Bariatric surgery is the only satisfactory treatment for loss and weight maintenance in obese patients. This loss is a result of several factors that go beyond the simple anatomical abnormality of the gastrointestinal tract and may be the product of post-op energy expenditure changes. The respiratory quotient (RQ) and diet-induced thermogenesis (DIT) are important components of energy expenditure and possible changes in them can perform an important role in weight loss after Roux-en-Y Gastric Bypass (RYGB). Previously, we conducted a cross-sectional study where we measured the RQ and DIT in pre and post-op RYGB patients, which was published in the Surgery of Obesity and Related Diseases (SOARD) journal. The objective of this study is to perform a prospective analysis of these same variables (RQ and DIT), evaluating them in the same patients, when pre and postoperatively.

Inclusion Criteria:

• Pre-op: Body mass index (BMI) ≥ 40 kg/m2 or BMI ≥ 35 kg/m2 with associated co-morbidities. Age > 18 years
• Post-op: Patients who underwent surgery ≥ 12 months by the Head Surgeon of the Gastrocirurgia de Brasilia Clinic;

Exclusion Criteria:

• Patients aged > 65 years. Severe heart and/or respiratory problems; Pregnant women.