UTILIZATION OF NONPROTEIN NITROGEN IN WHEY-DOMINANT FORMULAS BY LOW-BIRTH-WEIGHT INFANTS

1. The effects of increasing non-protein nitrogen intake on nitrogen b alance and alpha-amino nitrogen flux rate using [N-15]glycine were exa mined in 30 low-birthweight appropriate-for-gestational-age infants (b irthweight 1.5-2.0 kg). The compositions of the three whey-dominant fo rmulae were similar except for the ratios of non-protein nitrogen/prot ein 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-prote in nitrogen intakes were different as expected. Energy absorption (449 k3 day-1 kg-1, SD 13; n=3) did not differ significantly between group s. A similar weight gain was observed in all groups. 3. Nitrogen absor ption (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 s ignificantly 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 N-15 label in urina ry urea at isotopic steady state was significantly reduced in infants receiving the highest urea-containing formula, presumably due to the d ilution of N-15-labelled urea by dietary urea. No difference, however, was found in the enrichment of the N-15 label in urinary ammonia. Rat es of alpha-amino nitrogen flux, protein synthesis and protein breakdo wn calculated from the ammonia labelling did not differ significantly between groups. 5. The source of dietary nitrogen affects the enrichme nt of urinary nitrogenous end-products and must be taken into account when interpreting results of protein turnover studies using constant i nfusion of [N-15]glycine. 6. Based on the nitrogen balance results, th e nonprotein nitrogen content of formulae should not account for more than 12% of total nitrogen.