Amino Acids in the Blood and Urine of Human Subjects Ingesting Different Amounts of the Same Proteins

Abstract

Four college students were placed on diets supplying 25, 100 and 200 gm of protein daily contributed by meat, milk, and bread, and the urinary excretion and blood levels of amino acids were determined microbiologically. The excretion of histidine, serine, alanine, and tyrosine increased rather markedly with an increase in dietary protein, but not proportionally with the intake. Increasing the intake of protein had little effect on the excretion of most other amino acids or on the “fasting” levels of the individual amino acids in the plasma. The non-essential amino acids histidine, glutamic acid, cystine, aspartic acid, alanine and serine occurred most abundantly in acid-hydrolyzed urine. The essential amino acids were excreted in much smaller amounts. The amounts of the amino acids found in the plasma during the fasting state did not parallel the amounts excreted in the urine over a 24-hour period. Much individual variation in the excretion of amino acids was evident. This did not seem to be correlated with body weight, or with the excretion of creatinine, α-amino nitrogen or total nitrogen.