15N-tracer-kinetic studies on the nitrogen metabolism of very small preterm infants on a diet of mother's milk.

Protein synthesis and breakdown, nitrogen flux and other parameters of nitrogen metabolism were measured in five male preterm infants with a mean gestational age of 30.4 +/- 1.95 complete weeks of gestation and a mean body weight of 1592 +/- 517 g. The infants were fed on mothers' milk; the measurements were made at a post-conceptional age of 31.6 +/- 1.9 weeks of gestation (Ia) and were repeated at 34.4 +/- 1.9 weeks of gestation (Ib). [15N]-glycine (95 atom per cent) was used as a tracer, administered as a single enteral dose of 20 mg/kg. Whole-body protein parameters were calculated from an assumed three-pool model. The results were compared with data from moderately small preterm (II) and full-term (III) infants measured at post-conceptional ages of 36.1 +/- 1.4 and 48.0 +/- 2.8 weeks respectively. Protein synthesis rates tended to decrease with increasing post-conceptional age: 14.3 +/- 4.5 g/kg/d (Ia); 11.8 +/- 2.9 g/kg/d (Ib); 7.9 +/- 2.7 g/kg/d (II); and 7.7 +/- 1.4 g/kg/d (III). Protein breakdown and nitrogen flux showed the same trends. Possible consequences for the appropriate nutrition of very small preterm infants are discussed.