Definition

The Task Force of the American College of Gastroenterology defines irritable bowel syndrome (IBS) as "abdominal pain or discomfort associated with altered bowel function over a period of at least three months." Chronic abdominal pain and abnormal bowel function are characteristic of IBS, regardless of other comorbidities. The intensity of pain varies, and symptoms can fluctuate over time. Abnormal bowel function may include frequent bowel movements, an urge to defecate, incontinence, altered stool form (hard/lumpy or watery), feelings of incomplete defecation, straining, and the discharge of large amounts of mucus. 
IBS, although it shares some symptoms with inflammatory bowel disease and celiac disease, must be differentiated from these conditions. IBS is classified by stool consistency (according to the Rome IV criteria) into three types: 

• IBS-C: Predominant constipation 
• IBS-D: Predominant diarrhea 
• IBS-M: Mixed symptoms of both constipation and diarrhea 

The exact cause of IBS is unclear, but it is believed to involve a connection between the brain and gut (the gut-brain axis) that influences gut function, leading to IBS symptoms. Multiple factors appear to contribute to the pathogenesis of IBS, including: 

• The intestinal microbiome 
• Central and autonomic nervous system modulation 
• Altered viscerosomatic sensitivity (“visceral hyperalgesia”) 
• Changes in visceral motility 
• Immune regulation and inflammation 
• Epithelial permeability 
• (Epi)genetic influences 

IBS symptoms are diverse and can include bloating, flatulence, nausea, early satiety, dyspepsia, reflux, dysuria, musculoskeletal pain, dysmenorrhea, fatigue, and depression. Psychological stress and mental health disorders (such as anxiety and depression) can exacerbate symptoms. Although IBS is rarely cured, symptom improvement, enhanced social functioning, and improved quality of life can be achieved. The primary therapeutic goals are symptom relief through lifestyle and dietary modifications, stress reduction, and, where necessary, medication.

Impact on Nutritional Status 

Dietary issues contribute to the pathophysiology of IBS, making nutritional support a vital part of IBS management. Three key areas require focus: 

1. The impact of adverse food reactions on IBS symptoms
2. The nutritional consequences of IBS
3. The nutritional management of IBS patients 

While malnutrition is uncommon in IBS, some patients may attribute all symptoms to their “last meal,” leading to overly restrictive, unbalanced diets that can eventually cause malnutrition. Over 80% of people with IBS report food-related symptoms, especially linked to fermentable carbohydrates and fats.  Food can trigger IBS symptoms through several mechanisms: Primary effects: such as osmotic, chemical, immunological, mechanical, or neuroendocrine responses; Secondary effects: including fermentation by-products, changes in intraluminal pH, or effects on the gut microbiome. Due to the complex nature of food triggers in IBS, IgG antibody-based food-elimination diets are not recommended. Instead, nutritional support can complement pharmacological treatments. However, excessive dietary restrictions should be avoided, as they may lead to nutrient deficiencies in otherwise “healthy” IBS patients. 

Aims of nutritional therapy

• Maintain and improve nutritional status and body function 
• Prevent malnutrition and nutrient deficiencies 
• Avoid overly restrictive and unbalanced diets  

To identify malnourished patients early, nutritional risk screening should be conducted at least twice a year.

Energy and protein requirements should be met through oral nutrition whenever possible. If less than 75% of requirements are covered with fortification, snacks, or sip feeds, enteral nutrition should be added within five days at the latest. Complementary parenteral nutrition is indicated if oral and/or enteral nutrition covers less than 75% of requirements.

Oral Nutrition

In general, most patients with IBS can (and should) eat a balanced diet without restrictions. In clinical practice, three main dietary approaches have been considered beneficial for IBS management: individualized nutrition support, a low-FODMAP diet, and, in select cases, a gluten-free diet.

Individualized Nutrition

Individual nutritional support is considered the first line treatment – healthy eating habits, regular meals, adequate nutrition, limiting alcohol and caffeine intake, adjusting fiber intake, and reducing intake of fatty and spicy foods.

