Background:Although standard formulas for preterm infants promote intrauterine rates of weight gain, fat deposition in preterm infants fed these formulas has been reported to be considerably higher than that in the fetus. We hypothesized that a preterm infant formula with a higher protein:energy (P:E) ratio would promote accretion rates of fat, fat-free mass, and minerals closer to those of the fetus.Methods:As part of a larger study to determine whether accretion rates of fat and fat-free mass closer to those of the fetus can be achieved with a higher P:E ratio, we present a descriptive analysis of 72-h nutrient balance studies performed on a subset (n = 15/30) of the infants randomly assigned to be fed formula with a P:E ratio of either 3.2 g/100 kcal or 2.6 g/100 kcal.Results:Despite the higher intake and net absorption of nitrogen by infants fed the higher P:E formula, there was no statistically significant difference in net nitrogen retention between groups. There also were no statistically significant differences between groups in digestible energy, metabolizable energy, energy expenditure, or energy storage. Thus, partitioning of stored energy as protein and fat did not differ between groups. The retention of calcium, phosphorus, sodium, potassium, copper, and zinc also did not differ between groups, and nitrogen intake did not affect mineral retention.Conclusions:In this study, formula for preterm infants with a P:E ratio of 3.2 g/100 kcal vs. 2.6 g/100 kcal provided no apparent benefit in terms of the proportion of fat to lean tissue accretion as determined from nutrient balance data.