Plasma Amino Acid Response in Young Men Given Diets Devoid of Single Essential Amino Acids

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Abstract
Studies were conducted on the response of plasma amino acid levels in young, male MIT students to diets lacking in phenylalanine-tyrosine, threonine, lysine, valine, leucine or isoleucine. A purified L-amino acid mixture was the sole source of dietary “protein” and supplied a nitrogen level equivalent to about 0.5 g egg protein (N × 6.25) per kilogram body weight. The subjects were studied during a 3-day control period while consuming their usual diet and during a 12-day period of receiving the experimental diet. Blood samples were taken at 8 am after an 11-hour overnight fast and 3 hours after breakfast during the control period and again after 4, 8 and 12 days during the amino acid diet period. The initial fasting plasma levels of phenylalanine, lysine, isoleucine and leucine were reduced by less than 30% during the 12-day period of consuming diets deficient in the respective amino acid, whereas the levels of threonine and valine were reduced by approximately 50% within 4 days after initiation of the respective amino acid-free diet. The leucine-free diet resulted in a significant increase in the levels of valine and isoleucine in fasting and postprandial plasma whereas the valine- and isoleucine-free diets did not affect the fasting levels of the other branched-chain amino acids. These results are discussed in relation to the plasma amino acid approach for estimating human amino acid requirements.