Additional Study Information: The purpose of these studies is to gain an understanding of the mechanisms that promote weight loss and improved glucose control. The ultimate goal is to identify therapeutic pathways for obesity and diabetes as a way to optimize surgical procedures and discover novel non-surgical targets for safe and effective drug therapy. Participants with obesity will undergo weight loss by a low-calorie diet for 12 weeks or bariatric surgery. Treatment is not randomized but includes individuals who volunteer for the diet or those who are scheduled to undergo surgery. There will also be comparison with a lean control groups. Participants will undergo testing with administration of a test meal and blood draws. Testing will occur before weight loss, at 15% weight loss, and at one year.