Luminal digestion of human milk proteins in suckling and weanling rats

Recovery of 15N-lactoferrin is higher than that of 15N-casein in the small intestine of suckling, but not adult miniature pigs

Performance of biological functions of lactoferrin in the small intestine requires at least some resistance to degradation. Therefore, we studied prececal digestibility of lactoferrin in comparison to casein both in suckling and adult miniature pigs, applying 15N-labeled proteins. In study 1, 43 piglets (10-d-old), deprived of food for 12 h received 10 mL of sow's milk supplemented with 120 mg of 15N-labeled protein (porcine or bovine lactoferrin or bovine casein). Piglets were anesthetized 150 min later, after which the small intestine was excised, cut into three sections, and chyme was collected. In study 2, nine food-deprived boars fitted with T-canulae at the terminal ileum were given two semisynthetic experimental meals (204 g) in a cross-over design, 2 wk apart. One contained 7.5% (g/100 g) 15N-labeled bovine casein, the other 1.25% 15N-labeled bovine lactoferrin. Both were adjusted to 15% total protein with nonlabeled casein. Ileal chyme was collected from the canula over 33 h postprandially. All diets contained the indigestible marker chromic oxide. 15N-digestibility of lactoferrin, both porcine (84.4 +/- 3.2%) and bovine (82.3 +/- 4.8%), was significantly lower than casein digestibility (97.6 +/- 0.5%) in the distal small intestine of suckling piglets (P < 0.05). Based on immunoblotting after acrylamide electrophoresis, 4.5% of non- and partially digested lactoferrin was found in the last third of the small intestine of piglets. In adult miniature pigs there was no difference in 15N-digestibility of bovine lactoferrin compared to bovine casein (90.7 +/- 1.9% vs. 93.9 +/- 1.0%, P > 0.05). In suckling miniature pigs, the reduced digestibility of lactoferrin may provide the prerequisite for biological actions along the whole intestinal tract. The source of lactoferrin, porcine or bovine, made no difference in this respect.

Digestion of proteins in human milk, human milk fortifier, and preterm formula in infant rhesus monkeys

BACKGROUND

There is limited information in the literature on the capacity of the preterm infant to digest human and bovine milk proteins. We therefore studied in vivo the luminal phase of the hydrolysis of proteins in human milk, human milk fortifier, and preterm formula in preterm rhesus monkeys and in infant rhesus monkeys at 6 weeks and 7 months of age.

METHODS

Protein hydrolysis was followed by polyacrylamide gradient gel electrophoresis and electroimmunoassay. The serum level of absorbed unhydrolyzed human alpha-lactalbumin was measured by a radioimmunoassay method. Trypsin and elastase activities in duodenal contents were measured before and after the meal.

RESULTS

In 6-week-old monkeys, the enzyme activities decreased by 50% postprandially, whereas they increased in 7-month-old monkeys. In preterm and in 6-week-old monkeys, hydrolysis of human and bovine whey proteins was slow, and in 6-week-old monkeys, 30-50% of the proteins could still be detected immunochemically in duodenal contents after 60 min. At these ages, serum level of absorbed alpha-lactalbumin were high. At 7 months of age, no or small (lactoferrin and bovine serum albumin) amounts of the proteins could be detected in duodenal contents after 15 min. At this age alpha-lactalbumin was not measurable in serum.

CONCLUSIONS

The low capacity to digest whey proteins in suckling monkeys may depend upon an immaturity of the exocrine pancreas to respond to secretogogues.