1. The effects of human milk and two low-protein formulae on the protein metabolism of 30 preterm appropriate for gestational age infants, birth weight 1.5-2.0 kg, were studied. The compositions of the two formulae were identical except for the protein source, with either casein or whey predominant.

2. The three diet groups all received similar nitrogen (390 mg of N day−1 kg−1) and energy intakes (500 kJ day−1 kg−1).

3. Growth rates for length and weight were comparable and approximated to intrauterine growth. Nitrogen absorption and net nitrogen utilization were enhanced in the human-milk-fed group. However, there were no differences in nitrogen retention, which averaged 274 mg day−1 kg−1 and approximated to intrauterine accretion rates.

4. Rates of whole-body nitrogen flux, protein synthesis and breakdown in the human-milk group were increased by 43–54% over the values seen in either of the two formula-fed groups (P < 0.01). No differences were seen between the two formula groups.

5. No differences were seen in urine 3-methylhistidine excretion between the three study groups.

6. Our results suggest significant differences in the whole-body protein metabolism of formula-fed infants compared with human-milk-fed infants.

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