Obesity is a complex chronic disease, in which both genetic and environmental factors are involved, that shows a great heterogeneity in the response to different weight loss programs. Identifying patients as responder or no responder to the different obesity treatment options is a concept of great interest, both due to the high prevalence of obesity and its high consumption of resources. More than 500,000 surgeries are performed every year around the world, of which approximately 30% will present unsatisfactory results.

The general objective is to carry out a multi-omics approach for the discovery and validation of markers of weight response to bariatric surgery (BS) in a large sample of people with severe obesity (n=6,966 men and women who underwent sleeve gastrectomy or gastric bypass, including an additional external validation set). Thus, the investigators want to know the integrated contribution of several genomic markers (Genome Wide Association study, GWAs), new clinical and analytical variables (human exposome concept) and gender perspective to the prediction of response to the intervention at 12 month and its long-term longitudinal maintenance (3 years). The investigators intend, therefore, to provide new evidence to advance towards precision medicine. The investigators will focus our attention also on identifying those patients who, after being classified at the weight loss nadir as responders experienced weight regain.

Inclusion Criteria:

• Men and women between the ages of 18 and 65 at the time of surgery
• SG or RYBP as bariatric procedure.
• With a BMI≥35.0 kg/m2 with one or more comorbidities or a BMI>40.0 kg/m2 at the time of surgery.
• Provision of an informed consent form signed and personally dated by the patient.
• With available baseline data (anthropometric, analytical, surgical procedure, pharmacological therapies, and obesity-related comorbidities).

Exclusion Criteria:

• Any previous gastrointestinal surgery
• Treatment with any approved antiobesity drug in Spain (orlistat, liraglutide 3.0 mg and/or naltrexone/bupropion extended release) or systemic glucocorticoids for more than 4 consecutive weeks during the 3 years following BS
• End stage kidney disease (eGFR <15 ml/min/1.73m2) or liver cirrhosis at baseline
• Patients who developed any major medical conditions that limits the practice of a healthy life within normal limits (advanced cardiovascular disease or heart failure, stroke with neurological sequelae, chronic pulmonary obstructive disease with dyspnea at minimal exertion, severe joint pathology, end stage renal failure, or active cancer)
• Known type 1 diabetes or LADA diabetes
• Women who become pregnant during the follow-up period
• Current drug or alcohol abuse
• Uncontrolled psychiatric illness or eating disorders developed during the follow-up period.

This multicenter retrospective study will be aimed at 6,966 obese patients undergoing BS, whose surgical technique (sleeve gastrectomy vs gastric bypass) has been performed at least 3 years before, evaluating the best combination of genetic and exposomal characteristics (clinical and analytical) that will predict which patients will be responders and non-responders in terms of weight loss.