Lysine requirement of the enterally fed term infant in the first month of life

Background: Infant nutrition has a major impact on child growth and functional development. Low and high intakes of protein or amino acids could have a detrimental effect. Objective: The objective of the study was to determine the lysine requirement of enterally fed term neonates by using the indicator amino acid oxidation (IAAO) method. L-[1-13C]phenylalanine was used as an indicator amino acid. Design: Twenty-one neonates were randomly assigned to lysine intakes that ranged from 15 to 240 mg·kg-1·d-1. Breath, urine, and blood samples were collected at baseline and during the plateau. The mean lysine requirement was determined by using biphasic linear regression crossover analysis on the fraction of13CO2recovery from L-[1-13C] phenylalanine oxidation (F13CO2) and phenylalanine oxidation rates calculated from the L-[1-13C]phenylalanine enrichment of urine and plasma. Results: The mean (±SD) phenylalanine flux calculated from urine and plasma L-[1-13C]phenylalanine enrichment data were 88.3 ± 6.9 and 84.5 ± 7.4 μmol·kg-1·h-1, respectively. Graded intakes of lysine had no effect on phenylalanine fluxes. The mean lysine requirement determined by F13CO2was 130 mg·kg-1·d-1(upper and lower CIs: 183.7 and 76.3 mg·kg-1·d-1, respectively). The mean requirement was identical to the requirement determined by using phenylalanine oxidation rates in urine and plasma. Conclusions: The mean lysine requirement of enterally fed term neonates was determined by using F13CO2and phenylalanine oxidation rates calculated from the L-[1-13C] phenylalanine enrichment of urine and plasma. These methods yielded a similar result of 130 mg lysine·kg-1·d-1. This study demonstrates that sampling of13CO2in expired air is sufficient to estimate the lysine requirement by using the IAAO method in infants. This trial was registered at www.trialregister.nl as NTR1610.

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