Due to the development of new 'bedside' investigative methods, relatively abstract physiologic concepts such as energy cost of growth, efficiency of protein gain, metabolic cost of protein gain and protein turnover have been quantified in very low birthweight infants. 'Healthy' premature infants expend about 30% of their energy to cover the metabolic cost of growth. Stable isotope techniques using 15N-(or 13C)-labeled amino acids gave a new insight into this very high energy demanding process represented by the protein accretion in growing tissues. It has been demonstrated that the rate of protein synthesis (10-12 g/kg/day) greatly exceeds that necessary for net protein gain (2 g/kg/day). The postnatal growth and protein metabolism have different characteristics in 'healthy', 'sick' or 'intrauterine undernourished' very low birthweight infants.