OBJECTIVE To study metabolic and energy balances, growth and composition of increased body mass in healthy preterm infants fed control formula or control formula with three different nonprotein energy supplements. METHODS Growing preterm infants (birth weight < 1,500 g and gestational age < 31 weeks) were fed standard preterm formula (control group) or the same formula enriched with three different nonprotein energy supplements. An energy supplement of 23 kcal/kg/day was achieved by adding medium-chain triglyceride and dextrinomaltose in three different caloric ratios: 33:66 in group A, 66:33 in group B, and 85:15 in group C. Energy balance was determined by open-circuit continuous (5-6 hours) measurements of energy expenditure, with simultaneous measurement of 24-hour urinary nitrogen excretion. Metabolic balance was determined by measurements of energy intake, energy oxidation, and energy output in urine and stool. The composition of body mass accretion was determined as the accretion of fat and protein in the total weight gain. RESULTS The fat accretion (4.9, 5.9, 6.2, and 3.8 g/kg/day in groups A, B, C and D, respectively) correlated directly with fat intake. Infants receiving standard energy intake had a fat percentage of weight gain significantly lower (28%) than that of the high-energy intake groups (31%, 40%, and 38% in groups A, B, and C, respectively). This difference corresponded to the results obtained from skinfold thickness measurements. CONCLUSIONS Excess nonprotein energy is stored as fat regardless of its source (fat or carbohydrate). High caloric and medium-chain triglyceride intake in otherwise healthy growing preterm infants does not promote nitrogen retention.