RELATIVE KINETICS OF PHENYLALANINE AND LEUCINE IN LOW-BIRTH-WEIGHT INFANTS DURING NUTRIENT ADMINISTRATION

The effect of the route of nutrient administration on the relative rat es of leucine and phenylalanine kinetics was examined in 30 low birth weight (LBW) infants using L-[1-C-13]leucine, L-[H-2(5)]phenylalanine, and L-[H-2(2)]tyrosine tracers. The infants received special prematur e formula (PF, n = 10, 117 +/- 8 kcal . kg(-1). d(-1) and 3.2 +/- 0.2 g protein . kg(-1). d(-1)) or fortified human milk (HM, n = 10, 106 +/ - 6 kcal . kg(-1). d(-1) and 3.9 +/- 0.2 g protein . kg(-1). d(-1)), o r parenteral nutrition (PN, n = 10, 80 +/- 25 kcal . kg(-1). d(-1) and 1.8 +/- 0.3 g protein . kg(-1). d(-1)). The rate of appearance (Ra) o f leucine (Ra-Leu), was significantly higher in group PF as compared w ith groups HM and PN (434 +/- 51 versus 377 +/- 33 and 359 +/- 50 mu m ol . kg(-1). h(-1), p < 0.05). The Ra of phenylalanine (Ra-Phe) was lo wer in group HM as compared with group PF (94 +/- 18 versus 115 +/- 16 , p < 0.05), Ra-Phe in group PN (108 +/- 24 mu mol . kg(-1). h(-1)) wa s in between group PF and HM. The relative rate of Ra-Phe and Ra-Leu e xpressed as Ra-Phe/Rn-Leu ratio was lower in all groups than that expe cted from reported whole body protein composition and from that report ed in adults. The ratio of phenylalanine hydroxylation to leucine deca rboxylation was 0.202 in group PF, 0.212 in group HM, and 0.161 in gro up PN, suggesting a higher rate of decarboxylation of leucine relative to hydroxylation of phenylalanine. We conclude that: 1) the higher Ra -Leu, compared with the Ra-Phe may be the result of either a higher tu rnover of a body protein enriched in leucine or the consequence of hig her leucine intake in infant nutrition and 2) whole body protein kinet ics calculated from a single amino acid tracer do not adequately repre sent whole body protein metabolism.