Causes and Risk Factors of 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 like diarrhea, constipation, or both. For many people, IBS symptoms can disrupt daily routines and diminish their quality of life. 

IBS is a functional gastrointestinal disorder, meaning it affects the function of the digestive system but does not cause structural changes or inflammation. Experts have not identified a definitive cause for IBS, but a combination of factors likely plays a role in its development. 

What causes IBS is not fully understood, but researchers have several theories about the underlying mechanisms that contribute to its development.

The main belief is that IBS is a disorder of how the brain and gut work together. However, there are other factors besides a gut-brain interaction that could cause IBS.

Growing evidence suggests that IBS may develop due to abnormalities in gut microbiota makeup, gut motility, and immune system function. The role each of these factors plays in IBS varies from person to person.

1. Gut-Brain Axis Dysfunction 

The gut-brain axis is a communication network between the digestive tract and the brain. It regulates several bodily functions, including digestion. 

When the gut-brain axis functions optimally, normal sensations like food moving through the intestines typically go unnoticed. Problems with the communication system can lead to heightened sensitivity in the gut, which may explain why people with IBS experience discomfort or pain as food moves through the intestines.

Gut-brain axis dysfunction can also cause changes in gut motility (movement). The intestines may move food too quickly through the GI tract (causing diarrhea) or too slowly (leading to constipation).

Several factors can disrupt the balance of the gut-brain axis, including: 

• Stress: Acute (short-term) and chronic (long-term) stress can cause gut-brain axis dysfunction, leading to changes in gut motility. 
• Childhood trauma: Many people with IBS report experiencing trauma and stress in early life, which may play a role in gut-brain axis dysfunction and IBS development. 
• Anxiety and depression: Depression and anxiety are common among people with IBS, which can contribute to gut-brain axis dysfunction and cause more intense IBS symptoms. 
• Visceral hypersensitivity: Many people with IBS experience an exaggerated pain response to bowel activity and digestion, likely because their nervous system sends stronger pain signals to the brain as food moves through the intestines, leading to abdominal pain during digestion. 

2. Gut Microbiota Imbalance

The gut microbiome is a community of trillions of microorganisms, including bacteria, viruses, and fungi, that live in the gastrointestinal tract. These microorganisms play a role in digestion and immune function. Dysbiosis is an imbalance in this microbial community—when there are fewer beneficial bacteria in the gut and an overgrowth of harmful bacteria. 

Research shows that many people with IBS have imbalances in their gut microbiota, which may contribute to the development of IBS. Several factors can contribute to dysbiosis, including: 

• Antibiotic use
• GI bacterial infections, such as gastroenteritis (stomach flu)
• Low-level inflammation in the intestinal wall 
• Small intestinal bacterial overgrowth (SIBO), which is excess bacteria in the small intestine 

The gut microbiota interacts with the gut-brain axis and influences the communication between the gastrointestinal tract and the brain. Dysbiosis may disrupt these interactions, leading to increased gut sensitivity and gut motility problems.

3. Enteric Nervous System Hypersensitivity 

The enteric nervous system (ENS) is a network of nerves embedded in the intestinal wall that controls the muscles, hormones, and digestive secretions in the gut. The system coordinates how food is broken down, absorbed, and eliminated from the body, ensuring everything works smoothly during digestion.

People with IBS may have a hypersensitive ENS, which may contribute to the discomfort and pain associated with digestion when muscle contractions move food and waste through the digestive tract. 

Research shows that specific ENS receptors in the gut respond to signals from gut bacteria, which play a role in producing serotonin. Serotonin is a neurotransmitter, which is a chemical that carries messages from the brain throughout the body. This release of serotonin affects ENS signals, influencing the secretion of digestive fluids and gut movement.

Different types of IBS show varying levels of serotonin can affect how quickly food moves through the digestive system. For example, people with IBS with diarrhea (IBS-D) usually have higher serotonin levels, while those with IBS with constipation (IBS-C) have lower levels.

4. Immune System Dysfunction 

While IBS is a functional GI disorder, research suggests immune system dysfunction may contribute to its development. Some people with IBS have low-grade inflammation in the gut, which can occur in response to an overactive immune response to harmless substances like food or imbalances in gut microbiota. 

Many people with IBS have food intolerances or sensitivities—mainly lactose (in dairy), wheat, and eggs—that can trigger or worsen their symptoms. These sensitivities may prompt an overactive immune response that leads to low-grade inflammation and/or IBS symptoms like abdominal pain, bloating, and diarrhea.  

Genetics may play a role in irritable bowel syndrome, but exactly how remains unclear. Research shows that people with a family history of IBS may have a two to three times higher risk of developing the condition themselves.

Researchers have identified six specific genes associated with IBS. Four of these genes are related to mood and anxiety disorders, suggesting IBS and anxiety may have shared genetic links rather than one causing the other. While genetics may increase your risk of developing IBS, having a family history of the condition doesn’t necessarily mean you will develop it yourself. 

Genetics are likely just one piece of a complex puzzle, and more research is needed to understand what role genes play in the development and severity of IBS. 

Some people are more likely to develop irritable bowel syndrome than others. These factors may play a role:

• Age: IBS affects people of all ages, but symptoms typically appear before age 50.  
• Sex: Women are two times more likely to have IBS than men. 
• Ethnicity: IBS can affect people of all ethnicities but is most common in non-Hispanic whites.

Certain lifestyle and environmental factors can increase your risk of developing IBS, including:

• Family history: Having a family member with IBS may double or triple the risk of developing the condition yourself.
• Trauma history: People with a history of childhood trauma, stress, or abuse have higher rates of IBS than those without a trauma history.
• Mental health conditions: People with mental health disorders, including anxiety and depression, are more likely to develop IBS. 
• Food intolerances: People with sensitivities or intolerances to certain foods, including dairy, gluten, and eggs, may be more likely to develop IBS.  
• Infections: A history of severe gastrointestinal bacterial infection increases the risk of IBS.
• Lifestyle factors: Smoking and vitamin D deficiency are associated with a higher likelihood of developing IBS.

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that causes abdominal pain and changes in bowel movements. Some people experience recurring episodes of diarrhea, some experience frequent constipation, and others experience a mix of both.

IBS is believed to be a disorder of the gut-brain interaction, or how your gut and brain work together. While the exact causes of IBS are not fully understood, factors like gut microbiota imbalances, immune dysfunction, and genetics likely play a role. Anyone can develop IBS, but it most commonly develops in people under age 50, women, and those with a history of gastrointestinal infections or food intolerances.