Evaluation of Different 15N‐Tracer Substances for Calculation of Whole Body Protein Parameters in Infants

Abstract

SummaryThe validity of using different 15N‐tracer substances to measure whole body protein parameters, i.e., protein synthesis, protein breakdown, net protein gain, protein turnover, metabolic pool, and reutilization, was assessed by comparing the results obtained with: [15N]glycine, a mixture of 10 15N‐labeled amino acids, and a 15N‐labeled chicken egg protein in two infants, 9 and 12 weeks old, who were fed human milk. The tracer substances were fed orally as a single dose corresponding to a 15N‐excess quantity of 0.2 mmol‐kg‐1 body weight. 15N Excretion in the urine was measured cumulatively by emission spectrometry, and the data on the protein metabolism were calculated by means of a three‐pool model. All three tests yielded consistent net protein gains. The protein synthesis, protein breakdown, protein turnover, and nitrogen reutilization values produced by the [15N]glycine tracer study were higher than those produced by application of the 15N‐amino acid mixture and the 15N‐labeled egg protein. However, in our opinion, this discrepancy does not justify the replacement of [15N]glycine by expensive 15N‐amino acid mixtures as tracer substances.