1. The effects of increasing non-protein nitrogen intake on nitrogen balance and α-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 α-amino nitrogen flux, protein synthesis and protein breakdown calculated from the ammonia labelling did not differ significantly between groups.

5. The source of dietary nitrogen affects the enrichment of urinary nitrogenous end-products and must be taken into account when interpreting results of protein turnover studies using constant infusion of [15N]glycine.

6. Based on the nitrogen balance results, the nonprotein nitrogen content of formulae should not account for more than 12% of total nitrogen.

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