Plasma amino acids in term neonates after a feed of human milk or formula. I. Total amino acids and glycine/valine ratios as reflectors of protein intake.

Human milk and formulas with different quantities and qualities of protein were compared by measuring sequential postprandial changes in total amino acids and glycine/valine ratios in plasma of 23 healthy term neonates who had previously been breast-fed ad libitum. At the mean age of 5.5 days the infants received from a bottle 1/36 of their body weight of banked human milk (true protein 0.8 g/100 ml), or formula (1.5 g/100 ml of protein, whey-to-casein ratio 60 : 40 (adapted) or 18 : 82 (non-adapted), or 3.0 g/ml of protein, 60 : 40) after a 3.5-4 hour fast. After the feed, the sum of plasma free amino acids rose and the glycine/valine ratio fell, these changes being significant as soon as 30 min after the start of feeding and maximal at 30 min after human milk but at 60 min after the formulas. Both values returned to prefeeding levels, after human milk in 2-3 hours and after the two 1.5 g/100 ml formulas in about 4 hours, but after the 3 g/100 ml formula were still notably altered at 3.5 hours. The postprandial changes measured seemed to reflect dietary protein intake, and to be unaffected by protein quality (adapted vs. non-adapted formulas). The results show that in studies of plasma amino acids in young infants sampling times should be standardized exactly with respect to type of previous feeding and duration of fasting.