Total-body protein turnover in human premature neonates: effects of birth weight, intra-uterine nutritional status and diet.

1. The effects of birth weight, intra-uterine nutritional status and protein and energy intake on whole-body protein turnover, and skeletal muscle protein breakdown were examined in 40 premature infants. 2. Total-body nitrogen flux was 26% higher in the small-for-gestational-age compared with appropriate-for-gestation-age infants; similarly, whole-body protein synthesis and breakdown were increased by 26 and 35% respectively (P less than 0.01). 3. The lower-birth-weight neonates (less than 1500 g) had higher rates of skeletal muscle protein breakdown; 1.23 +/- 1.12 g day-1 kg-1, as compared with 0.54 +/- 0.28 g for the high-birth-weight group (P less than 0.05). 4. Protein intake was inversely correlated with the fraction of nitrogen flux coming from endogenous protein breakdown (P less than 0.05) and with skeletal muscle protein breakdown (P less than 0.05). There were no significant relationships found between energy intake and the parameters of protein metabolism. 5. On the basis of the turnover data, evidence is presented that the protein requirements for milk-protein fed premature neonates is less than 4.3 g day-1 kg-1.