Nutrition of the Low-Birth-Weight Infant

There are three major considerations affecting the quantity and quality of protein to be given to a low-birth-weight baby. These are (a) requirements for normal growth and body composition, (b) development of protein and amino acid metabolism, and (c) renal function. A number of different techniques have been used to estimate the requirements in adults, for example, nitrogen balance, body composition, amino acid turnover, fractional nitrogen excretion, and calorimetry. The application of these methods to the problem of determining the protein requirement of the small preterm infant is very difficult, because the optimal rate at which preterm infants ex utero should accumulate protein has yet to be determined. The classical factorial method has been used to estimate protein requirements. The results suggest that the amount of protein in human milk and in many formulas designed for normal babies is inadequate (33). The calculations assume that the in utero growth rate remains optimal ex utero and that the body composition of the developing fetus is adequately known. Also, many empirical studies have shown that babies fed expressed human milk gain weight at a slower rate than those receiving a cow's milk-based formula (5,8,20,32). But the question remains, does this difference in weight gain have any advantage? Is the difference in weight gain due to a difference in protein intake or to other factors, for example, a lower energy content or a lower sodium content in human milk compared with cow's milk-based formulas. In any case, it is not sufficient merely to estimate protein requirements on the basis of theoretical calculations from in utero accumulation rates or to administer large quantities of protein in order to attain intrauterine growth rates in the immediate extrauterine environment. The biochemical immaturity of the human preterm infant makes him nutritionally very vulnerable, and the margin between an adequate protein intake and protein undernutrition or overnutrition with possible adverse effects is small. There is an incomplete development of several amino acid metabolic pathways in the newborn infant, especially in the small preterm infant (22). Thus, many of the amino acids previously thought to be nonessential, for example, cysteine, taurine, may be essential, at least for the immature organism, and must be supplied in the diet. Also, amino acid catabolism is incomplete, and an administration of protein in quantities which exceed the needs for synthesis stress the infants metabolic