Gut mechanisms linking low-calorie sweeteners to impaired glycaemic control

Abstract

Habitual high consumption of beverages and foods containing low-calorie sweeteners (LCS) has been linked to an increased risk of developing type 2 diabetes (T2D) in humans, however the underlying mechanisms are unknown. It is known that sweet stimuli, including LCS, are sensed at broadly tuned sweet taste receptors (STRs) located on taste cells of the tongue, and in extra-oral sites including enteroendocrine cell (EEC) populations in the proximal intestine. Activation of STRs in intestinal cell lines and preclinical models triggers a signalling cascade leading to the release of gut hormones, including the ‘incretin’ hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) which augment pancreatic insulin release, as well as the intestinotrophic peptide glucagon-like peptide-2 (GLP-2). STR-dependent release of gut GLP-2 augments levels and function of the primary apical glucose transporter, sodium-glucose cotransporter 1 (SGLT-1), acting to increase the rate of glucose absorption; this evidence is, however, equivocal in humans. The potential for sweet stimuli (and LCS) to modify glucose absorption and disposal, and disrupt gut bacterial communities, is high, and habitual high intake of LCS, particularly in subjects with T2D, may exaggerate postprandial hyperglycemia and blood glucose excursions. Work in this thesis aimed to determine the glycaemic and thyroid hormone consequences of sub-acute LCS supplementation and acute STR blockade in healthy subjects, and the molecular pathways subserving LCS signals in human intestinal tissues. A clinical study was first undertaken to evaluate the impact of two-week, high dose LCS supplementation in capsules (combined sucralose and acesulfame-K) on glycaemic control in health. LCS supplementation augmented glucose absorption, disrupted glycaemic responses to intestinal glucose, tended to reduce glucoseevoked L-cell release of GLP-1, GLP-2 and peptide tyrosine-tyrosine (PYY) and had modest effects on the balance of thyroid hormones. Glycaemic changes were linked to shifts in microbiome composition and function toward microbiome features seen in T2D, while specific microbiome mediators (e.g., Eubacterium rectale) and moderators (e.g., Bacteroidetes uniformis) were identified. These provided first evidence that an individual’s basal microbiome composition, as well as LCS-induced changes, contributed to shifts in glycaemic response to LCS supplementation. A separate ex vivo study utilising a novel human intestinal tissue platform developed to interrogate LCS molecular signals, then added support that combined sucralose and acesulfame-K evoked more powerful GLP-1 release from the ileum than duodenum in humans, a response that was lower in the presence than absence of the STR blocker, lactisole. Finally, a clinical study was then undertaken to determine acute effects of the STR blocker, lactisole, on glycaemic responses to intraduodenal glucose infusion in healthy subjects, to evaluate any potential for postprandial benefits. Lactisole co-infusion augmented the rate of glucose absorption and attenuated late phase glucose-evoked GLP-1 release in health but had no effect on T1R2, SGLT-1 or GLUT2 transcript expression levels in the duodenum. Lactisole had none of these effects in subjects with T2D. These findings add support that intestinal STRs are acutely dysregulated and decoupled from absorptive function in the context of T2D, a finding that demands validation in a more chronic setting. Together, findings in this thesis highlight the fact that combined acesulfame-K and sucralose are not inert and can negatively impact glycaemic control via intestinal STR and, potentially, microbiome mechanisms. This evidence informs the public health debate over the merits of substituting these LCS for added sugar as well as the development of new targets for ‘next-generation’ anti-diabetic therapies that target gut control of glucose absorption/glycemia or the gut microbiome (precision pre- or pro-biotics), to optimise glycaemic control.