RELATIONSHIPS BETWEEN WATER, PROTEIN AND ENERGY METABOLISMS DURING EARLY POSTNATAL-GROWTH

Non-invasive methods, including stable isotope techniques indirect calorimetry, nutritional balance and skinfold thickness, have given a new insight into early postnatal growth in the neonates. Neonates and premature infants especially, create an unusual opportunity to study the fluid and metabolic adaptation to extrauterine life because their physical environment can be controlled, their fluid and energy balance can be mesured and the Link between their metabolism and the energetics of their postnatal growth can be assessed with accuracy. Thus the postnatal timecourse of total body water, heat production, energy cost of growth and composition of weight gain have been quantified in a series of ''healthy'' low birthweight premature infants. These results show that total body water is remarkably stable between postnatal day 3 to 21. Energy expenditure and heat production rates increase postnatally from average values of 40 kcal/kgxday during the first week to 60/kgxday in the third week. An apparent energy balance deficit of 180kcal/kg can be ascribed to a premature delivery. The cost of protein metabolism is the highest energy demanding process related to growth. The fact that nitrogen balance becomes positive within 72 hours after birth, places the newborn in a transitional situation of dissociated balance between energy and protein metabolism during early postnatal growth: skinfold thickness, dry body mass and fat decrease while there is a gain in protein and increase in supine length. This particular situation ends during the second postnatal week and soon thereafter the rate of weigth gain matches the statural growth. The goals of the following review are to summarize data on total body water and early postnatal growth in premature infants and to discuss how they correlate to physiological aspects of energy and protein metabolism directly related to the extrauterine adaptation and early postnatal growth.