Utilization of non-protein nitrogen in whey-dominant formulae by low-birthweight infants.

1. The effects of increasing non-protein nitrogen intake on nitrogen balance and alpha-amino nitrogen flux rate using [15N]glycine were examined in 30 low-birthweight appropriate-for-gestational-age infants (birthweight 1.5-2.0 kg). The compositions of the three whey-dominant formulae were similar except for the ratios of non-protein nitrogen/protein nitrogen, which were 6.5:93.5, 11.4:88.6 and 17.5:82.5. 2. Infants in the three diet groups each received similar total nitrogen intakes (395 mg of N day-1 kg-1, SD 2.6; n = 3). Protein nitrogen and non-protein nitrogen intakes were different as expected. Energy absorption (449 kJ day-1 kg-1, SD 13; n = 3) did not differ significantly between groups. A similar weight gain was observed in all groups. 3. Nitrogen absorption (76%, SD 4; n = 3) was not significantly different between groups. Apparent urea balance was significantly increased and became positive in the group receiving the formula with the higher proportion of non-protein nitrogen and urea nitrogen. Nitrogen retention, however, was significantly depressed in this group, indicating decreased efficiency of nitrogen utilization at this level of non-protein nitrogen despite an enhanced urea salvage. 4. The enrichment of the 15N label in urinary urea at isotopic steady state was significantly reduced in infants receiving the highest urea-containing formula, presumably due to the dilution of 15N-labelled urea by dietary urea. No difference, however, was found in the enrichment of the 15N label in urinary ammonia. Rates of alpha-amino nitrogen flux, protein synthesis and protein breakdown calculated from the ammonia labelling did not differ significantly between groups.(ABSTRACT TRUNCATED AT 250 WORDS)