The purpose of this study is to investigate the impact of bariatric surgery on the gut health. Patients operated with Roux-en-Y gastric bypass or sleeve gastrectomy and obese patients who want too loose weight with a traditional weight-loss program, will be followed up for 1 year. In these patients, the investigators will measure toxicity parameters to understand better the health status of their colon after surgery. In a next phase, the measured toxicity will be linked with certain players that might cause this toxicity. Protein metabolites, formed from undigested protein by microbiota in the colon, are expected to be toxic agents for the colon. Therefore, the investigators will investigate the fate of ingested protein once the surgery patients are metabolically stabilized.

To quantify protein assimilation after bariatric surgery, the investigators will perform in bariatric surgery patients (Roux-en- Y gastric bypass and Sleeve Gastrectomy) and in a control group a cross-sectional study with a single test day. On the test day, the study participants will consume a test meal that contains intrinsically labelled egg proteins and will take a capsule with 14C-glycocholic acid and a capsule with 3H-labelled polyethylene glycol. Participants will record a food diary 72h before and after the test day and during the test collect breath samples for 6h. Urine and stool will be collected respectively up to 24h and 72h after consumption of the test meal.

For the 1-year follow up study, the 3 patient groups (Roux-en-Y gastric bypass, Sleeve gastrectomy and control group with traditional weight loss therapy) will have 5 visits, a baseline visit and 4 visits after surgery or after starting traditional therapy. The time points after baseline include 2 weeks, 3 months, 6 months and 12 months. Each visit, a blood and stool sample will be collected, a 7-day food and stool diary will be registered and also weight loss will be recorded.

Inclusion Criteria:

• Surgery patients (toxicity and protein assimilation) : BMI > 40 kg/m² or > 35 kg/m² if combined with either obstructive sleep apnoea syndrome, high blood pressure not controllable with 3 different medications or type II diabetes, (3) exclusion of other underlying causes of obesity by an endocrinologist, (4) having tried to lose weight on a non-surgical way for at least 1 year without result and (5) a positive advice from a multidisciplinary team consisting of a surgeon, endocrinologist, psychologist and dietitian.
• Obese control group in weight loss program (toxicity) : obese but otherwise healthy patients on a weight loss diet (BMI > 30 kg/m²) in the context of the weight-loss program organised at the Obesity Clinic of University Hospital Leuven.
• Control group (protein assimilation) : normal weight (BMI 25-30 kg/m²) or overweight (BMI 25-30 kg/m²).

Exclusion Criteria:

• Intake of antibiotics 1 months before the start of the study. Pre-and probiotics, laxatives and anti-diarrhea drugs 2 weeks before the start of the study.
• Surgery patients and control group (toxicity) : Gastrointestinal disease or major abdominal surgery in the past (except from appendectomy and cholecystectomy).
• Surgery patients and control group (protein assimilation) : Lung, liver, kidney and gastrointestinal disease or major abdominal surgery in the past (except from appendectomy, cholecystectomy and bariatric surgery). Vegan, vegetarian, lactose-or gluten-free diet. Pregnant or lactating women and subjects who participated in the last year in a study with irradiation exposure.

• Surgery patient group was recruited at the University Hospital Leuven.
• Obese control group was recruited at the Obesity Clinic of the University Hospital Leuven.
• Normal and overweight control group was recruited among the employees and family members from employees of the University Hospital Leuven.