Efficacy of Lactose Reduction in Irritable Bowel Syndrome and Recurrent Abdominal Pain: A Systematic Review

Background/Objectives: Lactose intolerance (LI), irritable bowel syndrome in adults (IBS) and recurrent abdominal pain (RAP) in children are frequently diagnosed worldwide. This systematic review aimed to assess the effects of dietary reduction in lactose on symptoms, quality of life and healthcare visits in IBS and children with recurrent abdominal pain. Methods: PubMed, Cochrane Library, Embase and Lilacs were searched for studies on IBS and RAP investigating the effect of a reduction of dietary intake of lactose for at least 4 weeks on abdominal pain, stool characteristics, bloating and distension, and quality of life published up to 15 August 2026. No restriction in language, publication type or data limit was applied. Four reviewers independently screened the abstracts for eligibility, and risk of bias was assessed by two reviewers with ROBINS-2. Disagreements were resolved by consensus or consulting with a senior investigator. Results were summarised in tabular form. The systematic review was registered on PROSPERO (CRD42024617161). Results: Eighteen studies met eligibility criteria based on the title and abstract. Upon review of full-text articles, three studies in adults and six studies in children were retained for data extraction. Lactose breath tests (LBTs) were performed in all studies. All but one study prospectively compared patient cohorts. IBS was defined by Manning criteria in two studies and Rome II criteria in one study. Only patients with a positive LBT were subjected to dietary intervention in eight of nine studies. No control group was included in one of three and three of six of the studies in adults and children, respectively. None of the studies compared symptomatic evolution based on validated questionnaires. Improvement in pain was reported by two of three and five of six of the studies on IBS and RAP, respectively. The risk of bias was high for all studies. No study assessed quality of life. Conclusions: Current evidence in support of dietary lactose reduction in patients with IBS and RAP is limited due to a high risk of bias, the use of old criteria, the absence of a valid comparator group and poorly defined outcomes. A positive LBT was associated with improvement in pain after a lactose-free diet. Future studies on the role of lactose restriction in IBS and RAP should recruit patients based on current diagnostic criteria and assess outcome by validated measures, preferentially applying a randomised controlled design.