Low-FODMAP Diet (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols)

FODMAPS are short-chain fermentable carbohydrates found in fruits, vegetables, dairy products, artificial sweeteners, and wheat. They increase the volume of water in the small intestine (secretion) and gas production in the large intestine (fermentation) and can cause gastrointestinal symptoms in patients with visceral hypersensitivity. They can also trigger intestinal symptoms because they produce gases and short-chain fatty acids that lower the pH of the large intestine, which can lead to an expansion of the lumen and the onset of meal-related symptoms. Therefore, a low-FODMAP diet seems physiologically plausible and is recommended as a second-line diet to improve symptoms, especially for patients with predominant diarrhea and/or bloating. 

It is crucial to guide patients properly through all three phases of the diet. Almost all available studies have focused on the FODMAP restriction (first phase). Patients who respond to the restriction of FODMAPs can be identified within 2-6 weeks. 

• First Phase: Replace high-FODMAP foods with low-FODMAP options. 
• Second Phase: Gradually reintroduce foods to identify individual sensitivities. 
• Third Phase: Customize the diet to avoid symptom-triggering foods while allowing as much variety as possible for long-term adherence.  

Adopting a low-FODMAP diet requires a specialized dietitian and close monitoring to avoid nutrient deficiencies or the development of overly restrictive eating habits. Furthermore, a low-FODMAP diet can lead to harmful changes in the gut microbiota, resulting in a reduction in bifidobacteria and the total bacterial count, although the long-term consequences are unknown. 

Gluten-free diet

There is insufficient evidence to routinely recommend a gluten-free diet for IBS. Some patients report benefit from a gluten-free diet despite no objective evidence of celiac disease. It is suggested that the clinical benefit of a gluten-free diet is primarily due, not to the removal of gluten per se, but to a reduction in the content of fructans, which are a FODMAP.

Practical Advice if Indicated

Dietary Fibers 

Dietary fibers are carbohydrates that are neither digested nor absorbed in the small intestine. They are often categorized as either soluble (e.g. psyllium, oat bran, barley, and beans) or insoluble (e.g. wheat bran, whole grains, and some vegetables). 

Dietary fibers have a variety of effects in the gastrointestinal tract that have not yet been fully researched. These effects influence the intestinal microbiome, metabolism, transit time, stool consistency, and bile acid absorption. Insoluble dietary fibers generally increase stool water content and are less fermented in the large intestine. In contrast, soluble fibers lose their water-binding capacity, are mostly fermented in the colon, and can lead to gas production, which may worsen symptoms of bloating and gas. 

• Poorly fermentable dietary fibers may be beneficial for patients with irritable bowel syndrome. They tend to cause less bloating, help bind water, and prevent both excessive stool dryness and excess fluid. The lack of significant side effects makes them a practical therapeutic option. 
• Dosing Recommendation: Begin with low doses (3–4 g daily) of insoluble dietary fiber to reduce bloating, and gradually increase to a target dose of 20–35 g per day, based on individual tolerance. 

Monitoring / Special Considerations 

• Patients with mild symptoms respond to education and reassurance, and 40-70% of patients respond to placebo alone. 
• Ideally, the effectiveness of selected treatments should be reviewed after 3 months and discontinued / changed if there is no response. 
• Severe or Refractory IBS:
• Reassess diagnosis and consider further diagnostic tests. 
• Use an integrated, multidisciplinary approach to treatment, avoiding unnecessary opioid prescriptions, surgery, and unproven or unregulated treatments. 
• For more severe symptoms, consider augmentation therapy with gut-brain neuromodulators.
• Caution: Monitor for serotonin syndrome. 
• Psychological therapies: IBS-specific cognitive behavioral therapy or gut-oriented hypnotherapy for global symptoms; psychological therapies should be considered for persistent symptoms over 12 months without improvement from medication 
• All patients should be encouraged to engage in 20-30 minutes of physical activity on most days of the week

