What is IBS (Irritable Bowel Syndrome)? Complete guide 2026

What is Irritable Bowel Syndrome (IBS)?

Irritable Bowel Syndrome, known internationally as IBS, is a chronic functional gastrointestinal disorder that affects the large intestine. It is characterised by recurrent abdominal pain associated with changes in the frequency or form of bowel movements, without an identifiable structural or biochemical cause detectable through conventional tests.

In other words, the intestine of a person with IBS functions differently, but shows no visible lesions or abnormalities detectable in blood tests or colonoscopies. This does not mean the problem is not real: IBS has well-documented physiological underpinnings and can have a devastating impact on the quality of life of those who suffer from it.

IBS prevalence

IBS is one of the most common digestive disorders in the world. The numbers speak for themselves:

• Between 10% and 15% of the global population suffers from IBS, according to epidemiological studies.
• In Europe, this translates to more than 50 million people affected.
• In the UK, it is estimated that between 10% and 20% of the adult population lives with this disorder.
• It is twice as common in women as in men, and usually appears before the age of 50.
• IBS accounts for up to 25% of gastroenterology consultations, making it one of the leading reasons for visiting a digestive specialist.

Despite its high prevalence, many people with IBS do not receive a formal diagnosis or adequate treatment. A lack of information, social stigma and the invisible nature of the symptoms mean that millions of people suffer in silence.

IBS symptoms

IBS symptoms are varied and fluctuating. They can range from mild to severe and tend to appear in flare-ups that alternate with periods of relative calm. The most characteristic include:

• Recurrent abdominal pain: This is the cardinal symptom of IBS. It is usually located in the lower abdomen, although it can occur anywhere. It typically improves after a bowel movement and is related to changes in bowel habits.
• Bloating and abdominal distension: A feeling of fullness, pressure or inflammation in the abdomen. Many patients describe their belly "inflating like a balloon" throughout the day, especially after eating.
• Altered bowel habits: Episodes of diarrhoea, constipation or both in an alternating pattern. The frequency and consistency of stools can vary from one day to the next.
• Urgency: A sudden, overwhelming need to go to the toilet, which can generate social anxiety and limit daily activities.
• Feeling of incomplete evacuation: After using the toilet, the person feels they have not fully emptied their bowel.
• Excessive gas: Frequent flatulence that can be embarrassing and socially debilitating.
• Mucus in stools: The presence of white or transparent mucus in bowel movements.

In addition to digestive symptoms, IBS is frequently associated with chronic fatigue, headaches, back pain, urinary problems and sleep difficulties. The psychological impact is significant: anxiety, depression and reduced quality of life are common among people with IBS.

Types of IBS

IBS is not a homogeneous disorder. Based on the predominant stool pattern, it is classified into four main subtypes:

IBS-D (diarrhoea-predominant)

The most common subtype. It is characterised by frequent, loose or watery stools. Urgency is a prominent symptom. People with IBS-D often plan their activities around the availability of toilets, which significantly limits their social and professional lives.

IBS-C (constipation-predominant)

Characterised by infrequent, hard or pellet-like stools. The person may go several days without a bowel movement and experiences excessive straining, a feeling of blockage and incomplete evacuation. Abdominal bloating tends to be particularly severe in this subtype.

IBS-M (mixed)

Combines alternating episodes of diarrhoea and constipation. This is the most frustrating subtype for many patients, as the pattern is unpredictable: one day there may be urgency with watery stools and the next, severe constipation. This variability makes daily planning and management of the disorder difficult.

IBS-U (unsubtyped)

When symptoms do not fit clearly into any of the above subtypes. The person meets the diagnostic criteria for IBS but the bowel habit changes are not consistent enough to be classified into a specific subtype.

It is important to know that the subtype can change over time. A person diagnosed with IBS-D may evolve towards a mixed pattern or even a predominantly constipated one. This is why regular medical follow-up is essential.

