The effect of varying protein quality and energy intake on the nitrogen metabolism of parenterally fed very low birthweight (<1600 g) infants

Abstract

Net nitrogen retention (NNR) and rates of whole-body protein turnover (Q), synthesis, and breakdown (B) were measured in 24 intravenously fed premature infants, birthweight <1600 g, at the end of the first week of life. Four regimes were used: Amigen- glucose ± Intralipid; Vamin-glucose ± Intralipid. Mean protein intake was 2.7 g/kg/day. Mean energy intakes were 68 to 98 kcal/ kg/day. Vamin was a better protein source (P < 0.01), evidenced by a higher NNR; 72 ± 2%, cf. 56 ± 4% at high-energy intakes. The high-energy intake also improved (P < 0.01) protein retention (NNR); 64 cf. 50%. Infants receiving 2.9 g of Vamin (394 mg N)/ kg/day and 85 kcal/kg/day of nonprotein intake retained nitrogen at intrauterine rates (282 ± 7 mg/kg/day). Diet had no effect on Q, synthesis, or B. However, the protein source had a significant effect (P < 0.01) on the fraction of N-flux coming from protein breakdown (B/Q); 71.1% for Vamin, cf. 77.1% for Amigen. Similarly, energy intake had a significant effect (P < 0.01) on the fraction N-flux utilized for protein synthesis (S/Q); 91.3% high energy cf. 87.0% low energy. These results suggest that an increased energy intake improved N-retention by enhancing amino acid reutilization for protein synthesis, whereas a higher quality protein improved N-retention by limiting protein breakdown. Speculation Improved protein quality may inhibit breakdown by ensuring that a more complete and balanced mix of amino acids are available for protein synthesis. Where the protein quality is less ideal, endogenous proteins are catabolized to provide the necessary amounts and balance of amino acids at the ribosomal level for protein synthesis. © 1981 International Pediatric Research Foundation, Inc.