Diet and microbiome in the beginning of the sequence of gut inflammation

Abstract

Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract due, at least partially, to an aberrant and excessive mucosal immune response to gut bacteria in genetically-predisposed individuals under certain environmental factors. The incidence of IBD is rising in western and newly industrialized countries, paralleling the increase of westernized dietary patterns, through new antigens, epithelial function and permeability, epigenetic mechanisms (e.g., DNA methylation), and alteration of the gut microbiome. Alteration in the composition and functionality of the gut microbiome (including bacteria, viruses and fungi) seems to be a nuclear pathogenic factor. The microbiome itself is dynamic, and the changes in food quality, dietary habits, living conditions and hygiene of these western societies, could interact in a complex manner as modulators of dysbiosis, thereby influencing the activation of immune cells' promoting inflammation. The microbiome produces diverse small molecules via several metabolic ways, with the fiber-derived short-chain fatty acids (i.e., butyrate) as main elements and having anti-inflammatory effects. These metabolites and some micronutrients of the diet (i.e., vitamins, folic acid, beta carotene and trace elements) are regulators of innate and adaptive intestinal immune homeostasis. An excessive and unhealthy consumption of sugar, animal fat and a low-vegetable and -fiber diet are risk factors for IBD appearance. Furthermore, metabolism of nutrients in intestinal epithelium and in gut microbiota is altered by inflammation, changing the demand for nutrients needed for homeostasis. This role of food and a reduced gut microbial diversity in causing IBD might also have a prophylactic or therapeutic role for IBD. The relationship between dietary intake, symptoms, and bowel inflammation could lead to dietary and lifestyle recommendations, including diets with abundant fruits, vegetables, olive oil and oily fish, which have anti-inflammatory effects and could prevent dysbiosis and IBD. Dietary modulation and appropriate exclusion diets might be a new complementary management for treatment at disease flares and in refractory patients, even reducing complications, hospitalizations and surgery, through modifying the luminal intestinal environment.

Keywords: Diet; Inflammatory bowel disease; Microbiome; Pathogenia.

Figure 1

Commensal microbiota modulates the response of the immune system in the intestinal mucosa and participates in the metabolism of the epithelium. A: In the case of immune regulation, dendritic cells (DCs), activated in M cells, modulate regulatory T lymphocytes (Tregs) through anti-inflammatory signals (such as interleukin [IL]-10), which in turn inhibit other anti-inflammatory signals (such as IL-10 and transforming growth factor-beta [TGFβ]) of T helper cells 1-17 and macrophages. In the case of epithelial metabolism, the synthesis of short chain fatty acids (SCFAs), the secretion of prostaglandins (PGEs, such as PGE2) and the activity of nuclear factor-kappa B (NF-kB) contributes to an adequate secretion of defensins α and β, to the appropriate functioning of the mucus layer and even the microbiome state in homeostasis; B: The persistence of bacteria adhering to a deteriorated mucus film, the decrease in defensins and the invasion of the epithelium produces the activation of DCs and macrophages, that in turn activate the T helper (Th) cells, Th1 and Th17, by proinflammatory signals (IL-1, IL-6, IL-12, and IL-23), thereby promoting a low apoptosis, clonal expansion with loss of response to anti-inflammatory signals, and contributing to tissue damage mediated by interferon gamma (INFγ) and tumor necrosis factor alpha (TNFα). This figure is based upon data published in Reference 11.

Figure 2

Metabolic epithelial changes in dysbiosis due to loss of obligate anaerobes. A: The eubiosis microbiome contains a significant number of obligate anaerobes that convert dietary fiber into short chain fatty acids (SCFAs), contributing to a C2-type epithelial trophism with high oxygen consumption (O₂), which limits its diffusion to the intestinal lumen and maintains the epithelium in relative hypoxia; B: The deterioration of the microbiome with loss of obligate anaerobes and SCFAs orients the metabolism of the epithelium to a C1-type, with greater glucose fermentation, low O₂ consumption, and generation of lactic acid and nitric oxide (NO). The higher partial pressure of the O₂ not consumed by the C1 colonocytes and the conversion of NO to nitrate (NO₃^-) causes an overgrowth of facultative anaerobes; C: The persistence of the loss of strict anaerobes and their fermentation products ends up causing epithelial damage (via polymorphonuclear neutrophils R-spondin 2), the response of which is crypt hyperplasia and the multiplication of stem cells (SCs). The sustained increase in NO₃^- and O₂ contributes to the persistence of facultative anaerobes and dysbiosis. This figure is based upon data published in reference 39.