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Does a low FODMAP diet help IBS?


Irritable bowel syndrome (IBS) is a common condition that can have a significant impact on a person's quality of life.1 The cause of IBS is unknown but several mechanisms have been proposed including visceral hypersensitivity, central sensitisation, abnormal gut motility and altered gut microbiota.2,3 IBS is challenging to manage and many patients report insufficient symptomatic relief from treatment.2 Approximately 60% of patients identify food as a trigger for their symptoms,2 and there has been interest in exclusion diets for managing IBS.4 Dietary adaptation is a common self-management strategy for patients with IBS, with many self-diagnosing intolerance to specific foods. This may lead to patients adopting over-restrictive or inappropriate diets.5

In recent years, a diet low in poorly absorbed short-chain carbohydrates, known collectively as FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides and polyols), has been advocated for the treatment of IBS.2 Here, we discuss the background to the FODMAP diet and review the evidence supporting its use for people with IBS.

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Prevalence of IBS is estimated to be between 10-20% and it is twice as common in women.3 It is a chronic functional gastrointestinal disorder characterised by abdominal pain or discomfort, associated with abnormal bowel habit in the absence of any structural, physiological or biochemical abnormalities of the gastrointestinal tract.5

A national guideline suggests that a diagnosis of IBS should be considered in a person with abdominal pain relieved by defecation or associated with altered bowel frequency or stool form, accompanied by at least two of the following:6

  • altered stool passage

  • abdominal bloating, distension, tension or hardness

  • symptoms made worse by eating

  • passage of mucus.

Management options include dietary and lifestyle advice, psychotherapy, and pharmacological therapy targeted towards symptoms (e.g. antispasmodic agents, non-lactulose laxatives for constipation, loperamide for diarrhoea, and off-label use of either a tricyclic antidepressant or a selective serotonin reuptake inhibitor for gastrointestinal pain).6

Usual dietary advice is to adjust fibre intake and to reduce intake of insoluble fibre. Soluble fibre such as ispaghula or oats can be recommended; fresh fruit should be limited to three portions a day. People are advised to have regular meals and take time to eat, and to avoid the artificial sweetener sorbitol.6

What is the low FODMAP diet?

The low FODMAP diet, developed in Australia, is based on the observation that many short-chain carbohydrates are poorly absorbed in the small intestine and have been identified by patients as exacerbating IBS symptoms.5,7 The acronym FODMAP describes the fermentable carbohydrates fructans, oligosaccharides and galacto-oligosaccharides present in wheat, onions and legumes; the disaccharide lactose present in milk; the monosaccharide fructose (when consumed in excess of glucose) present in honey, apples and high fructose corn syrup; and polyols including sorbitol and mannitol present in many artificially sweetened gums and confectionery (see Appendix).8

The mechanism by which foods high in FODMAPs are thought to cause gastrointestinal symptoms in IBS patients is complex and depends on how well the individual is able to digest and absorb the different short-chain carbohydrates in the small intestine. Undigested FODMAPs increase water volume in the intestine and are rapidly fermented, producing hydrogen, methane and CO27 resulting in increased luminal distension. As many patients with IBS have visceral hypersensitivity, luminal distension may lead to sensations of pain, bloating and change in intestinal motility.7 The aim of the diet is not to exclude FODMAPs altogether, but to adjust consumption to a level that controls symptoms. The diet, which should be supervised by a trained dietitian, is intended to be introduced in two phases.8 The first, stricter phase eliminates all foods high in FODMAP carbohydrates, usually for 6 to 8 weeks. This phase establishes whether the patient's gastrointestinal symptoms are eased by a low FODMAP diet. The second phase involves a supervised liberalisation of the diet in a step-wise fashion, to determine the type and amount of FODMAPs that are tolerated by the patient without worsening symptoms.8

Ability to absorb lactose and fructose may be assessed by breath testing. These tests involve measuring hydrogen and methane in the breath, after challenge with lactose or fructose.9 The use of fructose breath testing is controversial. Some believe breath testing for malabsorption alongside symptom recording provides objective evidence of intolerance and may avoid the use of unnecessarily restrictive diets.10 Others think fructose hydrogen breath tests are unreliable and that the results don't correlate well enough with symptoms to be of use.2

Evidence of efficacy

A recent review4 identified six studies involving a total of 272 participants. Four of these studies were controlled trials, and are considered in more detail below. The authors of the review commented that there is ‘high quality evidence’ to support the use of a low FODMAP diet as an effective intervention for reduction of IBS symptoms.

