Metabolic balance studies in premature infants

Many investigators over the past three decades have successfully conducted traditional metabolic balance studies in efforts to determine the retention rates of key nutrients important to the optimal growth of preterm infants. Differences in methodologies discussed in this review may explain the inconsistent results of balance studies reported for some nutrients, particularly calcium. These methodologic differences include (1) variability in nutrient intake and nutritional course prior to the balance period, (2) differences in the method of stool collection (with and without markers), (3) use of single versus repeated balance periods, and (4) different durations of balance periods. The data presented here suggest that the variability of net fat absorptions among VLBW infants was decreased when an acclimation period of nutrient intake supportive of growth was provided prior to a metabolic balance study. In addition, the use of markers affected the estimates of net calcium absorption but not fat absorption. Additional factors that may influence net calcium absorption and methods for the estimation of calcium absorption in VLBW infants warrant further investigation. This review describes the methods of specimen collection for metabolic balance studies in VLBW preterm infants that demonstrated reproducible data. The recent application of stable isotope methodology to metabolic balance studies can be extremely advantageous in identifying the rates of nutrient absorption versus endogenous secretion in the GI tract.