Specific Aim 1: As shown in the literature, exercise can be a useful tool for treating and improving functional GI disorders. Therefore, the aim is to estimate the effectiveness of an exercise intervention on IBS symptoms as measured by a reduction of at least 50 score points on the IBS- Symptom Severity Scale (IBS-SSS) questionnaire.

Inclusion criteria:

Adult patients (18-65 years old) referred to the "Outpatient Clinic for Celiac Disease and Functional Disorders" who met the Rome III-IV criteria for IBS-D and were referred by their local General Practitioner.

Exclusion criteria:

Presence of severe heart, liver, neurological or psychiatric disease or GI disease other than IBS (e.g., inflammatory bowel disease, celiac disease) that could explain the current symptoms.

Patients who have previously followed a diet low in particular substances (e.g., low fermentable oligosaccharides, disaccharides, monosaccharides, and polyols - FODMAP -) diet, vegan diet, a gluten-free diet).

Antidepressant use All patients will receive detailed verbal and written information about the study before giving their written consent.

Methodology: V1. (Screening). Patients will be informed about the objective of the study; V2. (Day 0). Seven days after the screening visit, patients will return for the outpatient visit. At this time, patients will complete the symptom questionnaires. During this visit, blood and biological samples (urine, feces) will be taken; anthropometrical measurements and bioimpedance analysis (BIA) will be carried out. Additionally, ECG will be performed at the Cardiology Unit. V3. (pre-treatment). Seven days before the start of the exercise intervention, the enrolled subjects, only after submitting the certificate of physical fitness issued by their doctor or general practitioner (GP), will undergo three field physical tests for the initial assessment of their fitness. V4. (treatment). Treatment begins after assessing the baseline level of physical activity through analysis of the results of the three field tests. V5. (post-treatment). In the seven days following the end of the Physical Exercise intervention, the subjects are assessed again using the three field tests. V6. (Day 90).

End of treatment visit. During this visit, completed questionnaires will be collected, including the symptom diary and daily exercise diary; in addition, patients will complete the symptom questionnaires (IBS-SSS and GSRS). An anthropometric and bioimpedance analysis will also be performed, and potential adverse events during the intervention period will be assessed. Blood and biological samples necessary for the instrumental evaluations in the study will be taken.

Physical Exercise Intervention: The exercise intervention tool will be Fitwalking, which is structured, organized, supervised, and administered through a "Walking Group". Fitwalking is a motor-sport activity that has innumerable potential for health and physical well-being. Fitwalking is a type of training based on walking, carried out with a precise technique, which allows you to walk well and be faster, thus producing positive metabolic effects and improving physical efficiency. Fitwalking is an aerobic activity of moderate intensity in which the workload produces progressive adaptations and allows for improved recovery and load increase over time. An effective prescription includes a systematically designed and individualized exercise program in terms of Frequency, Intensity, Time, Type, Volume, and Progression (FITT-VP). These parameters are briefly described below:

Frequency. Exercise (walking) will be carried out outdoors on an urban route 3 times a week, on non-consecutive days, for 12 weeks.

Intensity. The intensity of the exercise (50/75% of HRmax) will be monitored through the use of the heart rate monitor and will be personalized through the Tanaka formula. In addition, to measure pace we will use the TALK TEST (a standardized and validated survey tool based on the ability of the exercising subject to holding a conversation), and to measure the perception of fatigue we will use the modified BORG scale (on a scale of 0-10, a perception of 5-6 is needed).

Type. The type of exercise is moderate aerobic with a speed ranging from 5 to 10 km/h. The single outing lasting 60' will be structured as follows:

• Warm-up: 5'
• Normal walk: 10'
• Sustained walking: 30'
• Fast walking: 10'
• Cool-down: 5' Time. Each walk will be led by a walking leader (Master's Degree in Science and Techniques of Preventive and Adapted Motor Activities).

Volume. Participants in the project will perform 180' minutes per week of aerobic exercise of moderate intensity, through walking in groups as per the ACSM (The American College of Sports Medicine) guidance on preventive health. However, participants may also increase their exercise volume through other activities outside the project. All exercise data, both within and outside the project, will be recorded in a daily diary for the assessment of energy expenditure (see appendix).

Progression. The FITT components (Frequency, Intensity, Type, and Time) can be modified and increased according to the subjects' abilities and improvements, to achieve the recommended volume of exercise as per the ACSM (The American College of Sports Medicine) guidance on preventive health.

Statistical analysis. This study aims to evaluate the effectiveness of exercise in symptom management of patients with IBS compared to standard dietary advice. The collected data will be grouped and summarized concerning demographic variables, baseline characteristics, and efficacy and safety assessments. Exploratory analyses will be performed using descriptive statistics. Data will be presented (as mean - standard deviation - SD, unless otherwise stated) for both the intention-to-treat population (i.e., all patients who exercised) and the per-protocol population (i.e., all patients who completed the study without major protocol violations). Patients who discontinued the study will also be listed, and their reasons described analytically. Categorical variables will be compared with the X2 test. Two-tailed p-values <0.05 will be considered statistically significant.

Statistical analysis will be performed using the Generalized Estimating Equation procedure to estimate the effect of interventions on the outcome (IBS-SSS). These models are instrumental in biomedical studies to evaluate the change in outcome in the presence of covariates. To overcome the non-normal distribution, a gamma distribution of the outcome will be assumed, and the identity link will be used. The structure of the covariance between the different measurements will be explored with special programs. The results will be expressed in the natural scale of the outcome measurement as the mean and the corresponding 95% confidence intervals. Marginal effects of interventions will also be estimated for different covariates or combinations of covariates. All statistical analyses will be performed using the statistical software Stata 17 (StataCorp, 4905 Lakeway Drive, College Station, Texas 77845 USA). In particular, the -xtgee- command will be used, and the statistical structure of the covariance will be explored with the -qic- command.

Inclusion Criteria:

• Adult patients referred to the "Outpatient Clinic for Celiac Disease and Functional Disorders" who meet the Rome III-IV criteria for IBS-D and referred by their local General Practitioner.

Exclusion Criteria:

• Presence of severe heart, liver, neurological or psychiatric disease or GI disease other than IBS (e.g., inflammatory bowel disease, celiac disease) that could explain the current symptoms.

Patients who have previously followed a diet low in particular substances (e.g., low FODMAPs, vegan diet, gluten-free diet).

Antidepressant use