A second review article summarised some of the same studies, but was more cautious in its conclusions.1 The authors, who declared a number of conflicts of interest related to work with pharmaceutical companies, noted that: “Long-term outcomes and safety of low-FODMAP diets remain to be demonstrated. It is also still unclear whether the low FODMAP intervention diet is beneficial to all IBS patients or whether selection, e.g. by hydrogen breath testing, can identify those patients who will benefit most.”

Clinical trials

In a small very short-term single-blinded randomised controlled trial in 30 people (15 patients with IBS, 15 healthy controls), participants received a high FODMAP diet (50g per day) or a low FODMAP diet (9g per day) for 2 days with a 7-day washout period before crossover to the other diet.11 All foods were provided by the study organisers. Outcomes assessed breath hydrogen and methane, and symptom diaries. All participants produced higher levels of hydrogen throughout the day while eating a high FODMAP diet. Participants with IBS produced much higher levels of hydrogen than healthy participants, on both diets. The study found that gastrointestinal symptoms and lethargy were higher in IBS participants on high FODMAP diets, while healthy participants noticed only increased flatus production. The two diets were assessed using a Likert scale for symptom severity (range 0 [no symptoms] to 3 [severe symptoms]). Of 15 participants with IBS on the low FODMAP diet, 6 participants rated their score for abdominal pain or discomfort as no symptoms, 8 participants rated symptoms as slight and 1 participant rated symptoms as moderate. This compared to the high FODMAP diet, where 2 said they had no symptoms, 4 rated them as slight, 7 rated them as moderate and 2 rated them as severe (p=0.006). Statistically significant differences were also seen for abdominal bloating, excessive flatus, nausea, heartburn, and tiredness or lethargy.

A non-randomised controlled trial (82 consecutive patients seen in an IBS outpatient department, 58 female, mean age 38 years) compared the effect of dietetic advice based on NICE guideline recommendations with the impact of advice on a low FODMAP diet following the introduction of a FODMAP dietetic service.12 The study found that significantly more patients who had been recommended a low FODMAP diet reported improvements in bloating (82% vs. 49% in the NICE group, p=0.002), abdominal pain (85% vs. 61%, p=0.023) and flatulence (87% vs. 50%, p=0.001). However, the study did not find statistically significant improvements for diarrhoea, constipation, nausea or energy levels between the two types of dietary advice. Although the study involved evaluation over 9 months, it did not report patients' duration of adherence to their diet. A sub-group analysis of those following a low FODMAP diet found the median time for symptom resolution was 3.5 weeks.

A third, unblinded randomised controlled trial (41 people with IBS symptoms that included bloating and diarrhoea but not constipation-predominant IBS) was designed to show changes to luminal microbiota, short-chain fatty acid production and faecal pH.13 However, it also measured gastrointestinal symptoms on a 4-point scale (absent, mild, moderate, severe) and overall symptom control (“Were your symptoms adequately controlled over the previous week?”). The intervention compared dietician instruction to restrict FODMAP consumption (19 people) with continuation of normal diet (22 people). After 4 weeks, 68% of patients in the intervention group reported adequate symptom control compared to 23% in the control group (p=0.005). The trial found a statistically significant difference between intervention and control group in the proportion of people whose mean daily symptom score improved after four weeks of treatment, for overall symptoms, bloating, borborygmi and urgency, but not for abdominal pain, flatulence, diarrhoea, constipation or tiredness (results presented graphically).

A randomised controlled single-blind crossover trial of 30 people with IBS and 8 healthy controls compared the effect of 21 days of eating a diet low in FODMAPs with a diet judged by the study organisers to represent a typical Australian diet.14 In both cases, most food was provided and adherence to the diet was assessed by food diaries. After completing the first 21 days, participants had a washout period where they followed their usual diet, then switched to the other study diet. The primary endpoint was the difference in overall gastrointestinal symptoms measured on a 100mm visual analogue scale (VAS). Baseline VAS for IBS patients was 36mm (95% CI 30 to 43). The average score in the last 14 days of dietary intervention was lower on the low FODMAP diet at 23mm VAS (95% CI 17 to 29) and greater on the Australian diet at 45mm (95% CI 37 to 53), suggesting that the ‘typical’ diet provided may have been higher in FODMAPs than the IBS participants usually ate. The difference between the diets was statistically significant (p<0.001). The researchers reported that it represented a clinically significant improvement (judged as >10mm) for 21 of 30 participants. They noted that the presence of fructose malabsorption (measured by breath test) “had no bearing on the benefit”. Healthy participants had low symptom scores, which did not differ by diet.

