This study is being done to compare the effects of a newer class of weight loss medications to weight loss surgery on the body's muscle, metabolism, and side effects over time. People with severe obesity often lose weight using either medications like GLP-1 receptor agonists (such as semaglutide or tirzepatide) or by having bariatric surgery, such as gastric bypass. While both approaches can lead to weight loss, they may affect the body in different ways. The investigators are inviting adult patients who are planning to either start one of these weight loss medications or undergo gastric bypass surgery to join this study. Participants will be asked to complete four body composition scans (called DXA scans) over the course of a year-at the beginning of the study and again at 3, 6, and 12 months. These scans will help us measure changes in fat and muscle. Participants will also complete surveys and provide information on side effects, such as nausea or fatigue, that may affect their eating or activity levels. The investigators' goal is to better understand how different weight loss treatments impact not just weight, but also muscle mass, energy levels, and side effects. This information may help doctors and patients choose the most appropriate treatment in the future.