There are two methods used for estimating the protein intake for ELBW infants necessary to maintain the intrauterine rate of protein accretion:
Factorial method, which includes an estimate of inevitable urinary nitrogen losses (i.e., the losses that occur in the absence of nitrogen intake) and an estimate of the amount deposited in utero corrected for efficiency of absorption and deposition.
Actual intake method, which determines the actual intake that supports intrauterine rates of growth and nitrogen accretion.
Interestingly, the two approaches do not result in the same estimate of protein requirement. The factorial method, depending on the assumptions made concerning inevitable nitrogen losses and efficiency of absorption and deposition, usually yields an estimate of approximately 4 g/kg per day to support intrauterine rates of growth and protein accretion. The actual intake method suggests that a protein intake of approximately 3 g/kg per day supports intrauterine rates of growth and nitrogen accretion.
Replicating the body composition of the fetus of the same postconceptional age as the preterm infant is as important a goal as achieving the fetal rate of weight gain. This strategy of promoting accretion of more lean mass but less fat deposition may have life-long implications. It appears with current strategies we may be promoting fat deposition and not enough lean mass. However, insufficient data are available concerning the body composition of infants fed different nutrition regimens.