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.