Weight loss surgery is very good at reducing body weight but it can also cause the loss of both muscle and strength. Some patients undergoing weight loss surgery do not achieve their weight loss goals and regain the weight they lost. When this occurs, the loss of muscle and strength combined with the regain of weight can impact the individual's quality of life and ability to remain active and mobile. The purpose of this study is to understand the behavioral, biological, and genetic factors that influence the success of weight loss surgery and its impact on muscle mass. Bariatric surgery patients participating in the trial will be monitored prior to, and for a year following weight loss surgery, with data collected about their eating habits, hand grip strength, and the loss of fat, muscle, and body weight following surgery. Some patients will be additionally invited to undergo detailed metabolic assessment, where we will measure how their body uses nutrients it consumes, the composition of their body (e.g. how much lean and fat tissue they have and where it is stored), identify the bacteria living in their gut, and determine their physical performance. In all patients a small sample of gut tissue will be collected at a routine endoscopy performed in advance of weight loss surgery to identify the expression (activity) of genes in their DNA. Healthy subjects will also be recruited to allow us to compare between healthy weight individuals and those undergoing weight loss surgery. On a single occasion, the healthy volunteers will undergo the same detailed metabolic assessment performed in patients as described above. We will not examine the bacteria living in the gut of the healthy volunteers, nor will we collect gut tissue from these individuals.