Performing gastric bypass surgery on patients severely changes their gut hormone response to glucose. The plasma concentrations of Insulin and a number of associated incretin peptides are significantly raised shortly after ingestion of glucose. Typically the plasma concentrations of these peptides are monitored using immunoassays. A simple and high throughput plasma peptidomics approach was used to simulatneously monitor a large number of these peptides. Comparison of gastresctomised and control individuals showed significant increases of insulin, glucagon gene products, PYY and neurotensin.