We developed a mass-spectrometry based platform for identification of peptides in humans and by applying this platform we characterized a peptide hormone oxyntomodulin secreted from the intestine in response to glucose. Our data suggest that oxyntomodulin is down-regulated in subjects with type 2 diabetes and up-regulated after bariatric surgery. In summary, the collected data indicate that oxyntomodulin may co-orchestrate appetite and glucose regulatory effects together with incretin hormones.