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

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.