The Low FODMAP Diet Improves Gastrointestinal Symptoms in Patients With Irritable Bowel Syndrome

A Prospective Study

R. H. de Roest; B. R. Dobbs; B. A. Chapman; B. Batman; L. A. O'Brien; J. A. Leeper; C. R. Hebblethwaite; R. B. Gearry


Int J Clin Pract. 2013;67(9):895-903. 

In This Article


In this observational study, we have prospectively assessed symptoms in IBS patients who received hydrogen/methane breath testing and dietary advice concerning a low FODMAP diet. Our study confirms that the low FODMAP diet offers symptomatic benefit for IBS patients and leads to a significant improvement for the majority of patients. Diet adherence is crucial to the success of the diet and fructose malabsorption is strongly associated with efficacy. The most common symptoms described at the initial consultation are representative of IBS patients described elsewhere.[2,5] Most IBS patients have visceral hypersensitivity and symptoms may be triggered by luminal distension.[16,17] A high FODMAP diet has been shown to lead to luminal distension through colonic fermentation and increased delivery of fluid to the colon.[14,15]

Previous studies of the effect of low FODMAP diet for IBS patients assessed a smaller range of symptoms such as bloating, abdominal pain, passing gas, diarrhoea, constipation and nausea.[7,18,19] To more fully describe the effects of a low FODMAP, we described a wider range of symptoms. In addition to confirming the beneficial effect of low FODMAP diet on core IBS symptoms, we have shown that related GI symptoms also improve.

While there was a significant improvement in most symptoms, burping, passage of mucus and the feeling of satiety did not improve with the low FODMAP diet. This may be caused by the low frequency of these symptoms in our population but, in addition, one might not expect a significant improvement in these given the proposed mechanism of action for FODMAPs. Our study has also shown consistent results between prospectively collected and the self-reported effects of low FODMAP diet that has been used by other investigators.[18] Furthermore, the degree of improvement is comparable with that described by Staudacher et al. demonstrating a consistent effect of the low FODMAP diet between populations.

The observation that constipation also improved on a low FODMAP diet may seem counterintuitive given the proposed mechanism of action for most FODMAPs. However, this may reflect other aspects of dietary advice, which ensure sufficient fibre and other dietary constituents as part of a balanced diet. A key aspect to the dietary advice is ensuring not only that trigger foods are removed but also that the resultant diet is balanced. It is conceivable that this may have led to more fibre in the diet of those who previously had low fibre diets and were constipated.

We have found a high rate of fructose malabsorption (75.6%) compared with others,[21] although there is a large variation in fructose malabsorption rates between populations.[20,25] The high rate in our population could be attributable to a hospital referral bias because many patients were referred by Gastroenterologists rather than General Practitioners. Furthermore, the cut-off used to define fructose malabsorption is controversial.[21,25–27] We used a 10 ppm rise of breath hydrogen on two occasions or else a rise greater than 15 ppm on one occasion that was accompanied by GI symptoms. This definition is on the aggressive side for diagnosing fructose malabsorption, but may be relevant given the strong association with efficacy of the low FODMAP diet. Despite the higher prevalence of fructose malabsorption, the prevalence of lactose malabsorption is comparable to other studies.[21]

The need for breath testing to identify individuals with fructose and lactose malabsorption prior to dietary interventions has been debated.[12,28,29] Parker et al. suggested that it is unnecessary to distinguish IBS patients with lactose malabsorption from those without.[28] Furthermore, other studies report that there is no difference in the prevalence of SIBO between patients with IBS and healthy volunteers.[29] Our study suggests that IBS patients with fructose malabsorption are significantly more likely than those without to respond to the low FODMAP diet. Therefore, we advocate the use of these tests, not only to direct dietary interventions but also to provide prognostic information. While lactose malabsorption and SIBO were less discriminatory, testing may play a specific role in a small number of patients. We hypothesise that the association between fructose malabsorption and efficacy of the diet may reflect large amounts of fructose in the diet of these IBS patients. This would make fructose more amenable to significant dietary reductions than other FODMAPs. Otherwise, this observation may reflect improved adherence among patients with fructose malabsorption. It is possible that a positive breath test result led to improved adherence to the diet, even though this was not shown in our study.

Not surprisingly, adherence to the diet is essential to its overall success.[19,24] In this study, we also found a positive correlation between adherence and symptom improvement. In addition, adherent patients had a significant improvement in 17/20 rated symptoms, compared with only 7/20 symptoms in less adherent patients. Several factors contribute to adherence with the diet, including its ease of use and the absence of barriers such as the perceived taste of the diet. Despite that, most participants do not find the diet easy to incorporate in their life, which may explain why only 12.2% of the patients report complete adherence. However, even with a mean follow up of 15.7 (±9.0) months, the majority of patients remain adherent suggesting that the perceived difficulties are worth the improvement in symptoms.

Most patients had two consultations with the dietitian and indicate that the consultation(s) and written information are the most important supporting factors in maintaining adherence. Because of similarities between the Australian and New Zealand diets and food choices, minimal adaptation of the diet was required for use in New Zealand. A minority of patients believe that the diet would be effective if given in written form only. It is also interesting to note that patients believe that the results of undergoing breath testing help to maintain adherence to the low FODMAP diet. Given that adherence to the diet is associated with efficacy, undergoing breath testing may improve outcomes.

Almost 75% of patients were satisfied with their symptoms after breath tests and dietary intervention – similar to that published by Staudacher et al. While this represents a significant improvement, one quarter are not satisfied with their symptoms. Therefore, while the low FODMAP diet may help many IBS patients, other approaches are needed. Targets for improving treatment include optimising adherence to this and other dietary approaches or else exploring other non-dietary strategies.

Although this study suggests that the low FODMAP diet is effective for many IBS patients, there are limitations to this study. First, the response rate of 46.9% was lower than hoped for and reflects high rates of migration out of our region since the Christchurch earthquakes. This lower response rate may reduce the generalisability of the results, particularly if those who took part in the study and those who did not are in any way different. However, when the demographic and symptom profiles of the repliers and non-repliers are compared, the only significant difference is a higher proportion of women among the repliers. This may have led to a small differential bias because male patients with abdominal pain were less likely than female patients to respond to a low FODMAP diet. However, this difference was not seen for any other symptom. Furthermore, if all of the non-repliers are included in the analysis as not having responded to the diet (i.e. assigned the same follow-up symptom scores as baseline), the results remained significant.

Secondly, this is a non-randomised observational study with the potential for a placebo effect from the intervention. However, the observed improvement in symptoms is biologically plausible because of the mechanisms that have been described and the symptoms with less biological plausibility (passage of mucous, early satiety and burping) did not show a significant improvement. Furthermore, a randomised placebo-controlled crossover rechallenge study showed efficacy of the diet in a small cohort.[7]

In conclusion, this study supports the efficacy of a low FODMAP diet in improving symptoms in IBS patients. Those with fructose malabsorption are most likely to benefit. Furthermore, the current strategy of dietary advice being delivered by trained dietitians following hydrogen breath testing provides a good base for patients to understand and adhere to the diet, which is essential for its success.