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

Abstract

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.

Trial registration: ClinicalTrials.gov NCT00957892.

Figures

FIGURE 1.

Mean (±SEM) ratings during the sensory evaluation of the formulas used during infant testing. A trained panel of 10 adults rated the sweetness, savoriness, saltiness, sourness, and bitterness of CMF (white bars), CMF+glu (gray bars), and ePHF (black bars) using the gLMS, a psychophysical tool that allows subjects to rate the perceived intensity of sensations that are arranged semilogarithmically and range from “no sensation” to “strongest imaginable” (eg, 0 = no sensation, 1 = barely detectable, 6 = weak, and 16 = moderate). Bars with different lowercase letters are significantly different, P < 0.05 (ANOVA with Fisher's least-significant-difference post hoc comparisons). CMF, cow milk formula; CMF+glu, cow milk formula with added free glutamate; ePHF, extensive protein hydrolysate formula; gLMS, general labeled magnitude scale.

FIGURE 2.

Amount of formula (mL; mean ± SEM) that the infants consumed while feeding CMF (white bars), CMF+glu (gray bars), and ePHF (black bars) during the first formula meal (A) and the amount of CMF (mL) that the infants consumed during the second formula meal (B). Note that whereas the first formula meal differed across the 3 testing days, the second formula meal was always CMF. Repeated-measures ANOVA with a priori planned comparisons between CMF+glu and CMF and between ePHF and CMF indicated that infants consumed significantly less CMF+glu (P < 0.02) and ePHF (P < 0.04) than CMF during the first formula meal. Infants’ intakes during the second formula meal did not differ across the 3 testing days. *Significantly different from CMF, P < 0.05. CMF, cow milk formula; CMF+glu, cow milk formula with added free glutamate; ePHF, extensive protein hydrolysate formula.