Kinetic tracer studies on protein synthesis during infancy using chemically defined diets containing 15N-lysine as tracer

A four-month old infant with exocrine pancreas insufficiency was fed exclusively on a chemically defined diet. The amino acid formulation of this diet corresponds to that of breast milk. The 15N-labelled lysine tracer (97.4 atom-%) was included in the diet without changing its chemical score. A trace dose of 2.22 mg/kg body weight was applied for 5 days after the infant had been maintained on a chemically defined diet for 11 weeks. Urine was collected over a period of 170 h. The compartment theory was used to calculate metabolic data. Cumulative renal 15N excretion showed that nitrogen retention was 90.8 %. Protein synthesis amounted to 10.34 g kg-1d-1 compared with a protein breakdown rate of 8.97 g kg-1d-1. Net protein gain in this infant was about 5.5 g d-1 corresponding to a rate of 1.38 g kg-1d-1 or 13.3% of the synthesis rate. The metabolic pool amounted to 4.96 g N kg-1d-1. 75.99 mg N kg-1h-1 was metabolized, mainly to protein, and only about 10% was excreted with the urine. (author)