Irritable bowel syndrome (IBS) is a common gastrointestinal disorder that is manifested by abdominal pain and altered bowel habits. Many IBS patients report meal-related exacerbations in symptoms, which may be due to true food intolerances but can be also due to visceral hypersensitivity or changes in gut microbiota. Gut microbiota can be significantly altered by changing the intake of fiber and fermentable oligosaccharides, disaccharides, monosaccharides and polyols, referred to by the acronym FODMAPs. High-FODMAP foods include those with excess fructose (e.g., honey, peaches, dried fruits), fructans (e.g., wheat, rye, onions), sorbitol (e.g., apricots, prunes, sweeteners), and raffinose (e.g., lentils, cabbage, legumes).
A high-FODMAP diet can result in increased gas and colonic distension from bacterial fermentation, and increased water in the small bowel due to the high osmotic load. In one study, a high-FODMAP diet was associated with higher levels of breath hydrogen compared with a low-FODMAP diet in both IBS patients and healthy controls, but it only induced GI symptoms and lethargy in IBS patients (J. Gastroenterol. Hepatol. 2010;25:1366-73). Dr. Murray and colleagues (Am. J. Gastroenterol. 2014;109:110-9) measured breath hydrogen as well as small bowel water content and colonic gas and distension using magnetic resonance imaging (MRI) scans of the abdomen in healthy volunteers. Intake of fructose, which has a high osmotic load, was associated with increased small bowel water content compared with glucose and inulin (osmotically inactive fructan). However, inulin increased breath hydrogen and colonic gas to a greater extent than fructose and glucose.
Studies have demonstrated a beneficial effect of a low-FODMAP diet in IBS patients. In one study, IBS patients who followed a low-FODMAP diet reported a better overall and individual symptom response (i.e., bloating, abdominal pain, and flatulence) compared with patients on a standard diet (J. Hum. Nutr. Diet. 2011;5:487-95). A recent crossover trial conducted in Australia compared a low-FODMAP diet to a typical Australian diet, which included high-FODMAP foods, for 21 days each (Gastroenterology 2014;146:67-75). Patients with IBS (n = 30) had lower GI symptom scores on a low-FODMAP diet compared with the Australian diet. Seventy-five percent of the IBS patients had evidence of fructose malabsorption but this did not have an effect on their response to a low-FODMAP diet.
A beneficial response to a low-FODMAP diet has been speculated to be primarily due to avoiding gluten, however this has not been supported by studies. Some IBS patients have reported significant improvement in GI and non-GI (e.g., tiredness) while on a gluten free diet. Dr. Biesikierski and colleagues (Gastroenterology 2013;145:320-8) conducted a double-blind crossover study in 37 IBS patients with nonceliac gluten sensitivity who were placed on a 2-week trial on a low-FODMAP diet and then randomly assigned to a high-gluten, low-gluten, or control (whey protein) diet for 1 week each. GI symptoms improved in all subjects while on a low-FODMAP diet. Symptoms worsened to a similar degree when the gluten or whey was added to their diets but there was no difference between these groups. The study found that there were no specific or dose-dependent effects of gluten in IBS patients who experienced symptom improvement on a low-FODMAP diet. At the recent Digestive Disease Week meeting, a study by Dr. Piacentino et al. (Gastroenterology suppl 2014;146:S-82) addressed this issue further by comparing GI symptoms (i.e., bloating, abdominal distension and abdominal pain) in IBS patients on a: 1) low-FODMAP, gluten-free diet, 2) a low-FODMAP and normal gluten diet, and 3) a normal-FODMAP and gluten-free diet (n = 20 in each group). A low-FODMAP diet with or without gluten was associated with a significant improvement in bloating, abdominal distension, and abdominal pain, compared to a normal FODMAP and gluten diet. There were no significant differences between the two low-FODMAP diets. The authors concluded that gluten avoidance did not add significant benefit to the low-FODMAP diet.
In summary, limited data, which is mainly comprised of studies with relatively small sample sizes, support IBS symptom improvement with a low-FODMAP diet. Fructose and fructans may have different mechanisms by which they cause symptoms in IBS. The beneficial effect of a low-FODMAP diet does not appear to be predominantly based on gluten avoidance. Lastly, there are no definite biomarkers as of now that are associated with symptom response.
Dr. Chang is professor of digestive diseases/gastroenterology, director of the Digestive Health and Nutrition Clinic and the University of California, Los Angeles, GI Fellowship Training Program, and co-director of the Center for Neurobiology of Stress, all at UCLA. Her comments were made at the annual Digestive Disease Week during the ASGE and AGA joint Presidential Plenary.