1. Indirect calorimetry and primed constant infusion of [U-13C]glucose were combined in 28 appropriate-for-gestational age newborn, parenterally fed infants, in order to measure glucose utilization and glucose oxidation and to estimate lipogenesis from glucose.
2. The infants were randomly allocated to either a group receiving glucose as the non-protein energy source or a group having one-quarter of the glucose energy replaced by intravenous fat. The energy intake (370 kJ day−1 kg−1) and protein intake (3.4 g day−1 kg−1) were similar in both groups.
3. Energy expenditure (P < 0.005), non-protein carbon dioxide production (P < 0.005) and non-protein oxygen consumption (P < 0.05) were lower in the lipid-supple-mented group.
4. The significant excess of glucose utilization over oxidation (P < 0.001) can be accounted for by lipid synthesis from glucose.
5. Fat synthesis from glucose was higher in the glucose/amino acid group (P < 0.02), but total fat storage was higher in the lipid-supplemented group (P < 0.02). Nitrogen balance was similar in both groups.
6. As lipogenesis from glucose is an energy- and oxygen-consuming and a carbon dioxide-producing process, the data suggest that the differences between the glucose-only group and the lipid-supplemented group are due to different rates of lipogenesis from glucose.