The preterm infant stomach actively degrades milk proteins with increasing breakdown across digestion time

Abstract

Aim: This study investigated the effect of time post-ingestion on gastric digestion and gastric hormones after feeding preterm infants unfortified and fortified human milk.

Methods: Human milk and infant gastric samples were collected from 14 preterm (23-32 weeks birth gestational age) mother-infant pairs within 7-98 days postnatal age. Gastric samples were collected one, two and three hours after beginning of feeding. Samples were analysed for pH, proteolysis, general protease activity and the concentrations of pepsin, gastrin and gastrin-releasing peptide (GRP). One-way ANOVA with repeated measures followed by Tukey's multiple comparisons test was used.

Results: Gastric pH was significantly decreased after each hour in the preterm infant stomach from one to three hours postprandial. Proteolysis increased significantly from human milk to gastric contents at one, two and three hours postprandial (by 62, 131% and 181%, p < 0.05). General protease activity increased significantly by 58% from human milk to the gastric contents at two hours postprandial. GRP was present in human milk, whereas gastrin was produced in the infant stomach.

Conclusion: Although preterm infants may digest human milk proteins to a lesser extent than term infants, we demonstrated that the preterm infant stomach actively degrades milk proteins with increasing breakdown over digestion time.

Keywords: Enzyme; Gastric proteolysis; Gastrointestinal hormones; Pepsin; Preterm milk.

Figure 1.

Schematic of the stimulation and release of GRP, gastrin and pepsinogen in the stomach. (A) GRP from nerve fibers stimulates the release of gastrin by G cells in the stomach. (B) Gastrin stimulates the secretion of HCl from parietal cells in the stomach. (C) Gastrin also stimulates chief cells to produce pepsinogen in the stomach. (D) HCl activates gastric pepsinogen to pepsin.

Figure 2.

(A) Change of pH in human milk (blue boxplot) to infant gastric contents at 1 (G-1h, pink boxplot), 2 (G-2h, red boxplot) and 3 h (G-3h, orange boxplot) after the beginning of feeding. Paired milk and gastric samples were collected in preterm infants (23–32 wk of birth gestational age, 7–98 days of postnatal age). Values are min, median and max, n = 14 for milk, G-1h, G-2h and G-3h. Letters a, b, c and d show statistically significant differences between groups (p < 0.05) using one-way ANOVA with repeated measures followed with Tukey’s multiple comparisons test. (B) Change of pH in human milk across postnatal age (23–32 wk) of preterm infants. (C) Change of pH in gastric contents at 2 h postprandial across gestational age (23–32 wk) of preterm infants. Values are p-values for n = 14 for postnatal age (B) and gestational age (C) via regression linear models. r, Pearson correlation coefficient.

Figure 3.

(A) Proteolysis and (C) general protease activity in human milk (blue bars) and infant gastric samples at 1 (G-1h, pink), 2 (G-2h, red) and 3 h (G-3h, orange) after the beginning of feeding. Paired milk and gastric samples were collected in preterm infants (23–32 wk of birth gestational age, 7–98 days of postnatal age). Values are min, median and max, n = 14 for milk, G-1h, G-2h and G-3h. Letters a, b, c and d show statistically significant differences between groups (p < 0.05) using one-way ANOVA with repeated measures followed with Tukey’s multiple comparisons test. (B) Proteolysis in fortified milk (blue boxplot) and unfortified milk (red boxplot) and gastric contents at 1, 2 and 3 h postprandial. Asterisks show statistically significant differences between fortified milk and unfortified milk using two-way ANOVA with repeated measures followed with Sidak’s multiple comparisons test (**: p < 0.01; * p < 0.05). Values are min, median and max, n = 9 for fortified and n = 5 for unfortified for milk, G-1h, G-2h and G-3 h.

Figure 4.

Concentration of (A) pepsin, (B) gastrin and (C) gastrin-released peptide (GRP) in human milk (blue boxplot) and infant gastric contents at 1 (G-1h, pink boxplot), 2 (G-2h, red boxplot) and 3 h (G-3h, orange boxplot) after the beginning of feeding. Paired milk and gastric samples were collected in preterm infants (23–32 wk of birth gestational age, 7–98 days of postnatal age). Values are min, median and max, n = 14 for milk, G-1h, G-2h and G-3 h. No statistically significant difference between groups (p > 0.05) using one-way ANOVA with repeated measures.