Obesity is now a problem of epidemic proportions within New Zealand. Gastric bypass surgery is well established as a treatment that provides sustained weight loss, remission of diabetes and improvements in cardiovascular risk at 5 years and later post-surgery. Changes in gut metabolism play a key role in the immediate post-operative period. Studying post-surgical patients several years after their gastric bypass would yield important information on the long-term effects of gastric bypass on gut metabolism, and other clinical outcomes associated with obesity such as diabetes, cardiovascular disease and overall mortality. It will contribute to understanding the magnitude and mechanism of these effects as well as the financial implications of gastric bypass surgery within the New Zealand healthcare system.