This study aimed to investigate host-microbe responses to bariatric surgery, weight loss, and glycaemic control in obesity, involving 156 adult participants at baseline, recruited from a university teaching hospital in London, the United Kingdom. Participants were recruited in 2015 through NHS clinical pathways, targeting individuals referred for bariatric surgery. All participants were aged 18 years or older, had a BMI over 30, and had previously failed lifestyle or dietary interventions. Of these, 49 participants underwent either Roux-en-Y gastric bypass (RYGB) or vertical sleeve gastrectomy (VSG), and a smaller subset of the surgical group was also followed up at 1 year post-surgery. Biological samples were collected at each follow-up point, and dietary data were gathered using 24-hour recall questionnaires.