In spite of the recognition of obesity as a serious public health problem due to its well-known increased risk for the development of type 2 diabetes hypertension, coronary heart disease, sleep-breathing disorders, and certain forms of cancer, among others, it is proving extraordinarily difficult to halt this pandemia. Strictly speaking obesity does not refer to an excess weight or weight to height ratio. In fact, the World Health Organization defines obesity as a state of increased adipose tissue of sufficient magnitude to produce adverse health consequences. Thus, in order to better define the effects and benefits of weight loss it is important to address the impact on body fat changes. Given the limited approved anti-obesity drugs, the main therapeutic strategies involve either conventional treatment or bariatric surgery. The main modifiable factors affecting energy balance are dietary energy intake and energy expended through physical activity. In spite of the difficulty in achieving relevant and sustained weight loss via the conventional approach, some patients are successful in reducing weight and obesity-associated complications. Bariatric surgery has proved to be the most effective long-term treatment for weight loss and comorbidity improvement. While some of the surgery-induced benefits are directly dependent on adipose tissue reduction, others are due to specific gastrointestinal changes that take place early on and before any significant effects on body weight are observed. Noteworthy, currently available bariatric procedures differ on their impact on these aspects. The present study contemplates the determination and comparison of the anthropometric and metabolic changes produced by the conventional and surgery-induced treatment modalities. Particular emphasis will be placed on the potential differential effects between conventional and surgical weight loss on energy intake, energy expenditure, body composition changes, circulating adipokines and gastrointestinal hormones together with their subsequent impact on cardiometabolic risk factors. The conventional weight-reduction program (CONV) will pursue a safe weight loss rate of 0,5-1,0 kg per week. The surgery-induced weight loss will be achieved by two of the most frequently used bariatric operations, the sleeve gastrectomy [SG (which implies a restrictive component)] and the Roux-en-Y gastric bypass [RYGB (which combines a restrictive and a malabsorptive component)].