The role of diet in controlling symptoms of irritable bowel syndrome (IBS) has gained much traction over the years,1 but until recently, diet therapy for IBS has been hindered by a lack of quality evidence, in part because of the challenges of conducting dietary clinical trials.
Several clinical trials have now been done that support a diet low in fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) for managing IBS. Although restrictive and difficult to follow, the low-FODMAP diet is gaining popularity.
This article provides an overview of dietary interventions used to manage IBS, focusing on the low-FODMAP diet. We discuss mechanisms of malabsorption of FODMAPs and the role of FODMAPS in symptom induction; highlight clinical trials that provide evidence of benefits of the diet for IBS; and discuss the steps to implement it. We also address the nutritional adequacy of the diet and its potential effects on the gut microbiome.
IBS IS A COMMON FUNCTIONAL DISORDER
IBS is one of the most commonly diagnosed gastrointestinal disorders, and it has a significant impact on quality of life.2 It is a functional disorder characterized by chronic abdominal pain and altered bowel habits in the absence of a structural or organic cause.
The Rome IV diagnostic criteria define IBS by the following:
- Recurrent abdominal pain or discomfort at least 1 day a week in the last 3 months, associated with two or more of the following:
- Symptoms improved by defecation
- Onset associated with a change in frequency of stool
- Onset associated with a change in form or appearance of stool.
IBS mainly arises during young adulthood but can be diagnosed at any age.3
The pathophysiology of IBS involves mechanisms such as bowel distention, altered bowel motility, visceral hypersensitivity, and disruption of mucosal permeability.4 Several therapeutic modalities targeting these mechanisms have been implemented in IBS management, including antispasmodics, laxatives, antidepressants, antibiotics, and behavioral therapy. Diet is only one line of treatment and is most effective when part of a multipronged approach.
TRADITIONAL DIETARY MANAGEMENT
Diet is important in inducing the symptoms of IBS—and in controlling them. Patients identify eating as a common precipitator of symptoms, but the complex diet-symptom interaction is not fully understood and varies widely among patients. Traditional dietary advice for IBS includes adhering to a regular meal pattern, avoiding large meals, and reducing intake of fat, insoluble fibers, caffeine, spicy and gas-producing foods, and carbonated beverages.5,6
Increase soluble fiber
Fiber and fiber supplements, particularly soluble fibers such as psyllium, calcium polycarbophil, and ispaghula husk are often recommended. A meta-analysis7 found that soluble fiber but not insoluble fiber (eg, wheat bran) is associated with an improvement in IBS symptoms (relative risk [RR] 0.84, 95% confidence interval [CI] 0.73–0.94). By improving stool consistency and accelerating transit, soluble fiber is especially useful in constipation-predominant IBS while posing a low risk for adverse outcomes.7 Fiber should be started at a low dose and gradually increased over several weeks to as much as 20 to 30 g/day.
Only about 4% of patients with IBS also have celiac disease, but estimating the prevalence of nonceliac gluten sensitivity is confounded by overlapping symptoms. There is some evidence implicating gluten in IBS: celiac disease and IBS overlap in their symptoms, and symptoms are often precipitated by gluten-containing foods in patients with IBS.8 The pathogenesis of gluten-induced (or wheat-induced) symptoms in IBS is unclear, and studies have had conflicting results as to the benefits of gluten restriction in IBS.9
In a study of patients with IBS whose symptoms improved when they started a gluten-free and low-FODMAP diet, symptoms did not return when gluten was reintroduced, suggesting that it is the fructan (a FODMAP) component of wheat rather than gluten that contributes to symptoms in IBS.10
Probiotics are increasingly being recommended as dietary supplements for people with IBS, as awareness increases of the importance of the gut microbiota. In addition to their effects on the gut microbiota, probiotics in IBS have been shown to have anti-inflammatory effects, to alter gut motility, to modulate visceral hypersensitivity, and to restore epithelial integrity.
