Infant regulation of intake: the effect of free glutamate content in infant formulas

Background: We recently discovered that infants randomly assigned to a formula high in free amino acids (extensive protein hydrolysate formula; ePHF) during infancy consumed less formula to satiation and gained less weight than did infants fed an isocaloric formula low in free amino acids (cow milk formula; CMF). Objective: Because ePHF and CMF differ markedly in concentrations of free glutamate, we tested the hypothesis that the higher glutamate concentrations in ePHF promote satiation and satiety. Design: In this counterbalanced, within-subject study, infants <4 mo of age (n = 30) visited our laboratory for 3 sets of 2 consecutive infant-led formula meals over 3 test days. Infants were fed 1 of 3 isocaloric formulas during each first meal: CMF, ePHF, or CMF with added free glutamate to approximate concentrations in ePHF (CMF+glu). When infants signaled hunger again, they were fed a second meal of CMF. From these data, we calculated satiety ratios for each of the 3 formulas by dividing the intermeal interval by the amount of formula consumed during that particular first meal. Results: Infants consumed significantly less CMF+glu (P < 0.02) and ePHF (P < 0.04) than CMF during the first meals. They also showed greater levels of satiety after consuming CMF+glu or ePHF: satiety ratios for CMF+glu (P < 0.03) and ePHF (P < 0.05) were significantly higher than for CMF. Conclusion: These findings suggest a role of free glutamate in infant intake regulation and call into question the claim that formula feeding impairs infants' abilities to self regulate energy intake. This trial was registered at clinicaltrials.gov as NCT00957892. Am J Clin Nutr 2012;95:875-81.