 Introduction: According to the National Health and Nutrition Examination Survey 2009-2010, 78 million American adults are obese. Bariatric surgery can provide for a sustained long-term weight loss and the metabolic change caused by the surgery seems to be the main cause of this loss. Cross-sectional, prospective and experimental studies, carried out during the post-operative period of Roux-en-Y Gastric Bypass surgery RYGBP have shown an increase of over 200 in energy expenditure after meals Diet-induced Thermogenesis DIT, a specific component of energy expenditure when compared with obese patients. However, despite this metabolic improvement, 20-50 of the patients can suffer weight regain about 2 years after surgery. So one question whether such metabolic benefits remain active following post-operative weight regain, or if the disappearance or decrease of these metabolic benefits may be seen as causes of this weight regain. 