Investigating the role of nutrient sensing in the gastrointestinal tract in energy and glucose homeostasis

Obesity is a major global health issue. As high-protein and high-fibre diets are associated with increased satiety and weight loss, uncovering the mechanisms by which dietary protein- and fibre-derived metabolites are sensed in the gut and regulate appetite may reveal targets for anti-obesity therapies. The work presented in this thesis sought to further this endeavour with a two-pronged approach, in vitro and in vivo. The in vitro intestinal organoid model recapitulates the major features of the in vivo intestinal epithelium. I cultured intestinal organoids from mouse duodenum, jejunum and ileum, and demonstrated that basal GLP-1 content increased distally along the small intestine, in line with the in vivo L-cell density pattern. Research within our group had identified that the amino acid L-Phenylalanine has anorectic properties and highlighted the calcium-sensing receptor as a potential target through which L-Phenylalanine mediates GLP-1 release. My work built on this by showing that L-Phenylalanine significantly stimulates GLP-1 release in mouse ileal organoids, an effect significantly attenuated by a calcium-sensing receptor antagonist. Investigation of the underlying signalling mechanisms indicated that L-Phenylalanine-induced GLP-1 secretion may not be mediated via a Gi/o-coupled pathway, and that Ca2+ influx through L-type calcium channels may be required. The in vivo dietary intervention study I concurrently ran in healthy humans led to observations that a high-protein, high-fibre test meal stimulated GLP-1 and PYY release that remained above fasting levels for the 6-hour sampling period. Furthermore, acute repeated exposure of the gut to a protein- and fibre-enriched diet may have beneficial effects in terms of sustaining post-prandial plasma PYY responses and modulating glucose homeostasis. Gastrointestinal contents from the terminal ileum and proximal colon were obtained from participants via a specialised nasoenteric sampling tube, and microbial profiling of these together with stool samples revealed region-specific differences within the distal gut, indicating that stool may not be an accurate proxy for either microbial diversity or composition of the terminal ileum or proximal colon. Additionally, acute temporal shifts in microbial composition, perhaps due to dietary intervention, were identified in the gut contents samples. The work in this thesis has contributed to our knowledge of how ingested protein and fibre affect anorectic gut hormone release, glucose homeostasis and appetite, and has shed light on how differences in microbial diversity and composition can exist in different regions of the distal gut. It has also enhanced our understanding of the mechanisms that sense L-Phenylalanine in the gut. Further investigations are required to determine the therapeutic potential of protein- and fibre-derived metabolites as appetite-suppressing agents, with an end-goal of developing novel strategies to prevent and treat obesity.