40 patients meeting the criteria for obesity surgery are enrolled. Fecal microbiota transplantation (FMT) will be administered in gastroscopy 6 months before obesity surgery. 20 of the patients will receive a fecal transplant of a thin donor and 20 will receive placebo - a transplant made of their on feces. Changes in weight, laboratory values, general well being and stool microbiota will be measured up to one year after the surgery; one and a half years after FMT. The hypothesis is that FMT and the change in gut microbiota will help to reduce and maintain lower weight.

Inclusion Criteria:

• Candidates for the bariatric surgery

BMI ≥ 40 or BMI ≥35 and at least two obesity-related co-morbidities such as type II diabetes (T2DM), hypertension, sleep apnea and other respiratory disorders, non-alcoholic fatty liver disease, osteoarthritis, lipid abnormalities, gastrointestinal disorders, or heart disease.

• Availability of consecutive fecal samples during one year
• Compliance to attend gastroscopy and FMT
• 18-65 years

Exclusion Criteria:

• Unable to provide informed consent
• Pregnancy
• Type I Diabetes Mellitus
• Severe renal insufficiency, GFR<30%
• Chronic or recurrent bacterial infection needing antimicrobial treatment
• Large hiatal hernia