People with severe obesity who are scheduled to undergo either primary gastric bypass or primary sleeve gastrectomy will be recruited from the Bariatric Clinics at University College London Hospital (UCLH) and the Whittington Hospital and asked to participate in an observational cohort study. Eligible patients will be given a participant information sheet (PIS) for BARI-LIFESTYLE Observational Study and informed consent will be sought. Recruited patient will attend for 4 research visits, timed to coincide with their clinic visits, at approximately 6 weeks before surgery (Visit 1), 3 month post-surgery (Visit 2), 6 months post-surgery (Visit 3) and 12 months post-surgery (Visit 4). Each research visit will include assessment of body weight, body composition (using bioelectrical impedance analysis), physical fitness and activity levels, obesity-associated co-morbidities, HRQoL and usual care laboratory investigations. Dual energy X-ray absorptiometry (DXA) scanning will be undertaken at Visit 1 and Visit 4, which is not a usual assessment for the post-surgery standard care. All of the research assessments will be done as part of this observational cohort study.