Preterm infants between 500 and 1000 g birth weight are surviving at increased rates. Most of their body growth and the associated development of functional capacity, therefore, take place outside of the uterus. Nutrition to support this growth and development must be provided by intravenous and enteral routes rather than by the placenta.

Many advances in intravenous and enteral nutrition of preterm infants have been developed over the past several years since the first edition of Neonatal Nutrition and Metabolism, but the increased survival at lower birth weights, advanced degree of immaturity, and increased dependence on extrauterine nutrition of these unique infants are providing renewed interest in the absolute importance of postnatal nutrition. Furthermore, the diminishing frequency and severity of other disorders in these infants means that their many adverse long-term outcomes cannot be blamed solely, or even primarily, on the consequences of other morbidities. Growth and development of sensitive organs, particularly the brain, clearly are dependent on unique, though variable, mixes of specific nutrients, provided at optimal rates and by safe and efficacious routes. There also is abundant evidence from animal experiments and human observational studies that prolonged undernutrition during critical periods of development (between 22–40 weeks postconceptional age for humans) adversely affects long-term growth and neurodevelopmental and neurocognitive outcomes. Despite the advances in nutrition of these infants, therefore, we now are at a new threshold of determining which specific nutrients should be provided to these infants, at what rates, in what mixtures, and by what means, to optimize their growth and development.