Energy requirements and utilization of the low birthweight infant.

Energy balance data have been obtained on 24 well grown immature (AGA) infants (mean BW 1554 g) and 16 small-for-gestational age (SGA) infants (mean BW 1210 g) using bomb calorimetry on milk and excreta over 7-10 days periods. Simultaneous measurements of energy expenditure were made, using a Kipp diaferometer. The main findings were: At any particular gestational age, digestible energy (the proportion of dietary energy absorbed) was similiar in AGA and SGA infants. Energy digestibility improved with maturation in all infants. Retained (metabolisable) energy was also similar in AGA and SGA infants, and averaged 65% of the intake at 30 weeks, 78% at 32 weeks, and 85% at 40 weeks post-conceptional age. Weight gain was linearly correlated with metabolisable energy in AGA infants, an energy cost of approximately 24 kJ/g (5.7 kcal). Maintenance energy requirement at zero weight gain in AGA infants was approximately 270 kJ/kg/d (64 kcal/kg/d). Resting energy expenditure was significantly higher in SGA than in AGA infants, but postprandial metabolism was less. In spite of these differences, rates of weight gain were similar in the two groups of infants. Energy expended in activity was probably less than 20% of the total energy turnover in both groups of infants. Energy balance was profoundly impaired by acidosis in all infants.