1. The measurement of whole body protein turnover in the preterm infant has been investigated with an intragastric infusion of L[1-13C]-leucine and with sampling of the urinary leucine pool.

2. Measurements have been made in seven preterm infants with body weights averaging 1733 g, fed with either human breast milk (n = 3) or proprietary formulae (n = 4), giving intakes of 541 (±25)kJ/kg and 465 (±42)mg of N day−1kg−1 and growing satisfactorily (14.7 ± 2.6 g day−1 kg−1).

3. The measurement of the enrichment of urinary leucine was well within the capability of gas chromatography-spectrometry, and similar values for the enrichment of plasma and urinary leucine were observed (plasma/urinary ratio was 0.93 ± 0.04, mean ± 1 sem, n = 13). Isotopic equilibrium, as indicated by a plateau in the urinary leucine and expired CO2 enrichment, was obtained within 8 h and was maintained for at least 48 h.

4. Nitrogen retention, measured by nitrogen balance, was similar to that calculated from the leucine retention (determined as the leucine intake — oxidation), i.e. 310 ± 45 and 301 ± 38 mg of N day−1 kg−1 (means ± 1 sem). Because of this it is suggested that in this specific type of study the direct measurement of nitrogen retention dispenses with the need for measurement of leucine oxidation, thereby simplifying the measurements.

5. From the leucine flux, leucine intake and nitrogen retention, rates of whole body protein synthesis and degradation were shown to be 11.32 (±0.78) and 9.54 (±0.55) g day−1 kg−1.

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