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Nybacka S, Törnblom H, Josefsson A, et al. A low FODMAP diet plus traditional dietary advice versus a low-carbohydrate diet versus pharmacological treatment in irritable bowel syndrome (CARIBS): a single-centre, single-blind, randomised controlled trial. Lancet Gastroenterol Hepatol. 2024;9(6):507-520. PMID 38643782

One-paragraph summary

Between 24 January 2017 and 2 September 2021, 1,104 people were assessed and 304 randomised at a specialist outpatient clinic at Sahlgrenska University Hospital, Gothenburg. Eligible participants had Rome IV IBS of moderate-to-severe intensity (IBS-SSS ≥175) and no other serious disease or food allergy. They were assigned 1:1:1 to (a) a diet low in fermentable oligosaccharides, disaccharides, monosaccharides and polyols plus traditional NICE dietary advice ("LFTD"), (b) a fibre-optimised diet low in total carbohydrate and high in protein and fat, or (c) optimised medical treatment selected according to the predominant IBS symptom. Participants were masked to the names of the diets; the pharmacological arm was open-label. The intervention ran four weeks, with six months of follow-up. In the modified intention-to-treat population (n=294), the proportion achieving an IBS-SSS reduction of ≥50 points was 73/96 (76%) with LFTD, 69/97 (71%) with the low-carbohydrate diet and 59/101 (58%) with optimised medical treatment (p=0.023). Completion was 95%, 95% and 90% respectively; five of 91 in the medical arm stopped early for side effects. No serious adverse events or treatment-related deaths occurred. Registered NCT02970591.

Key findings

  • Both diets outperformed optimised drug treatment on the primary responder endpoint at four weeks (76% and 71% vs 58%, p=0.023).
  • The comparator was not a single drug but symptom-directed optimised pharmacotherapy — the fairest available drug comparator.
  • A low-carbohydrate, high-fat, high-protein diet performed comparably to low FODMAP, which is not what the FODMAP mechanism alone predicts.
  • Dietary interventions were at least as well tolerated as drugs (95% vs 90% completion; two adverse-event discontinuations per diet arm versus five drug discontinuations for side effects).
  • Authors' conclusion: "Dietary interventions might be considered as an initial treatment for patients with IBS."

Limitations

  • Single centre, single-blind, specialist clinic; 82% women, mean age 38 years.
  • Four-week primary endpoint in a lifelong condition; the six-month follow-up was open.
  • Diets were masked only as to name; the pharmacological arm was fully open-label, so expectation effects are asymmetric between arms.
  • Delivery was dietitian-supported in a research setting, which is not the routine primary-care experience.
  • The two diets were not designed to isolate a mechanism, so the comparable performance of a low-carbohydrate diet is descriptive rather than explanatory.

Why it matters

CARIBS moved diet from adjunct to candidate first-line therapy by doing the comparison the field had avoided: diet against drugs, head to head, in the same patients, with a responder endpoint. Its result is reinforced from a different direction by DOMINO, in which a smartphone FODMAP-lowering app beat otilonium bromide in Belgian primary care (71% vs 61% responders, p=0.03; Carbone 2022, PMID 35483886). Together they are the empirical basis for the diet-first position, and they were both investigator-led and non-commercial — which is also why no guideline written before 2024 sequences diet ahead of pharmacotherapy.

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