We have shown that an E intake of 150 vs. 120 kcal/kg·d enhances protein utilization by LBW infants (Kashyap, et al., Pediatr Res 1994; 35:704). To determine the effect of quality of E intake on protein utilization, LBW infants (BW, 750-1600g were assigned randomly and blindly to receive one of 5 formulas differing only in quantity and quality of energy. Groups 1, 2 and 3 received formulas providing an E intake of 120 kcal/kg·d with 35, 50 and 65% of non-protein E, respectively, as carbohydrate (lactose + glucose polymers (CHO) and the remainder as fat; Groups 4 and 5 received an E intake of 150 kcal/kg·d with 35 and 65% of non-protein E, respectively, as CHO. Protein intake of all groups was 4g/kg·d. Leucine oxidation (i.e., the cumulative percent of a bolus dose of 1-13C-Leu recovered in breath from 0-8h) was determined in subsets of all groups (n=4-7) just prior to discharge. Leucine oxidation of Groups 1-5, respectively, was 24.2±5%a, 19.3±2.2%b, 14.8%c 13.7 ±2.9%c and 13.2±9,8%c of intake (values with different superscripts are statistically significant). Thus, both quantity and quality of concomitant E intake appear to affect protein utilization as measured by leucine oxidation. At E intakes of 120 kcal/kg·d, increasing the CHO content of non-protein E from 35% to 50% and from 50-65% increases protein utilization. A total E intake of 150 kcal/kg·d improves utilization relative to 120 kcal/kg·d with 35 and 50 but not 65% CHO. This suggests that increasing the CHO content of an E intake of 120 kcal/kg·d is as effective in promoting protein utilization as increasing E intake to 150 kcal/kg·d regardless of CHO content.