What Causes Irritable Bowel Syndrome (IBS)?

Irritable bowel syndrome (IBS) is a gastrointestinal (GI) disorder that causes symptoms like abdominal pain and bloating, as well as bowel movement changes such as diarrhea, constipation, or both. For many people, IBS symptoms can disrupt daily routines and diminish quality of life.

IBS is considered a functional gastrointestinal disorder, meaning it affects the function of the digestive system without causing structural changes or visible inflammation. While experts have not pinpointed a single cause for IBS, research suggests that a combination of factors likely contributes to its development.

Theories

The exact cause of IBS is not fully understood, but several theories explain its possible development. The most widely accepted belief is that IBS is linked to problems in how the brain and gut work together. However, other factors, such as gut microbiota changes, altered gut motility, and immune system imbalances, may also play significant roles.

Gut-Brain Axis Dysfunction

The gut-brain axis is a communication system between the digestive tract and the brain that regulates many bodily functions, including digestion. Under normal conditions, sensations like food moving through the intestines are barely noticeable. When this system malfunctions, the gut may become overly sensitive, causing discomfort or pain during digestion.

Gut-brain axis dysfunction can also affect how quickly the intestines move food. This may result in food passing too quickly, leading to diarrhea, or too slowly, leading to constipation.

Factors that may disrupt the gut-brain axis include:

• stress – Both short-term and long-term stress can affect gut motility.

• childhood trauma – Early life trauma is more common among people with IBS and may influence brain-gut communication.

• anxiety and depression – These conditions are frequently linked with IBS and can intensify symptoms.

• visceral hypersensitivity – People with IBS often experience stronger pain signals from the gut to the brain during digestion.

Gut Microbiota Imbalance

The gut microbiome consists of trillions of microorganisms, including bacteria, viruses, and fungi, that support digestion and immune function. Dysbiosis, or an imbalance in this community, occurs when there are fewer beneficial bacteria and more harmful ones.

People with IBS often have gut microbiota imbalances. This can be influenced by:

• antibiotic use

• gastrointestinal infections like gastroenteritis

• low-level inflammation in the intestines

• small intestinal bacterial overgrowth (SIBO)

The gut microbiota also interacts with the gut-brain axis, so imbalances may disrupt gut sensitivity and motility.

Enteric Nervous System Hypersensitivity

The enteric nervous system (ENS) is a network of nerves within the intestinal wall that controls muscle contractions, hormone release, and digestive secretions. It coordinates the breakdown, absorption, and movement of food through the digestive system.

In IBS, the ENS may become hypersensitive, leading to discomfort or pain as food and waste move through the intestines. Certain ENS receptors respond to gut bacteria by influencing serotonin production, which plays a role in gut movement.

Serotonin levels can vary by IBS type:

• IBS-D (diarrhea) – typically higher serotonin levels

• IBS-C (constipation) – typically lower serotonin levels

Immune System Dysfunction

Although IBS is not classified as an inflammatory disease, some individuals show signs of low-grade inflammation in the gut. This can result from an overactive immune response to harmless substances, such as certain foods or bacterial imbalances.

Many people with IBS have food intolerances—commonly to lactose, wheat, or eggs—which can trigger or worsen symptoms. These sensitivities may provoke immune reactions, leading to inflammation, abdominal pain, bloating, and diarrhea.

Is Irritable Bowel Syndrome Hereditary?

Genetics may contribute to IBS risk. People with a family history of IBS are two to three times more likely to develop the condition. Researchers have identified several genes potentially linked to IBS, some of which are associated with mood and anxiety disorders. This suggests a shared genetic influence between IBS and certain mental health conditions.

However, having a family history does not guarantee you will develop IBS. Genetics are likely only one part of a larger picture, with environmental and lifestyle factors also playing important roles.

Who Gets Irritable Bowel Syndrome?

Anyone can develop IBS, but certain factors increase the likelihood:

• age – Most people develop symptoms before age 50.

• sex – Women are about twice as likely to have IBS as men.

• ethnicity – IBS can affect anyone but is more common among non-Hispanic whites.

Risk Factors

Lifestyle and environmental factors can also influence IBS risk:

• family history – Having a close relative with IBS increases risk.

• trauma history – Childhood stress or abuse is linked to higher IBS rates.

• mental health conditions – Anxiety and depression are common among IBS patients.

• food intolerances – Sensitivities to dairy, gluten, or eggs may increase risk.

• infections – Past severe gastrointestinal infections raise the likelihood of IBS.

• lifestyle factors – Smoking and vitamin D deficiency are associated with increased risk.

A Quick Review

Irritable bowel syndrome is a functional gastrointestinal disorder causing abdominal pain and altered bowel habits. It is believed to stem from a mix of brain-gut communication issues, gut microbiota imbalances, immune system involvement, and possibly genetics. While anyone can develop IBS, it is most common in people under 50, women, and those with a history of digestive infections or food intolerances.