What Causes IBS?

The cause of IBS remains unclear; however, there are several explanations which are beginning to emerge to explain the symptoms of IBS.

In general, stress and emotional state along with social factors and gut motility and visceral sensitivity interact in a complex way to induce and exacerbate the symptoms of IBS.

A number of causes have been proposed to explain the symptoms. These include:

  • food sensitivity
  • intestinal inflammation
  • altered gastrointestinal motility
  • visceral hypersensitivity
  • alterations in gut bacteria
  • bacterial overgrowth in the small intestine (SIBO)
  • post-infectious
  • dysregulation of immune function
  • brain-gut axis interactions
  • genetic and environmental factors

It is likely that there is a complex interplay among these factors that results in the symptoms in people with IBS.

Three factors seem to be responsible for many IBS symptoms:

  • Altered gastrointestinal motility
  • Altered sensation within the gastrointestinal tract
  • Stress and coping strategies

Altered gastrointestinal motility

The traditional understanding of IBS centres around dysfunction in the movement of food particles through the colon and an exaggerated gut response to eating or drinking. There is increasing recognition that dietary factors may play a major role in IBS. Diet translates into altered gastrointestinal motility, hence why diet modification, eg: low FODMAP diet or gluten free diet, have become so helpful for some in controlling their IBS symptoms. The serotonin reuptake transporter (SERT), a complex microscopic mechanism at the cellular level in the bowel, is also involved in affecting gut motility. Too much serotonin contributes to diarrhea and too little contributes to constipation. There are medications which target this mechanism. Separately, bile acids in the bowel also effect gut motility in some individuals. Too much bile acids can contribute to diarrhea.

Altered sensation within the gastrointestinal tract

People with IBS tend to experience increased sensations from the gastrointestinal tract in response to stimuli, such as distension and visceral hypersensitivity. This has been demonstrated in studies involving balloon distension of the intestine where patients with IBS experience awareness and pain with a smaller balloon volume than patients without IBS.

Stress and Coping Strategies

Stress and coping strategies play a significant role in patients with IBS, tending to increase the frequency and severity of symptoms such as abdominal pain and diarrhea. It also influences the way a patient handles their symptoms, when they decide to see a physician and their beliefs and expectations regarding the illness and its treatment.

Emerging evidence on the role of gastrointestinal bacteria and the immune system in IBS

Gastrointestinal bacteria have an important role in the normal function of the gastrointestinal tract. There is a growing body of evidence suggesting that they play a complex role in a number of gastrointestinal disorders including IBS, however, it is unclear whether the alterations in bacterial flora are a cause or a consequence of the disorder. There is some evidence that low-grade inflammation and abnormalities of immune function may also have a role in the pathogenesis of IBS. Loss in function of the bowel epithelial barrier could lead to activation of mucosal immune responses and set in motion events that are closely related to the origin and clinical manifestations of IBS. This questions whether IBS is more akin to being a milder form of Inflammatory Bowel Disease (IBD), a digestive illness which is characterized by visible changes in the digestive tract. 

Evidence for bacteria contributing to the cause of IBS has shown that:

  • Alterations in the gastrointestinal bacteria affect gastrointestinal function such as changes in intestinal motility and visceral hypersensitivity
  • People with IBS appear to have an altered composition of gastrointestinal bacteria, specifically a reduction in diversity of bacterial species and an overabundance of some bacterial species 
  • Gastrointestinal bacteria responsible for fermentation of poorly absorbed fermentable carbohydrates may be altered in people with IBS resulting in looser stools and increased gas production
  • There may be increased permeability of the gut in people with IBS due to alterations in the gastrointestinal bacteria and mucosal inflammation
  • Gastrointestinal bacteria may also play a role in gut motility and the perception of pain due to factors such as stimulation of smooth muscle and bile acid malabsorption
  • There is a similarity in symptoms between people with small-intestinal bacterial overgrowth and those with IBS
  • Some people with IBS have increased levels of activated mast cells, histamine, intestinal epithelial lymphocytes, T cells and pro- and anti-inflammatory cytokines in the gastrointestinal tract

Source(s):
Irritable Bowel Syndrome in adults. BPJ 58 Feb. 2014
Gastroenterology 2016;150:1305–1318
Cedars-Sinai - Pimentel Laboratory