Partition of energy metabolism and energy cost of growth in the very low-birth-weight infant.

Energy requirements are partitioned between needs for maintenance (including resting metabolism, thermoregulation, and muscular activity) and needs for synthesis and storage of new tissue. The partition of energy utilization was evaluated by 22 metabolic and nutritional balance studies in 13 formula-fed (SMA 20/24), growing, appropriate-for-gestational age, very low-birth-weight infants (mean +/- SE birth weight, 1,155 +/- 39 gm; study weight, 1,271 +/- 60 gm; age at study, 21 +/- 2 days; weight gain, 16.8 +/- 1 gm/kg/day). Continuous open-circuit, indirect calorimetry was performed for periods of 6 +/- 0.25 hours in a thermoneutral environment. Results expressed as mean kilocalories per kilogram per day (+/- SE) were: energy intake, 148.6 (+/- 3.9); stool and urine losses, 18.2 (+/- 1.5); metabolizable energy, 130.4 (+/- 3.5); "basal" metabolic rate, 47.0 (+/- 0.75); energy cost of activity, 4.3 (+/- 0.9); thermic effect of food, 11.3 (+/- 0.65); energy stored in new tissue, 67.8 (+/- 3.0). These results provide a partition of energy utilization in very low-birth-weight infants under thermoneutral conditions. Increased activity and a thermal environment outside the neutral range will augment maintenance energy requirements, thus decreasing the amount of energy available for growth if metabolizable energy intake remains constant. The energy cost of growth (ie, for synthesis of, and storage in, new tissue) was determined as 4.9 kcal/gm of weight gain. To attain the equivalent rate of intrauterine weight gain, a metabolizable energy intake of approximately 60 kcal/kg/day in excess of maintenance requirements of 51.3 kcal/dk/day must be provided.