Non-invasive methods, including stable isotope techniques, indirect ca
lorimetry, nutritional balance and skinfold thickness, have given a ne
w insight into early postnatal growth in neonates. Neonates and premat
ure infants in particular, create an unusual opportunity to study the
fluid and metabolic adaptation to extrauterine life because their phys
ical environment can be controlled, fluid and energy balance can be me
asured and the link between metabolism and the energetics of their pos
tnatal growth can be assessed accurately. Thus the postnatal time cour
se of total body water, heat production, energy cost of growth and com
position of weight gain have been quantified in a series of ''healthy'
' low-birth-weight premature infants. These results show that total bo
dy water is remarkably stable between postnatal days 3-21. Energy expe
nditure and heat production rates increase postnatally from mean value
s of 40 kcal/kg/day during the first week to 60 kcal/kg/day in the thi
rd week. An apparent energy balance deficit of 180 kcal/kg can be ascr
ibed to premature delivery. The cost of protein metabolism is the high
est energy demanding process related to growth. The fact that nitrogen
balance becomes positive within 72 h after birth places the newborn i
n a transitional situation of dissociated balance between energy and p
rotein metabolism during early postnatal growth: skinfold thickness, d
ry body mass and fat decrease, while there is a gain in protein and in
crease in supine length. This particular situation ends during the sec
ond postnatal week and soon thereafter the rate of weight gain matches
statural growth. The goals of the following review are to summarize d
ata on total body water and energy metabolism in premature infants and
to discuss how they correlate with physiological aspects of early pos
tnatal growth.