Fibre 7th in a series
FIBRE
by Don Pearson, Technical Director BioBrew Ltd
The gut microbiome gets most of its nutrients from our diet and conversely, they help us digest some of the food we eat. It should come as no surprise that diet has a huge impact on the composition of the gut microbiome and, consequently, on function of our immune systems. Diet influences many aspects of the microbiomes interactions with the immune system. This includes, particularly, the permeability of the intestinal barrier, and how the immune system reacts to moderate stimuli.
Modern diets, particularly those high in fast food, are characterized by too much highly palatable energy-dense foods. 
These typically including high levels of animal protein, saturated fats, simple sugars and salt, but low amounts of plant-derived fibres. And this is exactly the dietary pattern that is being increasingly linked to immune dysfunctions associated with the gut microbiota. For example, a diet high in salt and long-chain saturated fatty acids may stimulate the harmful actions of some lymphocytes, which may increase the risk of autoimmune reactions (Kleinewietfeld, 2013)
Short-chain fatty acids (SCFAs), like acetate and butyrate, which are produced from dietary fibres by certain microbial species, promote the activity of regulatory lymphocytes. These suppress the abnormal activation of other immune cells. Fibre therefore plays a very important part in maintaining immune tolerance to the gut microbiome (Honda and Littman, 2016).
SCFAs also feed the epithelium directly and promote gut homeostasis by increasing intestinal secretions and decreasing inflammatory responses. SCFAs can reach other organs, such as the brain where they also decrease neuroinflammatory responses that are known to underlie many neurodegenerative diseases (Levy et.al. 2015) SCFAs are one of the reasons why fibre-rich diets support our immune system (Honda and Littman, 2016).
Summary
SCFA, are made when gut microbes ferment fibre. Eating a diet rich in plant foods that can be converted into SCFA helps keep the brain and immune system healthy.
Honda K, Littman DR. (2016). The microbiota in adaptive immune homeostasis and disease. Nature. 2016 Jul 7;535(7610):75-84. doi: 10.1038/nature18848.
Kleinewietfeld M, Manzel A, Titze J, Kvakan H, Yosef N, Linker RA, Muller DN, Hafler DA. (2013). Sodium chloride drives autoimmune disease by the induction of pathogenic TH17 cells. Nature. 2013 Apr 25;496(7446):518-22. doi: 10.1038/nature11868. Epub 2013 Mar 6
Levy, M., Thaiss, C.A. & Elinav, E. (2015). Metagenomic cross-talk: the regulatory interplay between immunogenomics and the microbiome. Genome Med 7, 120 (2015). https://doi.org/10.1186/s13073-015-0249-9