Bariatric Surgery and Gut Microbiota Changes Over Time

Inclusion Criteria (RYGB Group): * Age between 18 and 65 years. * Adults with a body mass index (BMI) ≥ 35 kg/m² with related comorbidities, or BMI ≥ 40 kg/m², who are candidates for Roux-en-Y gastric bypass (RYGB) surgery only. Exclusion Criteria (RYGB Group): * Treatment with antibiotics within the last 3 weeks, or probiotics/prebiotics within the last 4-6 weeks, or systemic corticosteroids within the 8 weeks prior to baseline (T0). * Participation in dietary regimens or calorie restriction programs in the weeks prior to enrollment. * Chronic inflammatory bowel diseases (e.g., Crohn's disease, ulcerative colitis), ongoing enteric infections, or active neoplasms. * Previous organ transplant or ongoing systemic immunosuppressive therapy. * Pregnancy or breastfeeding. * Previous major gastrointestinal surgery. * Chronic alcohol consumption above moderate levels or substance use disorder. Inclusion Criteria (Obese Control Group): * Age between 18 and 65 years. * Adults with BMI ≥ 30 kg/m² who are candidates for structured dietary treatment. Inclusion Criteria (Healthy Control Group): * Age between 18 and 65 years. * Normal weight (BMI between 18.5 and 24.9 kg/m²). * Normal metabolic function: fasting glucose below threshold, normal ALT/AST, no diagnosis of diabetes or metabolic syndrome. * No chronic gastrointestinal diseases or conditions that could alter the gut microbiota. Exclusion Criteria (Obese Control and Healthy Control Group): * Use of antibiotics within the last 3 weeks or probiotics/prebiotics within the last 4-6 weeks. * Recent gastrointestinal surgery. * Pregnancy or breastfeeding. * Extreme diets or recent dietary changes. * Recent international travel that could have affected the gut microbiota. * Medications known to influence the gut microbiota (e.g., PPIs, metformin, steroids). * Active smoking or excessive alcohol consumption. * Significant gastrointestinal symptoms (chronic diarrhea or constipation).