Protein Intake in Early Infancy: Effects on Plasma Amino Acid Concentrations, Insulin Metabolism, and Growth

Abstract

ABSTRACT: The effects of different protein intakes on wt gain, insulin secretion, and plasma concentrations of amino acids have been evaluated in a prospective study involving 30 normal term infants. The infants were studied from 4.0 to 6.0 mo of age. Ten infants were breast-fed (BF), the others were randomly divided into two groups of 10 infants. One group was fed a formula containing 1.3 g protein/100 mL (F 1.3), the other a formula with 1.8 g protein/100 mL (F 1.8). The formulas were isocaloric (72 kcaI/100 mL), and the fat concentrations were 3.5 g/100 mL (F 1.3) and 3.2 g/100 mL (F 1.8). AH infants received the same supplementary foods. The urinary C-peptide excretion in the infants fed the F 1.8-formula was 4.4 ± 2.1 nmol/mmol creatinine or 19.4 ± 12.9 nmol/m², significantly higher than that in the infants fed the F 1.3-formula (2.6 ± 1 .5 and 7.9 ± 5.1) or the BF infants (1.7 ± 1.4 and 6.3 ± 6.0). Gain in wt was 18.0 ± 4.3,19.9 ± 3.9, 22.8 ± 1.6 g/kg/wk and corresponded to protein intakes of 1.3 ± 0.2, 1.9 ± 0.3, and 2.6 ± 0.2 g/kg/d, in the BF, F 1.3, and F 1.8 groups, respectively. Gain in length was 6.7 ± 1 .8 (BFgroup), 6.2 ± 2.5 (F 1.3-group), and 7.6 ± 2.2 (F 1.8- group) mm/m/wk. Wt gain correlated with urinary Cpeptide excretion at 6.0 mo (r=0.51, p<0.01) and with protein intake (r=0.43, p<0.01). Furthermore, protein intake correlated with urinary C-peptide excretion (r=0.66, p<0.001). Caloric intake from carbohydrate and fat correlated both with wt gain (r=0.34, p<0.05) and with urinary C-peptide (r=0.44, p<0.05). A higher protein intake during a meal resulted in a higher postprandial excretion of urinary C-peptide. The difference between plasma C-peptide (δC-peptide) before and after a meal was highest in the groups of infants fed the F 1.8-formula. The intakes of the insulin-releasing amino acids (arginine, lysine, leucine, phenylalanine, valine, isoleucine, and threonine) were higher in the F 1.8-group than in the F 1.3- and BF-groups. The sums of the fasting plasma concentrations of these amino acids were 64.8 (BF), 83.6 (F 1.3), and 96.3 (F 1.8) /µ nol/100 mL. The plasma concentrations of the branched-chain amino acids, valine, leucine, and isoleucine correlated with wt gain and plasma valine-glycine ratio increased with higher protein intake. These results suggest that protein amino acid-induced insulin secretion could be a factor promoting growth in infants on high protein intakes.