The purpose of this study is to evaluate Non-Alcoholic Fatty Liver Disease (NAFLD) and Non-Alcoholic Steatohepatitis (NASH) changes in terms of steatosis and elasticity in patients with morbid obesity 1, 3 and 5 years after bariatric surgery.

In addition, genomics, microbiome and metabolomics analyses will be carried out.

After being informed about the study and potential risks, 300 eligible patients will be included.

All participants will undergo lifestyle intervention before bariatric surgery. Extensive cardiometabolic analyses will be carried out. At 5 different moments (twice before and 3 times after bariatric surgery), liver steatosis and elasticity will be evaluate using Fibroscan. For those with an elevated liver fibrosis measurement (TE ≥7.2 kilopascal with M probe or ≥7.95 kilopascal with extra large probe), a laparoscopic liver biopsy will be performed during surgery. Patients will undergo gastric sleeve resection, Roux-n-Y gastric bypass or gastric minibypass.

Inclusion Criteria:

Participants must fulfil all of the inclusion criteria (unless otherwise specified) for enrolment in the study.

1. Participants must fulfil the criteria of the International Federation for the Surgery of Obesity (IFSO) at screening.
2. Participants must be able to understand the nature of the study and written informed consent must be obtained before any study specific interventions are performed.

The cardiac function of a subset group of 30 subjects aged 35 years and older with low transient elastography measurements (≤F1) and no known cardiac disease, hypertension or diabetes mellitus will be compared with a similar number of patients with high transient elastography measurements (≥F3) and no known cardiac disease, hypertension or diabetes mellitus. These patients will be consecutively included.

The investigators will perform a liver biopsy during endoscopic bariatric surgery in the patients with an elevated transient elastography measurement (≥F2; suggesting fibrosis and therefore the presence of NASH) in order to correlate the histological diagnosis to the clinical diagnosis based on elastography. In those patients in whom the diagnosis of NASH is confirmed by histology, more detailed follow up in a separate program at the department of gastroenterology is indicated according to the current guidelines.

Exclusion Criteria:

Patients who meet any of the following criteria at screening (unless otherwise specified) will be excluded from the study:

1. Participants younger than 18 years or older than 65 years

2. Participants with an established diagnosis of liver pathology like, but not limited to:

   1. Hepatitis B
   2. Hepatitis C
   3. Autoimmune hepatitis
   4. Wilson's disease
   5. Hemochromatosis
   6. Primary biliary cholangitis
   7. Primary sclerosing cholangitis
   8. Alcoholic liver disease

3. Histologically documented liver cirrhosis (fibrosis stage F4), at screening or in a historical biopsy.

4. Participants with active HIV infection and/or treatment.

5. Participants with diagnosed malignancies with or without active treatment.

6. Participants with a history or evidence of any other clinically significant condition or planned or expected procedure that in the opinion of the investigator, may compromise the patient's safety or ability to complete the study.

7. The participant does not understand Dutch.

Participants with morbid obesity enrolled for bariatric surgery at the Franciscus Gasthuis, Rotterdam, the Netherlands.

Data are available upon reasonable request. The data that will be shared is individual participant data that underlie the results reported in the study, after deidentification (text, tables, figures and appendices). Also, the study protocol will be available. Data will be available beginning 9 months and ending 36 months following publication, for researchers who provide a methodologically sound proposal for any purpose. Proposals may be directed to w.theel@franciscus. nl up to 36 months following article publication. After 36 months, the data will be available in our hospital's data warehouse but without investigator support other than deposited metadata.