Preliminary results found that there is no difference on abdominal pain intensity between low-FODMAP and regular diet at 4 weeks. Moreover, both diets were difficult to adhere by patients. We do not expect any significant change in our findings.

This single-center, randomized, controlled, quadruple-blinded, superiority trial is performed to test the hypothesis that children with IBS and FAP-NOS who receive a low-FODMAP diet will have a lower mean abdominal pain intensity score compared with those who receive a regular diet after 4 weeks of intervention.

Introduction. Evidence from studies in adults documents that fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) may be triggers of symptoms in individuals with functional abdominal pain disorders. However, in children with FAPDs, there is a need for high-quality evidence regarding the dietary management. We aim to assess the effects of a low-FODMAP diet compared with a regular diet for the management of children with FAPDs.

Methods. In this trial, seventy-four children aged 8 to 18 years with a Functional Abdominal Pain Disorder (Irritable Bowel Syndrome or Functional Abdominal Pain-Not Otherwise Specified), diagnosed according to the Rome IV criteria, will be randomly allocated to receive either a low-FODMAP diet or a regular diet for 4 weeks.A primary outcome will be the percentage of the responders, defined as the participants who have at least 30% improvement in abdominal pain intensity on a Visual Analogue Scale (VAS) during the last week of the trial compared with baseline, that is at least equal to the Reliable Change Index (≥ 25 mm change on VAS).

Inclusion Criteria:

• functional abdominal pain - not otherwise specified (FAP-NOS) or irritable bowel syndrome (IBS) diagnosed according to the Rome IV Criteria,
• baseline average pain intensity at least 30 mm on a 100-mm Visual Analogue Scale,
• feeding via the oral route,
• ability to read and comprehend any employed questionnaires/scales,
• signed informed consent,
• stated availability throughout the study period.

Exclusion Criteria:

• receiving any other intervention/treatment with regard to FAP-NOS or IBS or those who received any other intervention during the last 3 months,
• an organic cause of symptoms or organic gastrointestinal disease,
• chronic illness, receiving medications which affect gastrointestinal motility,
• need for any other dietary management which could make the balancing or compliance with the diet troublesome,
• previously diagnosed carbohydrate intolerance without symptoms of FAPD after implementation of an exclusion diet,
• undernutrition (defined as World Health Organization [WHO] growth charts < -2 SD), decreased growth velocity (sharp decline in growth line), or overweight or obese (> 1 or > 2 SD on the WHO growth charts, respectively),
• unintentional weight loss greater or equal to 5% of subject' body weight within the previous 3 months,
• pregnancy,
• eating disorders,
• prior surgery of the gastrointestinal tract (within last 3 months),
• recurrent or unexplained fever,
• developmental disabilities which impair the ability of the child to understand or communicate.

The datasets used and/or generated during this study will be made available after the publication of results, no later than 3 years from the completion of data analysis.

Up to 3 years after completion of data analysis.

The datasets will be made available from the contact author upon reasonable request.

The research investigators, outcome assessor, and independent statistician (data analyst) will be blinded until the completion of the data analysis.The dietitian will remain unblinded. Success of blinding will be assessed after completion of the follow-up period.