Glutamic acid, serine, glycine, threonine, alanine, glutamine, lysine and tyrosine are excreted in the urine by normal infants. The urinary excretion of glycine and alanine, particularly, decreases with age, and probably this is related to development of renal function. The amount of histidine excretion is probably related to intake of dietary protein in most instances.
The role of proline and hydroxyproline in the urine of young infants is not explained.
Other amino acids may be excreted in small quantities, less than 25 µg/mg of creatinine. The presence of taurine,β-amino-isobutyric acid, leucine, valine, or Unknown No. 35 in larger quantities in the urine, is significant. In the young, apparently "normal" child the excretion of these substances in large amounts may indicate the presence of a metabolic disturbance even though no clinical symptoms are evident.
Use of the term "amino-aciduria" should be employed with great caution. The effects of drugs on amino acid excretion have not been established. The assessment of the significance of amino-aciduria, once established, whether generalized or specific, must await further investigation of metabolic interrelationships.