Obesity stretches the limits of metabolism, resulting in diseases such as type 2 diabetes and fatty liver . Bariatric surgery, also known as weight-loss surgery, is currently the most effective weight-loss treatment for obese and frequently sick patients. We found that surgery shifts metabolism to a new state, distinct from obese or lean metabolism. This state often leads to improvement in diabetes and other metabolic diseases independent of weight-loss. Understanding how bariatric surgery works presents opportunities for both studying organismal physiology as well as identifying new clinical approaches for obesity, diabetes and other diseases. We collaborate closely with physicians, make use of mouse models, and apply mathematical tools to interpret results and design new experiments.