What causes IBS?

The exact cause of IBS remains unclear, but several factors have been identified as potentially contributing to the development of the condition. See below for a summary of these factors.

Gastrointestinal motility

Gastrointestinal motility refers to the movement of food/waste through the digestive tract. In the intestinal tract, this occurs through the contraction of the intestine’s smooth muscle. In IBS, normal gut motility is often disturbed, that is, an otherwise healthy bowel is contracting and moving bowel contents in an irregular, abnormal way (Gut dysmotility). This can result in the common IBS symptoms of diarrhoea (Gut contractions are too fast) and constipation (Gut contractions are too slow). The precise mechanisms by which gut motility becomes abnormal are yet to be fully understood.

Changes in the gut microbiome

In our intestinal tract we have a complex arrangement of microorganisms (e.g., bacteria). These microorganisms are predominantly harmless –they coexist andplay a vital role in keeping us healthy, such as helping in the metabolism of nutrients and preventing harmful microbes from propagating and harming us. Evidence thus far has shown that changes in the gut microbiome (i.e., the collection of microorganisms in the gut) may play a role in IBS. The evidence for this can be found in the various studies that have linked changes in the gut bacteria to IBS. Research thus far has associated multiple factors with IBS that suggest the microbiome is a factor in IBS, including:

• Bacterial overgrowth in the small intestine has been associated with IBS
• Use of antibiotics or probiotics has been observed to improve IBS symptoms
• Intestinal or gastric infection (e.g., gastroenteritis) has been associated with post-infection IBS

Although changes in gut bacteria have been associated with IBS, it is too early to tell if these changes can cause IBS, or if they are simply an incidental by-product of IBS.

Post-infectious IBS

Approximately one in ten people with IBS believe their condition began with an infectious illness. It is not uncommon for infections involving the digestive tract to result in the emergence of IBS symptoms, with suggestions that around 10% of bacterial gastroenteritis cases result in IBS. Suggested causes of this include persistent,low-level (post-infection) inflammation that is normally not detected in screening, disruption of the normal gut microbiome, and sensitisation of the nervous system channels linking the gut to the central nervous system.

Visceral hypersensitivity

The research tells us that people with IBS appear to be hyper-sensitive to digestive sensations and symptoms that normally go unnoticed by those that do not have IBS. This mechanism, termed visceral hypersensitivity, can be understood as a lowering of the threshold for noticing, and in turn, being bothered by gut symptoms/sensations. It is not yet understood how or why visceral hypersensitivity develops, but it has been suggested to be caused by a complex interaction between the central and peripheral nervous systems.

Brain-Gut Axis

The digestive tract is packed with 100+ million nerve cells. This system, called the enteric nervous system, has a special bi-directional relationship with the brain (i.e., central nervous system), often referred to as the brain-gutor gut-brain axis.As your digestive tract is performing its’ usual functions of digesting food, the enteric nervous system is relaying information back to the brain (i.e., central nervous system). These signals relate to various kinetic (e.g., distension, motility), microbiological (e.g., intestinal bacterial flora), chemical and immune (e.g., inflammation) processes occurring in the digestive tract. These signals can have a profound impact on our thoughts, feelings and behaviours.

Feelings of gut discomfort that arise in the gut (e.g., pain, cramping), can instil feelings of anxiety or worry, which can cause increased gut motility resulting in further gastrointestinal symptoms and discomfort (e.g., diarrhoea). Pathways from the brain to the digestive tract can also facilitate or hinder the information travelling from the gut back to the brain, affecting the likelihood that a person will notice and act on digestive stimuli. The relationship between the gut and brain is reciprocal, complex, and occurs through various channels (e.g., vagus nerve, endocrine signals, autonomic nervous system). As the communication between the brain and gut become abnormal, so too does the function of the digestive tract.

Stress and psychological factors

Stress can cause alterations in the brain-gut axis resulting in changes in gut function (e.g., motility, permeability, secretion). It is well established that stress can cause gastrointestinal symptoms in healthy individuals (e.g., abdominal cramping, diarrhoea). Stress is similarly linked to the exacerbation of IBS symptoms, and stressful events are often associated with the onset of IBS. It is yet to be determined whether stress is a causative factor in the development of IBS, or if it simply co-occurs with IBS due to the discomfort and disruption caused by symptoms. Stress nonetheless remains as a significant symptom trigger for many people with IBS, and stress management has been shown to be an effective method for reducing IBS symptoms. People with IBS tend to report higher levels of depressive and anxiety disorders than people without IBS.Aside from their distressing nature and impact on quality of life, these conditions can exacerbate IBS symptoms and alter the ways in which someone with IBS copes with symptoms(e.g., increasing the likelihood of engaging in maladaptive coping strategies such as avoidance behaviours).

Comorbidity

IBS can co-occur with several different mental conditions. Many people with IBS report psychiatric illnesses, that is, mental issues that are severe or persistent enough to cause significant disruption in day-to-day life. Examples include major depressive disorder (MDD), persistent depressive disorder (aka dysthymia, also sometimes called chronic depression) and generalised anxiety disorder (GAD). Mental disorders are generally not seen as the cause of IBS but are widely accepted play a significant role in the triggering and exacerbation of symptoms. IBS has also been associated with a higher likelihood of showing somatisation, that is, a tendency to experience psychological distress in the form of physical symptoms. IBS patients are also more likely to have fibromyalgia, a condition where a person experiences pain and fatigue throughout the body without any apparent physical cause.

Familial aggregation and genetics

IBS appears to aggregate within families, that is, someone is 2 – 3 times more likely to have IBS if they have a relative with the condition. Twin studies have suggested that the development of IBS likely involves both genetic and environmental factors. Despite the apparent contribution of genes in IBS, research identifying the specific genetic variations associated with the condition is limited.

Menstrual cycle

The menstrual cycle is associated with fluctuations in female sex hormones (i.e., progesterone, oestrogen). Increased gastrointestinal symptoms (e.g., pain, bloating, diarrhoea, constipation) are common during the menstrual period, both for women with and without IBS, however, women with IBS tend to report more severe gastrointestinal symptoms during their periods. The symptoms of IBS can overlap with other gynaecological issues such as polycystic ovarian disease (PCOS) and endometriosis. If your symptoms are particularly severe around the time of your period, you may want to have your condition reviewed by a gynaecologist to rule out the possibility of any other issues.