Causes of IBS

IBS is a multifactorial disorder. There is no single cause, but rather a combination of factors that interact in a complex way. Scientific research has identified several key mechanisms:

The gut-brain axis

The gut and the brain maintain constant bidirectional communication through the enteric nervous system, the vagus nerve and hormonal and immune mediators. In people with IBS, this communication is disrupted. The brain may interpret normal intestinal signals as painful, and psychological stress can amplify digestive symptoms. This gut-brain axis dysfunction is now considered the central mechanism of IBS.

Visceral hypersensitivity

People with IBS have a lower pain threshold in the digestive tract. Stimuli that would be imperceptible to a healthy person (such as normal distension of the intestine by gas or food) cause pain, discomfort or urgency in someone with IBS. This hypersensitivity is not limited to the gut: many patients also show greater sensitivity in other organs.

Gut microbiota alterations

The composition of intestinal bacteria (microbiota or microbiome) differs between people with IBS and healthy individuals. Reductions in bacterial diversity, changes in the proportions of certain microbial groups and alterations in bacterial metabolite production have been observed. These changes may contribute to excessive fermentation of certain foods, producing gas, bloating and pain.

Post-infectious IBS

Approximately 10% of people who suffer an acute gastroenteritis (intestinal infection by bacteria, viruses or parasites) develop IBS in the following months. This phenomenon, known as post-infectious IBS, suggests that the initial inflammation can leave lasting changes in intestinal function, the microbiota and the sensitivity of the digestive tract.

Stress and psychological factors

Chronic stress, anxiety and depression do not cause IBS directly, but they are factors that can trigger flare-ups, worsen symptom severity and hinder recovery. The relationship is bidirectional: IBS generates stress and anxiety, which in turn worsen symptoms, creating a vicious cycle that is difficult to break.

Other factors

Genetic predisposition, altered intestinal motility (intestinal movements that are too fast or too slow), hormonal changes (which partly explain the higher prevalence in women) and certain dietary factors also play a role in the development and maintenance of IBS.

Diagnosing IBS

IBS is diagnosed primarily through clinical criteria, as there is no laboratory test, imaging study or endoscopy that can confirm it definitively. The current standard is the Rome IV criteria, published in 2016:

For an IBS diagnosis under Rome IV, the person must have recurrent abdominal pain at least one day per week over the past three months, associated with two or more of the following criteria:

1. The pain is related to defecation (improves or worsens with bowel movements).
2. There is a change in stool frequency.
3. There is a change in stool form or appearance.

In addition, symptoms must have begun at least six months before diagnosis.

Ruling out other conditions

Before confirming an IBS diagnosis, the doctor must rule out other diseases that can present similar symptoms. Standard tests include:

• Complete blood tests, including coeliac disease markers, thyroid function and inflammation markers.
• Faecal calprotectin: a protein in stool that helps distinguish IBS from inflammatory bowel disease (Crohn's, ulcerative colitis).
• Colonoscopy: especially recommended for people over 50 or those with alarm signs such as unintentional weight loss, blood in stools or a family history of colorectal cancer.
• Hydrogen breath test: to detect small intestinal bacterial overgrowth (SIBO) or lactose or fructose intolerance.

If all these tests come back normal and the Rome IV criteria are met, an IBS diagnosis is highly likely. It is important that the diagnosis is made by a qualified medical professional.

IBS treatment

IBS treatment is multifaceted and must be personalised according to the subtype, symptom severity and individual circumstances. The main strategies include:

Dietary treatment

Diet is the first line of treatment for IBS. The dietary interventions with the strongest scientific evidence are:

• Low FODMAP diet: The most studied and effective dietary intervention for IBS. Developed by researchers at Monash University, it has been shown to improve symptoms in up to 75% of patients. We cover it in detail in the next section.
• Soluble fibre: Supplements such as psyllium (Plantago ovata) can improve bowel transit, especially in IBS-C. Insoluble fibre, on the other hand, can worsen symptoms in some patients.
• Identifying individual triggers: Beyond FODMAPs, certain foods such as coffee, alcohol, fatty or spicy foods can trigger symptoms in some people.