Medications/Supplements 

First choice 

• Loperamide for diarrhea. Caution: May cause abdominal pain, bloating, nausea, and constipation. Careful dose titration can improve tolerability. 
• Spasmolytics for global symptoms and abdominal pain. Caution: Possible side effects include dry mouth, visual disturbances, and dizziness. 
• Peppermint oil for global symptoms and abdominal pain. Caution: May cause gastroesophageal reflux; gastric juice-resistant formulations can help.
• Laxatives for patients with IBS-C, with dose adjustment based on symptoms 
• Polyethylene glycol (PEG) for constipation. Caution: May cause abdominal pain and is not recommended for IBS-C; long-term efficacy is unknown

Second-Line Treatments 

Gut-Brain Neuromodulators   

• Tricyclic Antidepressants (TCAs): Used for global symptoms, abdominal pain, or concomitant anxiety. Start with a low dose, gradually increasing in 10 mg increments up to a maximum of 30-50 mg once daily. 
• Selective Serotonin Reuptake Inhibitors (SSRIs): Effective for global symptoms with associated anxiety. Note: Both TCAs and SSRIs commonly cause side effects, such as drowsiness and dry mouth. 
• Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs): Medications like duloxetine can improve symptoms and enhance quality of life. 
• Calcium Channel Alpha-2-Delta Ligands (e.g., pregabalin): Help reduce visceral hypersensitivity, as well as global symptoms, abdominal pain, diarrhea, and bloating. Caution: Side effects may include blurred vision, dizziness, and altered sensations. 

IBS-D (Diarrhea-Predominant IBS) 

• Eluxadoline: A mixed opioid receptor modulator that slows intestinal transit and reduces visceral hypersensitivity. Contraindications: Avoid in patients with a history of sphincter of Oddi dysfunction, cholecystectomy, alcohol addiction, pancreatitis, or severe liver disease. 
• 5-HT3 Receptor Antagonists: Slow gastrointestinal transit and reduce visceral hypersensitivity. Alosetron and ramosetron are not widely available; ondansetron (4-8 mg three times daily) is a reasonable alternative, with constipation as the most common side effect. 
• Rifaximin: A nonabsorbed antibiotic with limited effect on abdominal pain. Approved in the US but not widely available elsewhere for this indication. 
• Bile Acid Sequestrants: Not recommended for IBS-D. 

IBS-C (Constipation-Predominant IBS)

• Secretagogues: Stimulate ion channels on enterocytes, leading to water and ion efflux into the intestine, softening stool and accelerating transit.
• Linaclotide: A guanylate cyclase-C agonist, considered one of the most potent secretagogues, though diarrhea is a common side effect. 
• Plecanatide (not yet approved in Switzerland): Similar to linaclotide but with pH-dependent binding, limiting its effect to the proximal small intestine. 
• Lubiprostone: A prostaglandin E1 derivative and chloride channel activator, which tends to cause diarrhea less frequently than other secretagogues; nausea is a common side effect. 
• Tenapanor: A sodium-hydrogen exchange inhibitor that commonly causes diarrhea; not widely available for this indication. 
• Tegaserod: A 5-HT4 receptor agonist that acts as a prokinetic to accelerate transit; available only in the USA. Diarrhea is a frequent side effect.

Probiotics 

Probiotics may alleviate IBS symptoms through mechanisms such as:

1. Enhancing anti-inflammatory effects on mucosal tissues and reducing pro-inflammatory cytokines. 
2. Directly affecting gut pain. 
3. Inhibiting pathogenic bacterial effects on toll-like receptors in the immune system. 
4. Strengthening mucosal barrier function. 

The effectiveness of probiotics in IBS treatment remains uncertain. Due to variations in study designs, probiotic strains, and patient responses, specific recommendations are challenging. Current data suggest:

1. Lactobacilli alone have minimal impact on symptoms. 
2. Probiotic combinations may improve symptoms. 
3. Bifidobacteria show a trend toward symptom improvement. 

Patients interested in trying probiotics are advised to use them for up to 12 weeks, discontinuing if there is no symptom improvement.