Decreased Glucose Oxidation in Preterm Infants Fed a Formula Containing Medium-Chain Triglycerides

Several formulas for preterm infants contain medium-chain triglycerides (MCT) to enhance fat absorption. Although fat absorption with MCT was slightly higher in several studies in preterm infants, a beneficial effect on growth has only been reported in one publication. We hypothesized that when part of the fat blend of preterm formula is substituted by MCT oil, this might lead to a different metabolic pattern in which, due to the preferential oxidation of MCT, an increase in lipogenesis from glucose could lead to an increase in metabolic rate. To study the impact of MCT on glucose metabolism, 18 preterm infants were randomized to receive either an MCT or an LCT formula containing 38- and 6-by-weight medium-chain fatty acids, respectively, in their fat blend. At 4 wk of age, the metabolic rate, substrate utilization, glucose kinetics, and oxidation were measured by indirect respiratory calorimetry in combination with a constant-rate oral infusion of [U-13C]glucose. The “true” rate of appearance of glucose (Ra “true”) was measured from the dilution of the uniformly labeled (m 6) species of infused tracer, whereas “apparent” rate of appearance of glucose (Ra “apparent”) was measured from the dilution of infused tracer C (carbon). The latter was measured by an on-line combustion method using a gas chromatograph-isotope ratio mass spectrometer. At a carbohydrate intake of 8.4 mg·kg−1·min−1, total utilization of carbohydrate was equal in both groups at 7.6 mg·kg−1·-min−1. However, glucose oxidation, as measured by the appearance of 13C in breath CO2was significantly lower in the MCT group (4.5 ± 0.83 versus5.7 ± 0.67 mg·kg−1· min−1, MCT versusLCT). Therefore, it can be assumed that a larger proportion of the glucose intake in the MCT group was used in the nonoxidative pathway, e.g.synthesis of fat as compared with the LCT group. There was no difference in the rate of endogenous glucose production and glucose C recycling between the two groups. In addition, the metabolic rate calculated from the rate of oxygen consumption was also similar (59 ± 5 versus62 ± 4 kcal·kg−1·d−1MCT versusLCT). We conclude that, in preterm infants fed a formula containing 38 MCT in their fat blend, glucose oxidation is significantly decreased whereas lipogenesis is probably increased. These findings may provide a mechanism for the so-called metabolic inefficiency of MCT.