In vitro fermentation and immunomodulating characteristics of dietary fibres

Abstract

Dietary fibres are a diverse group of substances, indigestible by human digestive enzymes, but (partially) fermentable in the human large intestine by the resident microbiota. Many health beneficial effects of fibres such as lowering blood cholesterol levels or increasing stool bulk have been reported. For some fibres, immunomodulating properties have been shown. Other studies investigate the degradation fate of fibres by the bacteria. In this PhD thesis BMDCs from TLR2/4 knock out mice were validated to be unresponsive to naturally present contaminants like LPS and proved to be a good tool to analyse the immune response of dietary fibres. A variety of 44 fibres, was tested on these immune cells and all fibres were found to modulated the immune system differently. Also, different immunomodulating properties of an oat and barley β-glucan having rather similar chemical structures, were found. The insoluble fraction of the β-glucans induced highest amounts of cytokines. As a consequence, sample preparation such as drying, dispersing and heating were shown to affect the immunomodulatory properties. The in vitro fermentation characteristics of barley β-glucan and sugar beet pectin and the immunomodulatory properties of their degradation products on BMDCs were compared and shown to be substrate and degradation product specific. This study showed, that glycosidic degradation products of both fibres induced higher amounts of cytokines than their intact polysaccharide. An in vitro batch fermentation of soluble, indigestible maltodextrins by human faecal inocula was monitored and the activity of carbohydrate degrading enzymes, produced by the microbiota, was analysed. Results revealed that the maltodextrin was only slowly and incompletely fermented, despite the high potential of microbial enzymes present to degrade typical starch linkages.

Overall, this thesis showed that dietary fibres interact and influence the immune system dependent on their individual chemical fine structure. Additionally, an evaluation of the health impact of dietary fibres can only be complete when also glycosidic fermentation products are considered.