Protein quality and quantity in preterm infants receiving the same energy intake.

Fifteen low-birth-weight appropriate for gestational age infants, weighing 1.3 to 1.6 kg, were assigned to three isocaloric formulas differing in the quantity and form of nitrogen delivered. A balance study was done between 21 and 30 days after birth. Nitrogen retention expressed as milligram per kilogram per 100 kcal of metabolizable energy was greater with the 60:40 whey/casein and with the casein hydrolysate preparations than with the 20:80 whey/casein formula, which provided the same amount of energy (150 kcal/kg/day) but smaller quantities of protein (3.5 g/kg/day) than the two others (4.3 g/kg/day and 4.4 g/kg/day). Weight gain until discharge from the hospital and increment of height and head circumference over a three-month period showed an advantage of the two formulas providing higher intakes of protein. This study suggests that with high-energy intakes, protein quality does not affect nitrogen retention and growth unless the quantity of protein ingested falls below a critical level.