In a meta-analysis, Ford et al11 found that probiotics improved global IBS symptoms more than placebo (RR 0.79, 95% CI 0.70–0.89) and also reduced abdominal pain, bloating, and flatulence scores.
Which species and strains are most beneficial and the optimal dosing and duration of treatment are still unclear. Data from studies of prebiotics (nutrients that encourage the growth of probiotic bacteria) and synbiotics (combinations of prebiotics and probiotics) are limited and insufficient to draw conclusions.
FODMAPS ARE SHORT-CHAIN CARBOHYDRATES
The term FODMAPs was initially coined by researchers at Monash University in Australia to describe a collection of poorly absorbed short-chain fermentable carbohydrates that are natural components of many foods:
- Oligosaccharides, including fructans (which include inulins) and galacto-oligosaccharides
- Disaccharides, including lactose and sucrose
- Monosaccharides, including fructose
- Polyols, including sorbitol and mannitol.12
Intake of FODMAPs, especially fructose, has increased in Western diets over the past several decades from increased consumption of fruits and concentrated fruit juices, as well as from the widespread use of high-fructose corn syrup in processed foods and beverages.13
FODMAPs ARE POORLY ABSORBED
Different FODMAPs can be poorly absorbed for different reasons (Table 1). The poor absorption is related either to reduced or absent digestive enzymes (ie, hydrolases) or to slow transport across the intestinal mucosa. Excess FODMAPs in the distal small intestine and proximal colon exert osmotic pressure, drawing more water into the lumen. FODMAPs are also rapidly fermented by colonic bacteria, producing gas, bowel distention, and altered motility, all of which induce IBS symptoms.14
Fructans are fructose polymers that are not absorbed in human intestines. They have no intestinal hydrolases and no mechanisms for direct transport across the epithelium. However, a negligible amount may be absorbed after being degraded by microbes in the gut.15 Most dietary fructans are obtained from wheat and onion, which are actually low in fructans but tend to be consumed in large quantities.16
Galacto-oligosaccharides are available for colonic fermentation after ingestion due to lack of a human alpha-galactosidase. Common sources of galacto-oligosaccharides include legumes, nuts, seeds, some grains, dairy products, human milk, and some commercially produced forms added to infant formula.17,18
Lactose is poorly absorbed in people with lactase deficiency. It is mainly present in dairy products but is also added to commercial foods, including breads, cakes, and some diet products.19
Fructose is the most abundant FODMAP in the Western diet. It is either present as a free sugar or generated from the digestive breakdown of sucrose. In the intestine, it is absorbed via a direct low-capacity glucose transporter (GLUT)-5 and through GLUT-2, which is more efficient but requires the coexistence of glucose. Because of this requirement, fructose is more likely to be malabsorbed when present in excess of glucose, as in people with diminished sucrase activity. The main sources of fructose in the Western diet are fruits and fruit products, honey, and foods with added high-fructose sweeteners.13
Polyols such as sorbitol and mannitol are absorbed by slow passive diffusion because they have no active intestinal transport system. They are found in fruits and vegetables. Sugar-free chewing gum is a particularly rich source of sorbitol.20
QUANTIFYING FODMAP CONTENT
As interest in the low-FODMAP diet grew, studies were conducted to quantify FODMAPs in foods. One study used high-performance liquid chromatography to analyze FODMAP content in foods,21 and another evaluated fructan levels in a variety of fruits and vegetables using enzymatic hydrolysis.22 The Monash University low-FODMAP diet smartphone application provides patients and healthcare providers easy access to updated and detailed food analyses.23
Table 2 lists foods high in FODMAPs along with low-FODMAP alternatives. Total FODMAP intake is important, as the effects are additive.24 Readers and patients can be directed to the following websites for more information on the low-FODMAP diet: www.med.monash.edu/cecs/gastro/fodmap or www.ibsfree.net/what-is-fodmap-diet.