Premature Infants have Lower Gastric Digestion Capacity for Human Milk Proteins than Term Infants

Abstract

Objectives: Whether premature infants have lower gastric protein digestive capacity than term infants and the extent to which human milk proteases contribute to overall gastric digestion are unknown and were investigated in this study.

Methods: Human milk and infant gastric samples were collected from 16 preterm (24-32 wk gestational age) and 6 term (38-40 wk gestational age) mother-infant pairs within a range of 5 to 42 days postnatal age. For each pair, an aliquot of human milk was adjusted to pH 4.5 and incubated for 2 hours at 37 °C to simulate the gastric conditions without pepsin (milkinc). Their gastric protein digestion capacity was measured as proteolysis (free N-terminals) and protease activities. Two-way analysis of variance followed by Tukey post hoc test was applied to compare measurements between preterm and term infants as well as among human milk, milkinc, and gastric samples.

Results: Measurements of gastric protein digestion were significantly lower in preterm infants than term infants. Overall milk protease activity did not differ between human milk samples from term- and preterm-delivering mothers. As protease activity did not increase with simulated gastric incubation, milk proteases likely contributed minimally to gastric digestion.

Conclusions: Preterm infants have lower gastric protein digestion capacity than term infants, which could impair nutrient acquisition. Human milk proteases contribute minimally to overall gastric digestion. The limited activity of milk proteases suggests that these enzymes cannot compensate for the premature infant's overall lower gastric protein digestion.

FIGURE 1

(A) pH, (B) proteolysis, (C) general protease activity and (D) pepsin/cathepsin D activity in human milk (white bars), human milk incubated to simulate gastric conditions (milk_inc, gray bars) and infant gastric samples (black bars). Paired milk, milk_inc and gastric samples were separated by gestational age (GA) at birth, either premature (N = 16, 24–32 wk GA, 5–42 days of postnatal age) or term (N = 6, 38–40 wk GA, 14–28 days of postnatal age). Values are mean ± SEM. Letters a, b and c show statistically significant differences between groups (P < 0.05) using two-way ANOVA followed by Tukey’s honest significant difference post hoc test for pH, proteolysis, general protease activity and pepsin/cathepsin D activity.

FIGURE 2

Activity of pepsin estimated by subtracting the paired values from human milk incubated to simulate gastric conditions (milk_inc) from those in the stomach for (A) proteolysis, (B) general protease activity and (C) pepsin/cathepsin D activity. Paired milk_inc and gastric samples were separated by gestational age (GA), premature (N = 16, 24–32 wk GA, 5–42 days of postnatal age) and term (N = 6, 38–40 wk GA, 14–28 days of postnatal age). Values are mean ± SEM. Asterisks show statistically significant differences between variables (**, P < 0.01; * P < 0.05) using t-tests between preterm (white bars) and term (gray bars) infants.