Effects of type of dietary protein on acid-base status, protein nutritional status, plasma levels of amino acids, and nutrient balance in the very low birth weight infant.

Study Objective: To determine the effect of the type of dietary protein (3.3 gm/kg per day) on acid-base status, protein nutritional status, plasma amino acid concentrations, and nutrient (nitrogen, fat, mineral, trace element) balance.

Subjects: Preterm infants (birth weight less than or equal to 1250 gm, gestational age less than or equal to 32 weeks) with no evidence of systemic disease, who had achieved a minimal enteral intake of 110 kcal/kg per day by 21 days of age.

Interventions: Each infant was fed three study formulas that differed only with respect to the ratio of whey to casein (60:40, 35:65, 18:82). Each formula was given for 1 week. At the end each week, blood was drawn and a 48-hour balance was determined.

Main Results: Late metabolic acidosis, uremia, and hyperammonemia were not observed. No differences in pH or serum bicarbonate were noted. Base excess was greater with the casein-predominant formula (18:82 greater than 35:65, 60:40) but remained within normal limits for the preterm infant. Plasma concentrations of threonine (60:40 greater than 35:65 greater than 18:82), phenylalanine, and tyrosine (18:82 greater than 35:65 greater than 60:40) differed. Nitrogen absorption (60:40 less than 35:65, 18:82), nitrogen retention (60:40 less than 35:65, 18:82), fat absorption (60:40, 35:65 greater than 18:82), and phosphorus absorption (60:40 less than 35:65, 18:82) also differed.

Conclusions: At an intake of 3.3 gm/kg per day, the type of dietary protein had little effect on metabolic status. Differences in plasma amino acid concentrations and nutrient balance suggest that a formula containing protein with a whey/casein ratio of 35:65 may be preferable to that with a whey/casein ratio of 60:40 or 18:82 for the very low birth weight infant.