Inflammatory bowel disease (IBD) is a group of inflammatory conditions of the colon and small intestine, with Crohn's disease and ulcerative colitis (UC) being the principal types. Crohn's disease affects the small intestine and large intestine, as well as the mouth, esophagus, stomach and the anus, whereas UC primarily affects the colon and the rectum.
Signs and symptoms
In spite of Crohn's and UC being distinct diseases, both may present with any of the following symptoms: abdominal pain, diarrhea, rectal bleeding, severe internal cramps/muscle spasms in the region of the pelvis and weight loss. Anemia is the most prevalent extraintestinal complication of inflammatory bowel disease (IBD). Associated complaints or diseases include arthritis, pyoderma gangrenosum, primary sclerosing cholangitis, and non-thyroidal illness syndrome (NTIS). Associations with deep vein thrombosis (DVT) and bronchiolitis obliterans organizing pneumonia (BOOP) have also been reported. Diagnosis is generally by assessment of inflammatory markers in stool followed by colonoscopy with biopsy of pathological lesions.
Causes
IBD is a complex disease which arises as a result of the interaction of environmental and genetic factors leading to immunological responses and inflammation in the intestine.
Diet People living with IBD are keenly interested in diet, but little is known about the impact of diet on these patients. Recent reviews underlined the important role of nutritional counselling in IBD patients. Patients should be encouraged to adopt diets that are best supported by evidence and involve monitoring for the objective resolution of inflammation. A 2022 study found that diets with increased intake of fruits and vegetables, reduction of processed meats and refined carbohydrates, and preference of water for hydration were associated with lower risk of active symptoms with IBD, although increased intake of fruits and vegetables alone did not reduce risk of symptoms with Crohn's disease. A 2022 scientific review also found generally positive outcomes for IBD patients who adhered to the Mediterranean diet (high fruit and vegetable intake). Dietary patterns are associated with a risk for ulcerative colitis. In particular, subjects who were in the highest tertile of the healthy dietary pattern had a 79% lower risk of ulcerative colitis. Gluten sensitivity is common in IBD and associated with having flareups. Gluten sensitivity was reported in 23.6% and 27.3% of Crohn's disease and ulcerative colitis patients, respectively. A diet high in protein, particularly animal protein, and/or high in sugar may be associated with increased risk of IBD and relapses.
Bile acids Emerging evidence indicates that bile acids are important etiological agents in IBD pathogenesis. IBD patients have a consistent pattern of an increased abundance of primary bile acids such as cholic acid and chenodeoxycholic acid (and their conjugated forms), and a decreased abundance of secondary bile acids such as lithocholic acid and deoxycholic acid.
Microbiota The human microbiota consists of 10–100 trillion microorganisms. Several studies have confirmed that the microbiota composition is different in patients with IBD compared to healthy individuals. This difference is more pronounced in patients with Crohn's disease than in those with ulcerative colitis. In IBD patients, there is a decrease or absence of beneficial bacteria such as Bifidobacterium longum, Eubacterium rectale, Faecalibacterium prausnitzii, and Roseburia intestinalis, while harmful species like Bacteroides fragilis, Ruminococcus torques, and Ruminococcus are more abundant. The activation of reactive oxygen species and reactive nitrogen species leads to oxidative stress for both host cells and the gut microbiome. Consequently, in IBD, there is a microbial imbalance, known as dysbiosis, characterized by an increase in functional pathways involved in the microbial response to oxidative stress. This oxidative stress can promote the growth of certain species such as R. gnavus. Another opportunistic bacterium called A. muciniphila contributes to IBD development and is more prevalent in individuals lacking NOD-like receptor 6 (NLRP6). Both R. gnavus and A. muciniphila are bacterial species that are more abundant in IBD. Patients with IBD often exhibit stronger antibody and T-cell responses to microbial antigens. The gut microbiome employs various approaches to interact with the host immune system. For instance, B. fragilis, which is symbiotic in humans, can transfer immune regulatory molecules to immune cells through the secretion of outer membrane vesicles. This mechanism plays a protective role in IBD by activating the non-classical autophagy pathway, dependent on Atg16L1 and NOD2 genes. B. thetaiotaomicron induces the differentiation of T regulatory cells (Tregs) to modulate gut immunity, thus increasing the expression of Gata3 and FoxP3 genes. The colonization of Clostridium spp. can enhance the aggregation of RORγT+ FOXP3 Treg cells, which inhibit the development of Th2 and Th17 cells. Ultimately, this colonization could decrease the response of colonic Th2 and Th17 cells. Also F. prausnitzii attracts CD4 and CD8a (DP8α) regulatory T cells. E. coli Nissle 1917 has the capability to inhibit the growth of Salmonella and other harmful bacteria. It prevents these pathogens from adhering to and invading intestinal epithelial cells, which significantly reduces the likelihood of inflammation in the gut and may also prevent the onset of IBD.
Breach of intestinal barrier
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