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Gut Bacteria Changes Over Time After Weight-Loss Surgery

Follows gut bacteria in adults with obesity after gastric bypass surgery over time.

Key facts

Who runs it
Azienda Ospedaliera Specializzata in Gastroenterologia Saverio de Bellis
Where
Castellana Grotte, ItalySee the site
Trial code
Registry title
Bariatric Surgery and Gut Microbiota Changes Over Time
Status
Recruiting
People taking part
100 people, planned
Started
7 Jul 2026
Expected to end
Apr 2028
Type of study
Observational, 3 groups
Record updated
5 November 2026

Who can take part

AI summary

From the trial's eligibility criteria. Machine-read from the registry text on this page, so it can be wrong. The registry text below and the trial team decide who can take part.

The trial is looking for adults with obesity who are having Roux-en-Y gastric bypass surgery. It also takes adults with obesity on a diet alone, and healthy normal-weight adults.

Age
18 to 65 years
Sex
All sexes can join.
Disease stage
Surgery group: BMI 35 kg/m² or more with related health problems, or BMI 40 kg/m² or more, and a candidate for Roux-en-Y gastric bypass. Diet group: BMI 30 kg/m² or more, a candidate for structured diet treatment. Healthy group: BMI between 18.5 and 24.9 kg/m², normal metabolic function, no diabetes or metabolic syndrome.
Earlier treatment
Cannot have taken antibiotics in the last 3 weeks or probiotics/prebiotics in the last 4-6 weeks. Surgery group: no systemic corticosteroids in the 8 weeks before the start, and no diet or calorie restriction programs in the weeks before joining.

Main reasons for not taking part

  • Antibiotics, probiotics or prebiotics taken recently
  • Pregnancy or breastfeeding
  • Previous or recent gastrointestinal surgery
  • Inflammatory bowel disease, ongoing gut infections or active cancer (surgery group)
  • Medicines that affect gut bacteria, such as PPIs, metformin or steroids (diet and healthy groups)

The registry criteria, word for word

Show all 23 criteriaShow less

Who can take part

  • 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
  • Age between 18 and 65 years
  • Adults with BMI ≥ 30 kg/m² who are candidates for structured dietary treatment
  • 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

Who cannot take part

  • 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
  • Previous organ transplant or ongoing systemic immunosuppressive therapy
  • Previous major gastrointestinal surgery
  • Recent gastrointestinal surgery
  • Medications known to influence the gut microbiota (e.g., PPIs, metformin, steroids)
  • Chronic inflammatory bowel diseases (e.g., Crohn's disease, ulcerative colitis), ongoing enteric infections, or active neoplasms
  • Chronic alcohol consumption above moderate levels or substance use disorder
  • Pregnancy or breastfeeding
  • Use of antibiotics within the last 3 weeks or probiotics/prebiotics within the last 4-6 weeks
  • Pregnancy or breastfeeding
  • Extreme diets or recent dietary changes
  • Recent international travel that could have affected the gut microbiota
  • Active smoking or excessive alcohol consumption
  • Significant gastrointestinal symptoms (chronic diarrhea or constipation)

Check if you might fit

Answer for yourself. This is not a decision: only the study team can say whether you can take part.

  • Are you between 18 and 65 years old?

Where in Italy

  • Azienda Ospedaliera Specializzata in Gastroenterologia Saverio de BellisCastellana Grotte, ItalyContact: Domenica Mallardi, BiologistTaking people

About the trial

What the trial wants to find out

The goal of this observational study is to learn how bariatric surgery affects gut bacteria and gut-related metabolic products over time in adults with obesity. The study includes adults aged 18 to 65 years who are undergoing Roux-en-Y gastric bypass (RYGB) surgery, as well as adults with obesity treated with diet alone and healthy normal-weight adults.

Read full original descriptionShow less

The main questions it aims to answer are:

How does bariatric surgery change the composition and diversity of gut bacteria over time? How are these changes related to weight loss and improvement of obesity-related health conditions?

Researchers will compare people undergoing bariatric surgery with people with obesity treated with diet alone and with healthy normal-weight individuals to see if surgery leads to specific changes in gut bacteria and stool metabolites that are linked to better clinical outcomes.

Participants will:

Provide stool samples at scheduled time points over 12 months Provide blood samples and undergo routine clinical assessments Take part in follow-up visits to monitor weight, metabolic health, and gastrointestinal symptoms

The 3 groups in the trial

  • RYGB Group

    Adults aged 18-65 years with severe obesity (BMI ≥ 35 kg/m² with comorbidities or BMI ≥ 40 kg/m²) who are undergoing Roux-en-Y gastric bypass (RYGB) surgery.

  • Obese Control Group

    Adults aged 18-65 years with obesity (BMI ≥ 30 kg/m²) who are receiving structured dietary treatment without surgery.

  • Healthy Control Group

    Adults aged 18-65 years with normal weight (BMI 18.5-24.9 kg/m²) and normal metabolic function. This group helps identify normal variations in gut bacteria and metabolites over time, independent of obesity or surgery.

Questions to ask the study team

  1. How often do I need to come in, and for how long each time?
  2. Are travel costs paid?
  3. Can I keep my own doctor?
  4. What happens if I want to stop?

Contact the study team: see the registry record → Contacts and Locations

Print a page for my doctor

Data and sources

Registry record
Listed on ClinicalTrials.gov.
Official title
Longitudinal Evaluation of Gut Microbiota and Fecal Metabolome Following Roux-en-Y Gastric Bypass: Clinical Implications and Identification of Predictive Biomarkers
Plain-language title
Gut Bacteria Changes Over Time After Weight-Loss SurgeryGenerated from the registry record. The registry title is the official one.
AI summary
From the trial's eligibility criteria. Read by machine on 13 November 2026.
Fit questions
Built from NCT07381140, who can take part
Record updated
5 November 2026