Longitudinal studies on metabolic rate, heat loss, and energy cost of growth in low birth weight infants.

Longitudinal studies on total and resting metabolic rate and total heat loss were made in 14 LBW infants, age 1-58 d. Metabolic rate was calculated from indirect calorimetry, heat loss was measured by direct calorimetry. Total metabolic rate and total heat loss were lowest during the first week of life, 178.9 +/- 18.0 and 171.8 +/- 15.5 kJ X kg-1 X 24 h-1, respectively. During the age period of 8-58 d both total metabolic rate and heat loss showed a very slight increase with age: mean total metabolic rate was 278.8 +/- 2.6 and mean total heat loss 257.0 +/- 3.4 kJ X kg-1. during the first week of life and 248.0 +/- 2.5 during d 8-58. The energy cost of components of new tissue was calculated from the energy balance equation. Ecomponents during the second week of life was 25.5 +/- 4.9 kJ/g weight gain and 11.9 +/- 0.4 kJ/g weight gain over subsequent weeks. The net energy cost of tissue synthesis, calculated from the difference between indirect and direct calorimetry was 3.2 +/- 1.1 kJ/g weight gain during the second week and 1.1 +/- 0.1 kJ/g weight gain in the following weeks. A neonate who receives a caloric intake of 535 kJ X kg-1 X 24 h-1 and is growing at a rate of 17 g X kg-1 X 24 h-1 will use 42% of the caloric intake for maintenance and thermoregulation, 6% for activity, 38% for the components of new tissue, 4% for tissue synthesis and 10% for loss in faeces and urine.