.84; and histidine−0.27, 0.25. When pigs were fed graded levels of lysine for only a short period before blood samples were taken, a linear increase in plasma-free lysine was found. This indicates that a period of metabolic adjustment is necessary before plasma-free amino acids can be used to evaluate lysine, and perhaps other amino acid, needs. Plasma-free amino acid values indicated the well-known leucine-isoleucine and valine antagonism at dietary levels of leucine less than that at which the antagonism affected rate of gain or feed intake. At low levels of leucine, plasma free valine was high, but both plasma-free valine and isoleucine were drastically reduced by increasing levels of dietary leucine.

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