Bypass or resection of the stomach and oesophagus, has long been recognised to result in profound changes in the handling of ingested nutrients. This results in significant morbidity after radical surgery for oesophago-gastric cancer, in particular post-prandial hypoglycaemia, altered appetite, early satiety and noxious post-prandial symptoms. By profiling and challenging the gut hormone axis in healthy volunteers and patients who had undergone total or subtotal gastrectomy, or oesophagectomy, this thesis explores the possible causative mechanisms for the challenges faced by this patient population. In the surgical groups, an oral glucose tolerance test (OGTT) resulted in enhanced secretion of satiety and incretin gut hormones (GLP-1, GIP, PYY) and insulin, followed by hypoglycaemia in a cohort of patients. Continuous glucose monitoring of gastrectomy participants over two weeks of normal lifestyle identified an increased incidence of day and night time hypoglycaemia. RNAseq and mass spectrometry based peptidomics of human and murine enteroendocrine cells in the pre- and post-operative populations revealed no significant change in the underlying cellular pathways for nutrient sensing and gut hormone secretion, indicating that the altered hormone secretion is primarily driven by accelerated nutrient transit, rather than adaptive changes in the gut. Finally, specific blockade of the GLP-1 receptor in post-gastrectomy patients using Exendin 9-39 normalised insulin secretion and prevented reactive hypoglycaemia after an OGTT. In conclusion, profound changes in gut hormone secretion as a result of enhanced nutrient transit after foregut surgery likely underlie the early and late post-prandial symptoms seen in this group, and therapies specifically targeting the gut hormone axis, and GLP-1 in particular, could be the first targeted treatments for post-gastrectomy syndromes.