Figure 1.

Hypothesis for effects of diet in relation to the development of chronic inflammatory diseases (CIDs). Diet (meat,⁹⁷ fibre,⁹⁷ animal fat,⁹⁸ n-3³⁰ and n-6 polyunsaturated fatty acids, vitamins A⁹⁹ and D,¹⁰⁰ carotenoids,¹⁰¹ smoking, gluten¹⁰²) may affect the immune system^{103 104} either directly or indirectly via, for example, the activity and composition of the gut microbiome.^{105 106} The effect of low intake of fibre/high intake of red and processed meat is shown at left: in short, low intake of fibre (which could otherwise serve as a nutrient for the microbes) may lead to the microbial metabolism of mucus and to decrease of the intestinal mucus layer.^{61 107 108} A high intake of red and processed meat may render the mucus layer penetrable to, for example, bacteria by reducing the disulphide bonds in the mucus network.^{8 105 109} Thus, microbes may reach the epithelium^{8 110 111} and activate the immune system.^{8 112 113} There is some support for such a mechanism in CIDs,⁸ including findings of; high amounts of sulphate-reducing bacteria in patients with inflammatory bowel disease (IBD)^{8 111 114}; association of high-fibre intake with low risk of IBD among 170 776 participants from the prospective Nurses’ Health Study I^{8 27}; and association of high intake of red meat and total protein and risk of developing inflammatory polyarthritis in the population-based prospective cohort of 25 630 participants from the European Prospective Investigation of Cancer in Norfolk.^{8 39 40} IEC, intestinal epithelial cell; MAMPs, microbe-associated molecular patterns; NFkB, nuclear factor kappa-light-chain-enhancer of activated B cells; TLRs, toll-like receptors; TNF, tumor necrosis factor. Figure from Christensen et al ⁸ (copyright 2018 by Vibeke Andersen).