The aim of this study is to assess whether activity thermogenesis increases in patients undergoing bariatric surgery post-operatively compared to pre-operatively. The investigators will assess non-exercise activity thermogenesis using a validated Physical Activity Monitoring System and doubly labeled water, immediately pre-operatively and again at 6 and 18 months post-operatively. PAMS has been validated in lean and obese individuals. This is to assess energy expenditure, activity thermogenesis and NEAT during the phase of greatest weight loss and once weight loss has plateaued. This will permit us to address our primary hypothesis that activity thermogenesis as a fraction of total daily energy expenditure increases in patients undergoing bariatric surgery 6 and 18 months post-operatively, compared to pre-operatively.