Efficacy and Acceptability of Dietary Therapies in Non-Constipated Irritable Bowel Syndrome: A Randomized Trial of Traditional Dietary Advice, the Low FODMAP Diet and the Gluten-Free Diet

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  5. Efficacy and Acceptability of Dietary Therapies in Non-Constipated Irritable Bowel Syndrome: A Randomized Trial of Traditional Dietary Advice, the Low FODMAP Diet and the Gluten-Free Diet

Irritable bowel syndrome (IBS) is a common functional bowel disorder characterized by chronic abdominal pain, bloating, and altered bowel habit. Dietary therapies are frequently recommended in IBS, given that over 80% of individuals report food-related symptoms.

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Traditional Dietary Advice (TDA) is the first-line dietary therapy recommended in the UK, based on guidance by NICE and the BDA. The principles of this diet include sensible, healthy eating patterns whilst reducing intake of fatty/ spicy food, caffeine, alcohol and limiting intake of fresh fruit and gas-producing foods. The low FODMAP diet (LFD) is the second-line dietary therapy for IBS within the UK, involving a reduction in consumption of short-chain, fermentable carbohydrates found in fruit, veg, dairy products, artificial sweeteners and wheat. A gluten-free diet has also grown in popularity for the management of functional gut symptoms, with approximately 10% of the population reporting that gluten-based products provoke IBS symptoms.

In the absence of any pragmatic head-to-head trials comparing the LFD, GFD and TDA for the management of IBS, this randomised trial sought to investigate the efficacy, acceptability, nutritional and stool microbial changes associated with these diets.


Patients were recruited from 2 secondary care centres in the UK, inclusion criteria were: adults ≥ 18 years, Rome IV IBS-Diarrhoea (IBS-D) or mixed type (IBS-M), and an IBS-symptoms severity score of >75. Patients were randomized to be educated to follow a TDA, LFD or GFD. Participants were seen face-to-face by specialist dietitians and dietary advice was provided using a standardised 45-60 minute presentation, dietary info sheets and time for questions. During the Covid-19 pandemic, this process became a virtual consultation, using the same materials. Participants followed their diets for 4 weeks, with outcomes at week 4 compared with baseline. Questionnaires completed pre- and post-intervention, included:

  • IBS symptom severity score (IBS-SSS)
  • Hospital Anxiety & Depression Scale
  • Patient health questionnaire
  • IBS quality of life (QoL) questionnaire
  • Acceptability of dietary restriction questionnaire
  • Food-related QoL questionnaire
  • Comprehensive Nutrition Assessment Questionnaire

Stool samples for assessment of dysbiosis were collected for 50% of patients (Covid-19 prevented collection of remaining samples)

Key Findings

A total of 99 participants, 33 per arm, completed the study. There was no difference in base-line variables across groups. Mean age was 37 years, 71% female, 88% white, 75% IBS-D, 25% IBS-M. Nine percent of participants had mild IBS, 47% moderate IBS and 45% severe IBS (p=0.5 across all groups).

The diets did not significantly differ in clinical efficacy. The primary endpoint of ≥50-point reduction in IBS-SSS was met by 42% taking TDA, 55% with LFD, and 58% with GFD, with no significant difference across groups; p=0.43.

The modes of dietary education, either face-to-face or virtual, were equally effective: A ≥50-point reduction in IBS-SSS was seen in 52% receiving face-to-face consult vs. 51% receiving live virtual consult; p=0.98.

There was no statistical difference in response rates between IBS-D vs. IBS-M based on a particular dietary therapy. A ≥50-point reduction in IBS-SSS was seen in 54% (n=40/74) with IBS-D vs. 44% (n=11/25) with IBS-M, with no difference between groups; p=0.38.

Individuals found TDA cheaper, less time-consuming to shop, and easier to follow when eating out. Individuals found TDA and GFD easier to incorporate into their life than the LFD (p=0.02). The proportion of individuals who would consider continuing the diets were 70% for TDA, 67% for LFD and 61% for GFD, with no difference across groups (p=0.73).

FODMAP intake was reduced across all groups. Significant within-group reduction in total FODMAP intake occurred with all three diets. The greatest reduction was seen with the LFD (27.7g/day pre-intervention to 7.6g/day at week 4) compared with TDA (24.9g/day to 15.2g/day) and GFD (27.4g/day to 22.4g/day); p<0.01.

Changes in dysbiosis index (DI) did not differ across groups. 22-29% of participants experienced an improvement, 35-39% had no change, and 35-40% had worsening DI. Changes in DI did not differ between responders and non-responders.


Discussion & conclusions

TDA, GFD and LFD are effective approaches in non-constipated IBS. The authors of this study recommend TDA as the first-choice dietary option due to its widespread availability and patient friendliness. The LFD or GFD are alternative options based on specific patient preferences and with specialist dietetic counselling. Current national guidelines do not recommend a GFD for the management of IBS, however, the results of this study suggest that this deserves future re-evaluation. It would also be of interest to evaluate their efficacy in patients not responding to TDA.

The diets implemented in this study reduced total FODMAP intake, mostly in the LFD group compared with TDA and GFD. This suggests a degree of overlap and that moderate FODMAP restriction, as seen with TDA and a GFD, may be similarly effective as a strict LFD. The LFD is a 3-stage process and data from some countries suggests that patients may fail to move from the strict elimination phase, in to the reintroduction and personalisation phases, therefore putting them at risk of overly restrictive eating patterns and nutritional inadequacies1. There are suggestions that a ‘bottom-up’ or “FODMAP-gentle” approach to the LFD may overcome its extensive 3-phase program. For example, in the long-term, many patients on a personalised LFD reduce fructan intake to manage their symptoms, and facilitate this through purchasing gluten- or wheat- free products2. This raises the hypothesis that a GFD might be an option before considering the complete LFD programme. Furthermore, this study, amongst another recent publications, suggests that a GFD in IBS does not need to be strict as that seen in coeliac disease. Future studies should determine the level of gluten restriction required to derive symptom benefit.


Link to original paper

Further references:

  1. Tuck CJ, Reed DE, Muir JG, Vanner SJ. Implementation of the low FODMAP diet in functional gastrointestinal symptoms: A real-world experience. Neurogastroenterol Motil. 2020;32(1):e13730.
  2. Rej A, Shaw CC, Buckle RL, et al. The low FODMAP diet for IBS; A multicentre UK study assessing long term follow up. Dig Liver Dis. 2021 Nov;53(11):1404-1411