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Total sulfur amino acid requirement and metabolism in parenterally fed postsurgical human neonates^1,2,3

¹ From the Research Institute, The Hospital for Sick Children, Toronto, Canada (GC-M, KPC, AMM, JHK, and PBP); the Department of Nutritional Sciences, University of Toronto, Toronto, Canada (GC-M, KPC, ROB, and PBP); and the Department of Agriculture, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada (ROB and PBP)

Background:Except for tyrosine, the amino acid requirements of human neonates receiving parenteral nutrition (PN) have not been experimentally derived.

Objectives:The objectives were to determine the total sulfur amino acid (TSAA) requirement (methionine in the absence of cysteine) of postsurgical, PN-fed human neonates by using the indicator amino acid oxidation (IAAO) technique with L-[1-¹³C]phenylalanine as the indicator.

Design:Fifteen postsurgical neonates were randomly assigned to receive 1 of 18 methionine intakes ranging from 10 to 120 mg · kg^–1 · d^–1, delivered in a customized, cysteine-free amino acid solution. Breath and urine samples were collected for the measurement of ¹³CO₂ and amino acid enrichment. Blood samples were collected at baseline and after the test methionine infusion for the measurement of plasma methionine, homocysteine, cystathionine, and cysteine concentrations.

Results:Breakpoint analysis determined the mean TSAA requirements to be 47.4 (95% CI: 38.7, 56.1) and 49.0 (95% CI: 39.9, 58.0) mg · kg^–1 · d^–1 with the use of oxidation and F¹³CO₂, respectively.

Conclusions:This is the first study to report the TSAA requirement of postsurgical, PN-fed human neonates. The estimated methionine requirement expressed as a proportion of the methionine content of current commercial pediatric PN solutions was 90% (range: 48–90%) of that found in the lowest methionine-containing PN solution.