A comparison of the estimates of whole-body protein turnover in parenterally fed neonates obtained using three different end products.

Protein turnover rates in neonates have been calculated largely by measuring urinary [15N]urea enrichment following administration of [15N]glycine. Although ammonia has been increasingly recognized as an end product of nitrogen metabolism, in neonates it yields a different estimate of protein turnover than does urea. Comparisons of ammonia and urea end products in parenterally fed neonates have not previously been reported. A third and independent way of estimating protein turnover, developed for adults, is to use breath 13CO2 as an end product following administration of [1-3C]leucine. We therefore carried out simultaneous measurements of protein turnover in 10 parenterally fed neonates, using the three end products. The infants were clinically stable, weighed 2.6 ± 0.2 kg, and received 3.1 ± 0.2 g∙kg−1∙d−1 of amino acid, 2.2 ± 0.1 g∙kg−1∙d−1 of lipids, and an energy intake of 90 ± 4 kcal∙kg−1∙d−1 (1 kcal = 4.186 kJ). The turnover estimates derived from the 13CO2 and [15N]urea end products were very simi...