Published online by Cambridge University Press: 10 December 2009
Very-low-birth-weight (VLBW) infants face many diseases during the neonatal period that may affect their growth. The weight gain in utero between 24 and 36 weeks of gestation is higher than at any other time during life. In this period the average weight gain rates equal 15–17 g kg−1 day−1 with a more than 3-fold increase of weight over this period. Adequate nutrition in the neonatal period is therefore extremely important for growth.
As protein depletion is one of the factors limiting survival, accretion of body protein is the most important factor for growth if there is an excess of nutrients. Quantifying the magnitude of protein deposition or loss is, therefore, vital if one wants to understand how the various diseases the very premature neonate faces directly after birth affect survival. The purpose of this chapter is to describe the available methods to follow dynamic changes in protein metabolism in the newborn and to interpret the results obtained from these methods. Noteworthily, as proteins or amino acids differ from glucose or fat only in the presence of nitrogen, studies on changes in body protein mass should focus on the nitrogen atom of amino acids.
The conventional method to follow changes in body protein status is the nitrogen (N) balance method. This routine method has been the golden standard for defining minimum levels of dietary protein and essential amino acid intake in humans of all ages, including infants.
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