The 1987 Borden Award Lecture.: Protein metabolism in premature human infants

Abstract

Our studies have focused on the regulation of whole body and skeletal muscle protein metabolism in premature infants. Net deposition of protein is the result of a positive balance between protein synthesis and breakdown. To measure protein metabolism we have employed end-product studies with [15N]glycine and 13[C]leucine. Myofibrillar protein degradation was estimated by measuring the excretion of N-t-methylhistidine in urine. Energy expenditure and substrate utilization were also measured. Premature infants have high rates of protein synthesis (12 g∙kg−1∙d−1), twice those measured in children and four times those found in adults. Intrauterine malnourished babies have increased rates of protein turnover. Very low birth weight infants (< 1500 g) have higher myofibrillar protein turnover than larger babies. Intravenous feeding decreases whole body protein turnover, and we estimate visceral protein synthesis to be approximately 4 g∙kg−1∙d−1. Suboptimal energy intake worsens nitrogen utilization by reducing the reutilization of endogenous amino acids for protein synthesis. We have also examined the effects of varying the source of nonprotein energy (i.e., glucose only versus glucose plus lipid) at requirement levels and have shown there is no effect on protein metabolism. Recent improvements in technology have opened the way to detailed study of individual amino acid metabolism in neonates in the future.