The goal of this observational study is to learn whether routine stool tests can detect changes in gut function 6 months after two common types of weight loss surgery: sleeve gastrectomy and gastric bypass (including mini gastric bypass and Roux-en-Y gastric bypass). The main questions it aims to answer are:

Do stool tests show more signs of undigested food or carbohydrate malabsorption after gastric bypass compared to sleeve gastrectomy?

Do signs of gut inflammation in stool (like fecal calprotectin) decrease after surgery, and does this relate to improvements in blood sugar and weight loss?

Can simple stool test results at 6 months predict how much weight a person loses or how well their diabetes improves?

Researchers will compare stool test results between the two surgery groups (sleeve gastrectomy vs. gastric bypass) to see if the type of surgery leads to different changes in gut health.

Participants will:

Provide a stool sample before surgery and again 6 months after surgery

Provide a blood sample at the same time points to measure weight, blood sugar, and cholesterol

Undergo their planned weight loss surgery as part of their regular medical care

Background:

Obesity is a global pandemic with rising incidence, contributing significantly to morbidity, mortality, and healthcare costs worldwide. Bariatric surgery, particularly sleeve gastrectomy (SG) and gastric bypass procedures (including Mini Gastric Bypass [MGB] and Roux-en-Y Gastric Bypass [RYGB]), remains the most effective and durable treatment for morbid obesity, achieving sustained weight loss and remission of metabolic comorbidities such as type 2 diabetes.

The gastrointestinal tract, particularly the gut microbiome, has emerged as a key metabolic regulator. Obesity is associated with dysbiosis-reduced microbial diversity and altered predominance of bacterial phyla-which contributes to systemic inflammation, insulin resistance, and metabolic syndrome. Bariatric surgery induces profound and durable changes in the gut environment, including alterations in gastric emptying, intestinal transit time, bile acid signaling, and the composition of the gut microbiota.

However, advanced microbiome sequencing is expensive, time-consuming, and not readily available in many clinical settings. Routine stool analysis, by contrast, is a simple, inexpensive, and universally available test that can provide valuable preliminary data on gut health. Parameters such as stool pH, the presence of leukocytes, undigested food particles, and quantitative markers like fecal calprotectin and reducing substances can offer indirect evidence of maldigestion, mucosal inflammation, and microbial activity.

Objective:

The primary objective is to compare changes in routine stool parameters (physical examination, microscopic examination, undigested food particles, fecal calprotectin, and reducing substances) from baseline to 6 months postoperatively between patients undergoing SG and those undergoing Gastric Bypass (MGB or RYGB).

Secondary objectives include: correlating postoperative changes in stool parameters with clinical outcomes (%TWL, diabetes remission, changes in HbA1c and lipid profile); comparing the prevalence of specific stool findings between groups; evaluating whether baseline stool parameters can predict postoperative outcomes; and assessing the utility of routine stool analysis as a simple, low-cost tool for monitoring gut health after bariatric surgery.

Methods:

This is a prospective, two-arm, observational cohort study conducted at the Department of General Surgery, Kasr Al-Ainy University Hospitals, Cairo, Egypt. The study period is from March 2026 to December 2026.

Study Timeline and Data Collection:

Patients will be assessed at two predefined time points:

T0 (Baseline): 1-2 weeks preoperatively - informed consent, demographic data, medical history, anthropometric measurements, blood samples, stool sample

T1 (6 months): 6 months ± 2 weeks postoperatively - anthropometric measurements, blood samples, stool sample, assessment of comorbidities, final outcomes

Clinical and Anthropometric Measurements:

Body weight (kg) and height (m) for BMI calculation (kg/m²)

Percentage total weight loss (%TWL) at 6 months = [(initial weight - 6-month weight) / initial weight] × 100

Presence and status of comorbidities (diabetes, hypertension, dyslipidemia)

Diabetes remission defined as HbA1c < 6.5% off all antidiabetic medications at 6 months

A total of 46 patients with morbid obesity (BMI ≥40 or ≥35 kg/m² with comorbidities) scheduled for primary laparoscopic bariatric surgery will be enrolled and allocated into two groups: Group A (SG, n=23) and Group B (Gastric Bypass [MGB or RYGB], n=23). The type of gastric bypass (MGB vs. RYGB) will be determined by the surgical team based on patient anatomy, comorbidities, and surgeon preference, reflecting real-world clinical practice.

Blood Sampling and Biochemical Analysis:

After an overnight fast (8-12 hours), venous blood will be collected at both time points for:

• Fasting plasma glucose (mg/dL)
• Glycated hemoglobin (HbA1c %)
• Lipid profile: total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides (mg/dL)
• High-sensitivity C-reactive protein (hs-CRP) (mg/L) as a marker of systemic inflammation

Stool Sample Collection and Analysis:

Patients will be provided with a sterile, leak-proof stool collection container and written instructions. They will collect a fresh morning stool sample at home. Samples should be transported to the hospital laboratory within 2 hours of collection. If immediate transport is not possible, samples should be refrigerated (4°C) and transported within 12 hours.

Stool analysis parameters include:

Physical Examination: Color, consistency, odor, mucus, pH

Microscopic Examination: White Blood Cells (WBCs), Red Blood Cells (RBCs), trophozoites, cysts, ova, larva, flagellates, ciliate

Undigested Food Particles: Meat fibers, vegetable particles, starch granules, fat globules

Quantitative Tests: Fecal calprotectin (ELISA), Reducing substances (Clinitest tablets)

Inclusion Criteria:

• Male and female patients aged 18 to 60 years
• Body Mass Index (BMI) ≥ 40 kg/m², OR BMI ≥ 35 kg/m² with at least one obesity-related comorbidity (type 2 diabetes, hypertension, dyslipidemia, osteoarthritis, obstructive sleep apnea)
• Patients scheduled for primary laparoscopic bariatric surgery (either SG or Gastric Bypass [MGB or RYGB]) at Kasr Al-Ainy University Hospitals
• Provision of written informed consent

Exclusion Criteria:

• Age < 18 years or > 60 years
• Previous bariatric surgery or major gastrointestinal surgery (gastric, intestinal, or colorectal resection)
• Chronic kidney disease (eGFR < 60 mL/min/1.73m²) or previous renal transplant
• Chronic liver disease (cirrhosis, chronic hepatitis) or liver failure
• Chronic inflammatory bowel disease (Crohn's disease, ulcerative colitis), celiac disease, or chronic pancreatitis
• Active malignancy or history of chemotherapy/radiotherapy in the past 5 years
• Pregnancy or breastfeeding
• Chronic alcohol abuse or substance abuse
• Use of antibiotics, probiotics, or prebiotics within 4 weeks prior to stool sample collection (to avoid confounding effects on gut flora)
• Acute gastrointestinal infection at the time of sample collection

Patients with morbid obesity scheduled for primary bariatric surgery at Kasr Al-Ainy University Hospitals, Cairo, Egypt.