Gastrointestinale Prozessierung und physiologische Bedeutung von Oligosacchariden der Humanmilch

Abstract

Gastrointestinal processing and physiological relevance of oligosaccharides from human milk

Human milk is especially rich in complex oligosaccharides providing an extraordinary structural diversity. Neither their fate after intake by the human infant nor their biological functions are known. Therefore, the aim of the present study was to answer the following questions:

Are the secreted pancreatic enzymes and/or the brush border bound hydrolases in the small intestine capable of hydrolysing the free oligosaccharides of human milk ?
Our in vitro as well as in vivo experiments demonstrated an exceptional enzymatic stability of the oligosaccharides against hydrolases of the gastrointestinal tract. Even fucose and sialic acid in terminal positions of the oligosaccharides were highly resistant to the enzymatic attack. A complete hydrolysis could only be achieved by applying intracellular or lysosomal enzymes of pancreatic tissues. However, these enzymes are not or only in trace amounts inherent in pancreatic secretions. Therefore, human milk oligosaccharides most likely survive passage along the human small intestine as intact molecules. As shown recently, the milk oligosaccharides are submitted to fermentation in the infants large intestine comparable to other non starch polysaccharides. In this way these oligosaccharides may serve primarily as the soluble fiber fraction of human milk.

Are oligosaccharides of human milk absorbed in their intact form or after partial digestion?
Supported by the results obtained in our in vitro systems, oligosaccharides of human milk may permeate the small intestinal epithelia in intact form with an absorption rate of 2 - 3 %. Within this range of low permeation rates smaller uncharged oligosaccharides show enhanced absorption when compared to the negatively charged or larger saccharides. As there were no differences in the permeation rates between oligosaccharides and mannitol, the oligosaccharides most likely use the paracellular permeation route; similar to mannitol. This implies that intracellular hydrolysis in epithelia may not be of great importance. When oligosaccharides were administered to young mice, small amounts were detected in the liver as well as in the urine, partly in intact form. However, the absorption rates in this animal model were similar to those determined in Caco-2 monolayer cultures. As human milk oligosaccharides mostly escape absorption, larger amounts are submitted to the microflora of the human infant. This might be important for the selection of a particular colonic flora. Furthermore, the fermentation products of the oligosaccharides such as butyrate may serve as energy fuel in colonic cells and provoke the growth of the epithelium.

Do oligosaccharides of human milk affect the cell cycle of intestinal epithelial cells ?
The high stability and low absorption rate of the oligosaccharides provide an intense contact with the intestinal epithelial cells. The results of our studies do not indicate a profound effect of milk oligosaccharides on proliferation, differentiation or apoptosis of epithelial cells. Although proliferation rates of Caco-2 and HT-29 cells were significantly reduced, especially by high concentrations of acidic or high molecular weight oligosaccharides, this was not accompanied by an increase in differentiation or induction of physiological cell death. The reduced proliferation rates seen with some human milk oligosaccharides seem to be of no great importance under physiological conditions and in comparison to other food ingredients.

Do free or peptide bound milk oligosaccharides possess anti-adhesive properties ?
According to the literature several milk oligosaccharides have anti-bacterial as well as anti-viral properties in acting as soluble receptors in the respiratory tract, the gastrointestinal as well as the urinary tract. To test this assumption for the milk oligosaccharides available to us, we developed different model systems. In spite of several methods to test the adherence of bacterial strains to epithelial cells or isolated brush border membranes, none of the human milk oligosaccharides showed reproducible effects on adhesion. Only a cell culture model of Caco-2 cells provided consistent results showing distinct anti-adhesive effects of the glycosylated part of bovine k-casein at higher concentrations but not of free milk oligosaccharides. These results, along with the low absorption rates, do not support an important role of human milk oligosaccharides as anti-adhesive molecules in extraintestinal tissues such as the respiratory or urogenital tract.

In summary, we conclude with respect to the fate and biological role of human milk oligosaccharides, that - based on their extraordinary stability against gastrointestinal hydrolysis and their low rates of absorption in the small intestine - they seem to function primarily as the soluble fibre fraction of human milk. As there might be the possibility of further biological functions of human milk oligosaccharides in infant metabolism, they clearly need to be investigated further.