For this trial, sham intervention will mimic the osteopathic care according to the "touch equality assumption". Therefore, it mimics a clinical examination and treatment. The sham clinical evaluation involves a full evaluation of the position and mobility of the spine.This screening of the patient will be performed using conventional osteopathic manual tests, without any intention to make a diagnosis. Sham treatment will involve light manual contact, with the subject lying supine on the treatment table.

In addition to the real or sham osteopathic care, all participants will follow a 4-week flexi-FODMAP diet delivered online. Participants will receive structured dietary instructions via a customized online platform, based on the NHS Gentle-FODMAP diet guidelines and informed by expert input from dieticians. This modified "FODMAP-gentle" approach involves a targeted, less restrictive elimination of selected high-FODMAP foods.

The diary asks for dietary intake information of two weekdays and one day in the weekend. Collected data with the 3-day food diary will allow us to find out approximate calorie intake, micronutrient intake, macronutrient intake of each participant.

The patients will record all bowel movements over a 10-day period using a stool diary, based on the Bristol Stool Form (BSF) scale. The diary will document stool frequency (number of stools per day), mean stool consistency on a 7-point scale, and will be used to classify IBS subtypes and evaluate bowel habits. Sub-types of IBS are divided into four categories based on the predominant stool form; IBS with predominant constipation (IBS-C, ≥25% hard stools (BSF 1 or 2) and <25% loose stools (BSF 6 or 7)), IBS with predominant diarrhea (IBS-D, ≥25% loose stools and <25% hard stools), IBS with alteration between constipation and diarrhea (IBS-M, ≥25% of reported stools hard and ≥25% loose); and un-subtyped IBS (IBS-U, insufficient abnormality of stool consistency to meet criteria for IBS-C, -D, or -M).

The "Brief Pain Inventory" allows to rate the worst pain in the last 24 hours, the least pain in the last 24 hours, the average pain and the pain intensity at the moment of filling out the questionnaire. This tool also assesses the interference of pain with seven daily activities, including general activity, walking, work, mood, enjoyment of life, relations with others, and sleep via an 11-point numeric rating pain scale, ranging from 0 (no pain) to 10 (worst pain)

The Multidimensional Fatigue Inventory-20 (MFI-20) assesses the severity of general fatigue, physical fatigue, reduced activity, reduced motivation, and mental fatigue. Each dimension contains 4 questions, with a range of scores between 4 and 20, generating a total score between 0 and 100. A higher score indicates more severe fatigue. The MFI-20 is considered a valid and reliable scale, with a Cronbach's Alpha = 0.93.

The IBS quality of Life (IBS-QoL) is an IBS-specific QoL questionnaire that measures 10 domains that have been found to be relevant to patients with IBS: emotional health, mental health, health belief, sleep, energy, physical functioning, diet, social role, physical role, and sexual relations. The minimum score is 30 and the maximum score is 160. The higher the score, the better the QoL. The IBS-QoL is considered a valid and reliable scale, with Cronbach alpha=0.95.

The Visceral Sensitivity Index measures GI-specific anxiety, that is, anxiety originating from fear of GI symptoms, which is related to the unpredictable symptom pattern commonly found in IBS. Each item can be scored from one, 'strongly agree', to six, 'strongly disagree'. High-sum scores represent more severe GI-specific anxiety. The Visceral Sensitivity Index is considered a valid and reliable scale, with a Cronbach's Alpha = 0.92.

The Patient Health Questionnaire (PHQ-9) will be used to diagnose depressive disorders. The PHQ-9 measures the severity of a depressive disorder with nine questions resulting in a sum score between 0 and 27 points. A score of 5 or more indicates mild depression, from 10 moderate, from 15 moderate-severe and above 20 severe depression. The internal and test-retest reliabilities of the PHQ-9 were both excellent, with a Cronbach's α of 0.89.

Sleep quality experienced in the previous month were measured using the 19-item self-reported Pittsburgh Sleep Quality Index (PSQI). The 19-items generate seven component scores: subjective sleep quality, sleep latency, sleep duration (i.e., quantity), habitual sleep efficiency, sleep disturbances, use of sleep medication, and daytime dysfunction. Each of these seven component scores is weighted equally on a scale ranging from 0 to 3, 0 indicating no difficulty and 3 indicating severe difficulty. Overall, sleep quality (global PSQI score) was calculated by summing the seven component scores, which range from 0 (better) to 21 (worse); a global PSQI score > 5 is consistent with poor sleep quality. The internal and test-retest reliabilities of the PSQI were both excellent, with a Cronbach's α of 0.83.

Perceived stress was measured using the PSS, which also measured participants' current stress levels and which situations in their lives were appraised as stressful. The PSS is a 10-item questionnaire where individuals are asked about their feelings in the past month; it is scored on a 5-point scale ranging from 0 (Never) to 4 (Very often). Four PSS responses are then reverse-scored, and a sum is calculated to obtain a total, where higher scores suggest higher perceived stress. The scale validity is strong (r = 0.72), as is the test-retest reliability (r = 0.93, p < 0.001).

The Work Productivity and Activity Impairment Questionnaire-IBS measures whether IBS symptoms affect the ability to work and perform everyday activities with four different variables: absenteeism, presenteeism, overall work impairment, and activity impairment. Each variable ranges from 0% to 100%, with higher values indicating higher impairment. The Work Productivity and Activity Impairment Questionnaire-IBS is a valid and reliable scale with high test-retest correlations (r = 0.97 - 0.99).

Fecal microbiota analysis

Dietary intervention (4 weeks): Adherence assessed through participant-completed food diaries and questionnaire. Patients will rate the number of times they forgot to follow the diet on a Likert scale (0 = never to 6 = constantly)

Osteopathy care (4 weeks): Adherence monitored using attendance records of scheduled sessions.

Long-term follow-up: Adherence evaluated through patient diaries focusing on maintenance of dietary recommendations and osteopathy-related practices.

Treatment satisfaction was scored as an improvement in IBS symptoms compared with baseline and ranging from 'a lot worse' (zero) to 'extremely better' (seven).

Additionally, patient attending the osteopathic care, the level of satisfaction will be assessed at the end of the study by asking patients their global appreciation of osteopathic care and whether they would agree to pay for subsequent manipulative procedures.