Fibre fermentation in the ileum : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nutritional Sciences at Massey University, Palmerston North, New Zealand
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Date
2022
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Massey University
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Abstract
Recently, several studies have suggested that the microbes present in the ileum (i.e., the end of the small intestine) can ferment dietary fibre resulting in organic acid production and contribute to the overall gastrointestinal tract (GIT) fermentation. However, studying human ileal fermentation is challenging due to inaccessibility of the small intestine.
The aim was to validate a newly developed and optimised in vivo/in vitro ileal fermentation assay based on the growing pig as an animal model for human adults. After the assay was validated, this method was used to quantify ileal fermentation and compare this with large intestinal fermentation. In addition, the effect of diet on ileal fermentation and which factor was a greater contributor to in vitro ileal fermentation (inoculum or substrate) were studied.
Firstly, in vitro ileal organic matter (OM) fermentability was similar to in vivo fermentability in the conventional grown pig. Artificially rearing and inoculating young pigs with an infant faecal inoculum did not improve the model. Secondly, the ileal microbiota from pigs and human ileostomates was found to have similar in vitro OM fermentability and organic acid production for arabinogalactan, fructooligosaccharides and pectin, even though some differences were found in the ileal microbial community. Therefore, the in vivo/in vitro ileal fermentation assay using conventional pigs is a preferred and valid model for studying ileal fermentation in the adult human. It was found that ileal fermentation was quantitatively significant and similar in magnitude to hindgut fermentation when using this validated assay. However, the microbial community and organic acid production (mainly acetic acid) in the ileum differed. It was also found that partly replacing cellulose with more fermentable fibres in the diet affected the ileal microbial community and its fermentative capacity in growing pigs. Lastly, the substrate (i.e., different fibre sources) was found to have a greater effect on ileal fermentation than the inoculum (i.e., different ileal microbiota obtained by feeding pigs different diets).
In conclusion, this work has demonstrated the quantitatively significant contribution of ileal fermentation to overall GIT fermentation, and that the in vivo/in vitro ileal fermentation assay using the growing pig is a valid assay for studying ileal fermentation in the adult human. Dietary intervention can be used to shape ileal microbiota and fermentation.
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Figures 2.1 and 2.2 are used with permission, and Chapters 5 and 6 are published under license.
Keywords
Ileum, Microbiology, Fermentation, Fiber in human nutrition, Fiber in animal nutrition