The current work aims to directly observe food intake to test the hypothesis that after gastric bypass (traditional RYGB and OAGB) food intake changes in a manner which leads to beneficial outcomes on body weight when compared to weight stable control participants. Patients (n=32; n=16 traditional RYGB, n=16 OAGB) with a planned gastric bypass procedure will be recruited from Phoenix Health (Ireland and England) and Letterkenny University Hospital (Ireland), alongside control participants (n=16) with no planned weight loss. All subjects will attend the Human Intervention Studies Unit (HISU), Ulster University on four occasions (1-month pre-surgery and 3, 12 and 24 months post-surgery, with controls being time-matched). Study visits will be fully residential involving two overnight stays within the facility during which participants' 24-hour food intake will be covertly measured (7am-11pm Day 2 and breakfast Day 3) and the following procedures undertaken; basal metabolic rate, body composition, bone health, assessment of liking/wanting high fat foods (Visit 1, 2 & 4 only) and post-meal gut hormone responses. On each study visit participants will have ad lib access to a range of foods of varying macronutrient composition and which are compatible with their stated food preferences (assessed prior to the start of the study). Changes in all ingestive behaviours will be evaluated over time as compared to the control participants and between groups ((traditional RYGB vs OAGB).