Glycine, leucine, and phenylalanine flux in low-birth-weight infants during parenteral and enteral feeding

Abstract

Kinetics of three amino acids with different sites and characteristics of metabolic regulation were studied in low-birth-weight infants during enteral and parenteral feeding regimens typical of clinical practice. Primed constant infusions of [15N]glycine, L-[1-13C]leucine, and L-[1-13C]phenylalanine were administered simultaneously by the same route as the feeding, with isotope enrichment measured in urine over 12 h. The effect of feeding regimen was specific to each amino acid (mean +/- SD): glycine flux was lower during parenteral feeding (470 +/- 15 vs 561 +/- 69 mumol.kg-1.h-1, P less than 0.05), leucine flux was unaffected (360 +/- 77 vs 388 +/- 78 mumol.kg-1.h-1), and phenylalanine flux was higher (106 +/- 29 vs 56 +/- 6 mumol.kg-1.h-1, P less than 0.01). Kinetics were influenced by the interaction of several factors, including amino acid intake and routes of feeding and tracer administration. Glycine was most affected by route of feeding and phenylalanine was most affected by intake whereas leucine was little affected. Estimates of whole-body protein turnover calculated from leucine and phenylalanine were different; thus calculations of protein turnover from kinetics of a single amino acid should be interpreted with caution.

Threonine kinetics in preterm infants fed their mothers' milk or formula with various ratios of whey to casein

“…Isotopic enrichment of threonine was similar in plasma and urine (plasma-to-urine enrichment ratio of 0.966 ± 0.010; x -± SEM, n = 14), thus validating the measurement of [ 13 C]threonine enrichment in urine as opposed to plasma (23,30). The mean (± SEM) CV of urinary threonine enrichment above baseline at plateau was 5.2 ± 1.6% (n = 14).…”

“…Threonine flux was measured from the dilution of the infused tracer in the metabolic pool at isotopic steady state, as described previously (23,29,30).…”

“…The oral administration of the amino acid tracer doses simulated the primed, constant-isotope-infusion technique (23). On the day of the tracer-infusion study, the L-[1-…”

Phenylalanine Kinetics in Sick Preterm Neonates with Respiratory Distress Syndrome

“…Limited data from research in seven normal growing newborns aged 20 d have been reported with endogenous production rates of phenylalanine equal to a mean of 87 kmol/kg/h (23). In 12 low-birth-weight infants, the phenylalanine flux, calculated from infusions of ~-[ l -~~~]~h e n~l a l a n i n e , during enteral and parenteral feeding was 106 and 56 kmol/kg/h, respectively (24).…”

“…In relation to values for term infants, previous work has demonstrated similar hydroxylation rates of 22 kmol/kg/h for conversion of phenylalanine to tyrosine. This constituted 25% of the total phenylalanine flux (24).…”