It was hypothesized that low-molecular-weight products of carbohydrate fermentation would contribute only a small percentage to the total fecal excretion of nonfat, nonnitrogenous energy (carbohydrate energy) in premature infants. Infants born at 28-32 weeks' gestation who were 2-4 weeks of age were randomized to receive a formula with lactose as the sole carbohydrate (n = 7) or the same formula with 50% of the carbohydrate as glucose polymer (n = 8). The percent contribution (X +/- SD) to total carbohydrate energy of sugars (glucose, galactose, lactose, glucose polymer), short-chain fatty acids (acetate, propionate, butyrate, isobutyrate, valerate, and isovalerate), and D- and L-lactate was 9.4% +/- 2.9% for the 15 subjects and was not significantly different between groups. The percent contribution of all four sugars was 5.8% +/- 1.7% and did not differ between the two groups. Doubling the lactose intake resulted in significant increases in fecal excretion (kilocalories per kilogram per day) of acetate (77% increase; P = 0.03), total short-chain fatty acids (54%; P = 0.04), and galactose (188%; P = 0.03). These data suggest that as much as 90% of fecal carbohydrate energy may be in the form of large-molecular-weight compounds, presumably bacterial in origin.