A controlled trial that was not included in the two review articles, randomised patients to a low FODMAP diet, probiotic supplementation or a normal Danish diet (control group) for 6 weeks.15 The unblinded trial (123 adults with IBS, 42 assigned to a low FODMAP diet, 41 to Lactobacillus rhamnosus GG supplement, 40 to a normal Danish diet) found a statistically significant reduction in IBS severity scoring system (IBS-SSS) from week 1 to week 6 for both interventions compared with the normal diet (IBS-SSS reductions: low FODMAP diet 133 [±122]; probiotic supplement 68 [±107], control 34 [±95; p<0.01 for both interventions). The IBS-SSS includes five items on a 0–100 scale, with total scores ranging from 0 to 500. A reduction of 50 points was considered to be a clinically significant improvement. Notably, 15 people dropped out of the study, 8 from the low FODMAP diet group because of difficulty adhering to the diet.

Other considerations

Potential harms from restrictive or exclusion diets could arise from nutritional inadequacy or harmful changes to the gut microbiota. We are not aware of any published data that has assessed the nutritional adequacy of patients' intake while on the low FODMAP diet.2

Some FODMAPs have been referred to as ‘prebiotics’ because they promote the growth of probiotic bacteria thought to be beneficial, such as bifidobacteria.13 Reducing FODMAPs in the diet could affect the growth of these beneficial bacteria. Although one trial demonstrated a lower concentration and proportion of bifidobacteria,13 a second trial did not observe a reduction in bifidobacteria compared to the participants' normal diet.16 The effects of the observed changes to gut microbiota are unclear, and it is also unclear whether these persist or are a temporary response to the low FODMAP diet.

A reduced FODMAP intake is likely to be associated with a reduced fibre intake, unless FODMAP fibres are replaced by low FODMAP alternatives such as rice bran or oats.2

The strict low FODMAP diet is intended as a temporary measure for 6 to 8 weeks, before types of FODMAPs are re-introduced. Liberalisation of the regime to the level of adequate symptom control should be promoted and the diet should not be recommended for asymptomatic populations.16 None of the randomised controlled studies we reviewed have examined the impact of the low FODMAP diet over more than a 6 week period.


We found three UK guidelines offering advice on dietary management of IBS. The British Dietetic Association suggests that low FODMAP diets should be discussed second line, after a clinical and dietary assessment, checking for food intolerance, general healthy eating and lifestyle advice including advice on lactose and non-starch polysaccharides.17 It says there is ‘limited good evidence’ that fermentable carbohydrates (including FODMAPs) increase bloating in people with IBS, and that fructose can precipitate worsening of pain and flatulence. It adds: “Avoidance of fermentable carbohydrates, particularly FODMAPs, is an emerging treatment and requires specialist dietetic knowledge… Following symptom resolution (2–8 weeks), planned and systematic reintroduction of foods high in fermentable carbohydrates verifies individual tolerance to specific fermentable carbohydrates.”

The British Society of Gastroenterology guidelines, published in 2007, do not specifically mention FODMAPs18 and pre-date much of the evidence for this dietary approach. The guidelines note that dietary treatment relies on ‘low’ or ‘very low evidence’ and suggest taking a history, assessing fibre intake and considering whether to recommend increase or decrease, trialling exclusion of wheat bran or lactose, and considering systematic modifications of diet to identify intolerances.

The most recent revision of the NICE guideline for the management of patients with IBS, suggests offering advice on further dietary management if patients' symptoms persist while following general lifestyle and dietary advice. Such advice should include single food avoidance and exclusion diets (e.g. a low FODMAP diet).6 It states that this advice should only be given by a healthcare professional with expertise in dietary management.


Evidence for the efficacy of the low FODMAP diet to improve symptoms of irritable bowel syndrome (IBS) is based on a few relatively small, short-term unblinded or single-blinded controlled trials of varying duration. All the trials provide some evidence of efficacy in terms of improved symptom score with few reports of adverse events. One study has shown that a low FODMAP diet can have an impact on gut microbiota, although the clinical implications of this are unclear. The long-term effect of this type of dietary manipulation is unknown.

Some guidelines suggest that a low FODMAP diet may be appropriate for motivated patients for whom other therapies have not offered sufficient symptomatic relief; and that advice on a low FODMAP diet should be provided by a dietitian with specialist knowledge of this type of intervention. However, we believe that patients should be advised that there is very limited evidence for its use, the ideal duration of treatment has not been assessed in a clinical trial and its place in the management of IBS has not been fully established.

Appendix: Examples of high and low FODMAP foods (not a complete list)18,19


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