Irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic gastrointestinal disorder characterized by a combination of abdominal pain or discomfort and altered bowel habits over a period of at least 3 months that is not explained by structural, histologic, or biochemical abnormalities. Irritable bowel syndrome affects 7% to 10% of the population worldwide. It is considered a functional bowel disorder. Individuals are classified into one of three subtypes: irritable bowel syndrome with constipation predominance (IBS-C), irritable bowel syndrome with diarrhea predominance (IBS-D), and irritable bowel syndrome with mixed constipation and diarrhea (IBS-M).

Up to two thirds of patients who suffer from irritable bowel syndrome consider their gastrointestinal symptoms to be food related and modify their diets to avoid symptom triggers. Among these patients, approximately 12% overly restrict their intake and consume inadequate or unbalanced diets. Commonly identified culprit foods include milk products, raw vegetables (especially onions, cabbage, and beans), fatty foods, spicy foods, coffee, and alcohol. These items have been linked to excessive gas-bloat symptoms and abdominal pain, followed by dyspepsia and loose stools.

Food intolerance refers to adverse food reactions resulting from various nonimmune mechanisms, including direct effects of toxins, pharmacologic agents in foods (e.g., caffeine, tyramine), malabsorption caused by host enzyme or transport deficiency (e.g., lactase, fructose), and idiosyncratic reactions.

Carbohydrate malabsorption of lactose, fructose, and sugar alcohols (e.g., sorbitol, xylitol) has been implicated as an underlying cause or trigger for irritable bowel syndrome symptoms.

Fructose and sorbitol malabsorption also have been demonstrated in patients with irritable bowel syndrome. Fructose is a naturally occurring monosaccharide abundant in fruit and honey and the preferred sweetener in sodas and juices. Sorbitol is a sugar substitute used in many dietetic foods. 

Dietary counseling may be a helpful adjunct to pharmacologic treatments for irritable bowel syndrome. Pharmacologic treatments used are based on the individual symptom pattern and include antidiarrheal agents, stool softeners, and antispasmodic agents (e.g., dicyclomine). Dietary assessment can identify potential food triggers and dispel preconceived notions of food intolerances that may result in unnecessary restriction. In general, patients with irritable bowel syndrome should eat a balanced diet with few restrictions.

Dietary modifications should be based on the dominant gastrointestinal symptom. Reductions in specific food items known to exacerbate symptoms should be advised, such as decreased caffeine consumption in patients with IBS-D. Increased intake of food items with therapeutic potential should be considered, such as fiber intake for constipation.

Although generalized exclusion diets are not advised, diets specifically avoiding unabsorbed, fermentable shortchain carbohydrates, collectively termed fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FOD-MAPs), may be of benefit to patients with irritable bowel syndrome. FOD-MAPs include fructose and lactose when malabsorbed, poorly absorbed polyols (sorbitol, xylitol), and fructooligosaccharides (fructans) and galactooligosaccharides (raffinose) not cleaved by human hydrolases and thus poorly absorbed.

Fiber supplementation is a commonly prescribed dietary remedy for irritable bowel syndrome. Its use depends on the patient’s dominant symptom. Fiber supplementation is most commonly used for patients with constipation and, to a lesser extent, for those with diarrhea. Fiber is believed to alleviate constipation by adding bulk to the stool, which accelerates oral–anal transit and decreases intracolonic pressures. For patients with diarrhea, soluble fiber creates a more viscous stool and can improve stool output. When pain, gas, and bloating predominate, use of fiber may actually be counterproductive because fermentation of fiber generates increased gas production.