N-15-TRACER STUDIES IN FORMULA-FED PRETERM INFANTS - THE ROLE OF GLYCINE SUPPLY IN PROTEIN-TURNOVER

In preterm infants, protein-turnover rates obtained by [N-15]glycine a s a tracer are known to be overestimated. This may reflect the insuffi cient supply of dietary glycine. In this randomized study, the influen ce of a glycine-rich diet on whole body protein turnover rates in eigh t male preterm infants (29-32 weeks, 1,200-2,540 g birthweight) using the N-15-tracer technique on days 21 and 28 of life was investigated t o evaluate the necessity of supplementing preterm infant formulas with proteins rich in glycine. Before and during the study, the infants we re alternately fed with a commercial available preterm infant formula (I, 2% protein, 40 mg glycine/dl) and a variety of this formulation wi th glycine-rich proteins (II, 2% protein, 130 mg glycine/dl). The prot ein-turnover rates were computed after N-15-single-pulse labeling with the help of the three-compartment model (TCM) and the urinary ammonia end-product method (AEPM). The tracer used was [N-15]glycine (dosage: 2 mg N-15/kg). For the determination of N-15-excess-excretion kinetic s, fractionated urine specimens were collected over a 36-h period. The protein-turnover rate calculated by TCM was 8.8 +/- 1.6 g/kg/day (for mula I) and 7.7 +/- 2.0 g/kg/day (formula II); using AEPM, the rate wa s 8.7 +/- 2.5 g/kg/day and 7.5 +/- 1.5 g/kg/day, respectively. We conc lude that the presaturation of the precursor pool by an adequate glyci ne intake minimizes drawbacks that may arise when using [N-15]glycine as a tracer in preterm infants, and a protein concentration of 2%, as in formula I, and consequently, a 170% glycine content when compared w ith the same human milk volume, meets the glycine requirement.