Few studies were performed that assessed beta cell mass after bariatric surgery. In Goto-Kakizaki rats duodenal jejunal bypass was found to increase pancreatic concentrations of vesicular monoamine transporter type 2 (VMAT2). VMAT2 was used as a biomarker for beta cells, however later VMAT2 was found to be expressed on pancreatic polypeptide cells (PP cells) as well. Furthermore, an increase in beta cell mass, beta cell number and extra islet beta cells was found after RYGB in a porcine model and GK rat model. Studies in humans showed conflicting results. Long term after gastric bypass, cases of hyperinsulinemic hypoglycemia have been described, which are related to pancreatic islet hyperplasia or nesidioblastosis, while this does not occur after purely restrictive surgery. However, beta cell mass was assessed in six patients after gastric bypass by histological analysis of the pancreas after pancreatectomy and did not find an increase in beta cell mass. These studies were performed on pancreas specimen and not in vivo.