Medium-chain triglycerides, with a chain length of eight and 10 carbon atoms, form up to 50% of the total fat content in some preterm infant formulas. In 20 small preterm infants (birthwt: 1153 +/- 227 g; mean +/- SD) fed a special formula containing 40% MCT, a primed constant oral infusion study of 1-13C-potassium octanoate was conducted to quantify the oxidation of MCT. A plateau in 13C enrichment in breath CO2 was reached in all patients within 1-3 h. Simultaneously, substrate utilization was measured using a closed system indirect calorimeter. No significant difference was found between appropriate for gestational age (n = 8) and small for gestational age (n = 12) infants in the percentage of the administered tracer that was oxidized (44.9 +/- 9.1% versus 48.5 +/- 11.0%). In all patients, the recovery was calculated to be 47.1 +/- 10.2%, which is less than previously estimated and corresponds to a mean MCT oxidation of 1.26 +/- 0.27 g/kg/d. With indirect calorimetry, a total fat oxidation of 1.42 +/- 0.84 g/kg/d in appropriate for gestational age and 2.00 +/- 0.85 g/kg/d in small for gestational age infants was found, indicating that MCT accounted for around 85% of the total fat oxidation in appropriate for gestational age versus 65% in small for gestational age infants.