Obesity is a growing problem worldwide and its prevention has become one of the leading priorities for the World Health Organisation. Obesity results from chronic imbalance between energy intake and energy expenditure. Although early prevention of obesity is preferable, surgical treatment is often required for severely obese people.

Bariatric surgery has been shown to be the most effective therapy for severe obesity. Weight loss following bariatric surgery results in significant improvements in coexisting comorbidities, such as diabetes and hypertension but there is controversy whether bariatric surgery also improves aerobic capacity.

The purpose of this study is to investigate the effects of bariatric surgery on cardiopulmonary function and on daily physical activity. It is hypothesized that bariatric surgery will improve aerobic capacity and result in beneficial lifestyle changes from sedentary to more active.

Inclusion Criteria:

• Ages 18 years to 50 years
• Patients enrolled in the bariatric surgery program of St George's Hospital with BMI > 40 kg/m2, or 35-40 kg/m2 in the presence of other obesity-related comorbidities such as hypertension or type- 2 diabetes

Exclusion Criteria:

• Locomotor difficulties which would prevent participants from completing the cardiopulmonary exercise testing
• Weight > 190 kg (due to weight restrictions of equipment used to transfer patients in the event of medical emergencies)
• Cognitive impairment
• Patients unable to follow instructions in English

Patients with scheduled bariatric surgery at St George's Healthcare NHS Trust.