Pharmacological treatment

Medications that can complement dietary measures include:

• Antispasmodics: such as mebeverine or otilonium bromide, to reduce pain and intestinal spasms.
• Osmotic laxatives: such as polyethylene glycol (PEG), for constipation in IBS-C.
• Anti-diarrhoeals: such as loperamide, to control diarrhoea in IBS-D.
• Probiotics: certain specific strains have shown modest benefits in some patients, although the evidence is variable.
• Neuromodulators: low-dose antidepressants (tricyclics or SSRIs) can be effective for chronic visceral pain, acting on the gut-brain axis.

Psychological treatment

Given the importance of the gut-brain axis in IBS, psychological therapies are a fundamental pillar of treatment:

• Cognitive behavioural therapy (CBT): helps modify thought and behaviour patterns that amplify symptoms.
• Gut-directed hypnotherapy: one of the interventions with the strongest evidence. Studies demonstrate significant and lasting improvements in IBS symptoms.
• Mindfulness and relaxation techniques: meditation and diaphragmatic breathing can reduce stress and enteric nervous system reactivity.

Physical exercise

Regular physical activity has been shown to improve IBS symptoms in multiple studies. Moderate exercise (walking, swimming, yoga, cycling) improves intestinal motility, reduces stress, promotes sleep and can positively modulate the microbiota. At least 150 minutes of moderate activity per week is recommended.

The Low FODMAP diet for IBS

The Low FODMAP diet is currently the dietary intervention with the strongest scientific evidence for managing IBS. Developed by researchers at Monash University in Australia, it has transformed the treatment of Irritable Bowel Syndrome worldwide.

What are FODMAPs?

FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides And Polyols: short-chain carbohydrates that are poorly absorbed in the small intestine. In people with IBS, these carbohydrates ferment excessively in the colon, producing gas, drawing water into the intestine and triggering the characteristic symptoms of pain, bloating, diarrhoea or constipation.

Scientific evidence

The numbers are compelling: controlled, randomised clinical trials have shown that up to 75% of IBS patients experience significant symptom improvement when following the Low FODMAP diet. This makes it the most effective dietary intervention available for this disorder, far surpassing other elimination diets.

The three phases of the Low FODMAP diet

The Low FODMAP diet is not a lifelong diet, but a structured process in three phases:

1. Elimination phase (2-6 weeks): All high-FODMAP foods are reduced to achieve maximum symptom relief. This is the most restrictive phase and should not be maintained indefinitely.
2. Reintroduction phase (6-8 weeks): FODMAP groups are reintroduced one by one, systematically, to identify which ones trigger symptoms and in what quantity. The goal is to determine your individual tolerance.
3. Personalisation phase (long-term): With the information obtained, a personalised diet is designed that avoids only the FODMAPs that are problematic for you, while maintaining the widest possible food variety.

How to follow the Low FODMAP diet

To follow the Low FODMAP diet safely and effectively, it is recommended to:

• Work with a registered dietitian specialising in digestive disorders.
• Use the Monash University app as a reference for the FODMAP content of foods.
• Plan meals in advance to avoid falling into an overly restrictive or monotonous diet.
• Keep a symptom diary to identify patterns and triggers.
• Do not extend the elimination phase longer than necessary.

For more information about FODMAPs and how to implement this diet, see our complete guide to the Low FODMAP diet.

Complete nutrition with IBS

One of the greatest challenges for people with IBS is maintaining nutritionally complete eating habits. Dietary restrictions, fear of eating certain foods and the elimination phase of the Low FODMAP diet can lead to:

• Nutritional deficits: Restricting food groups can cause shortfalls in fibre, calcium, iron, B vitamins and other essential micronutrients.
• Insufficient caloric intake: Fear of symptoms leads many people to eat less than they need, losing weight and energy.
• Inadequate protein intake: By eliminating common protein sources (legumes, certain dairy products), it can be difficult to meet daily protein requirements.
• Negative relationship with food: Anxiety around eating can lead to restrictive behaviours and loss of enjoyment at mealtimes.

For these reasons, it is essential to have nutritional options that are safe for the digestive system and at the same time nutritionally complete. Adequate nutrition is not only important for general health, but also supports gut recovery and long-term wellbeing.

Frequently asked questions about IBS