Creative Touch Permanent Makeup and Aesthetics

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted primarily by enteroendocrine L-cells in the ileum and colon in response to nutrient arrival – the hormonal endpoint of the ileal brake mechanism. Its core physiological roles are glucose-dependent insulin secretion, glucagon suppression, delayed gastric emptying, and appetite reduction via hypothalamic and vagal signalling. Endogenous GLP-1 has an active half-life of approximately two minutes before cleavage by the enzyme DPP-4, making its satiety effect meal-contingent and pulsatile rather than sustained. GLP-1 receptor agonist medications – including semaglutide and tirzepatide – replicate this signal at suprathysiological and continuous concentrations by engineering resistance to DPP-4 degradation. The same satiety pathway is activatable through food composition: meals prioritising fat and protein over rapidly absorbed carbohydrate deliver substrate to the ileum, stimulating endogenous GLP-1 release and a post-meal satiety response that lasts two to four hours without pharmacological intervention. nature.com

Glucagon-like peptide-1 is a 30-amino-acid incretin peptide derived by post-translational processing of proglucagon, a precursor protein whose cleavage products vary by tissue: in pancreatic α-cells, proglucagon produces glucagon; in intestinal L-cells and brainstem neurons, the same gene produces GLP-1 and GLP-2. This tissue-specific processing is the biological reason GLP-1 and glucagon have opposing metabolic effects despite sharing a common genetic origin. nature.com

What GLP-1 Is and Where It Comes From

GLP-1 is synthesised and secreted primarily by enteroendocrine L-cells – open-type secretory cells concentrated in the ileum and colon, with a smaller population in the duodenum and jejunum. L-cells express nutrient-sensing receptors on their luminal surface: free fatty acid receptors (FFAR1/GPR40 and GPR120) detect long- and medium-chain fatty acids from dietary fat; peptide transporter PEPT1 detects di- and tripeptides from protein digestion; electrogenic glucose transporters detect luminal glucose. Binding of any of these ligands depolarises the L-cell membrane and triggers calcium-dependent exocytosis of pre-formed GLP-1 vesicles into the portal circulation. GLP-1 is therefore a nutrient sensor as much as a satiety hormone – a real-time report from the intestinal mucosa on what is present in the gut lumen.

A smaller but clinically relevant population of GLP-1-producing neurons exists in the nucleus of the solitary tract (NTS) in the brainstem, which contributes to central appetite regulation independently of the gut-derived signal. This central GLP-1 source helps explain why GLP-1 receptor agonist medications produce appetite suppression even when administered between meals, outside the pulsatile meal-stimulated release window of gut L-cells. nature.com

The DPP-4 Problem: Why Endogenous GLP-1 Is Short-Lived

Once secreted into the portal circulation, GLP-1 is rapidly cleaved and inactivated by dipeptidyl peptidase-4 (DPP-4), a serine protease that removes the two N-terminal amino acids from GLP-1, rendering it biologically inactive. DPP-4 is ubiquitous – present on vascular endothelial cells, in plasma, and on lymphocytes – meaning GLP-1 encounters it almost immediately on entering the circulation. The resulting active half-life of intact GLP-1 is approximately two minutes in plasma, meaning the majority of meal-stimulated GLP-1 is inactivated before it even reaches the systemic circulation from the portal vein. ^[3]

This is not a design flaw. The short half-life creates a precisely meal-contingent signal: GLP-1 is active when nutrients are present in the gut, and absent – appropriately – between meals. The pulsatile, meal-matched nature of endogenous GLP-1 is part of its normal physiological architecture, coordinating insulin secretion and appetite suppression with actual nutrient arrival rather than anticipation of it.

How GLP-1 Works: Receptor Signalling

GLP-1 exerts its effects by binding to the GLP-1 receptor (GLP-1R), a class B G-protein-coupled receptor expressed across multiple tissues. When GLP-1 binds GLP-1R, it activates Gαs proteins, increasing intracellular cyclic AMP ( cAMP). cAMP then activates protein kinase A (PKA), which drives insulin synthesis and secretion from pancreatic β-cells. Simultaneously, cAMP can activate EPAC (exchange protein directly activated by cAMP), a second pathway also involved in insulin secretion regulation. GLP-1 also activates the PI3K/Akt pathway, supporting the survival and functional integrity of β-cells – a mechanism that may partly explain why GLP-1 receptor agonist use is associated with preserved β-cell function in type 2 diabetes over time. ^[1]

The satiety effects operate through separate receptor populations. GLP-1R on vagal afferent neurons transmit satiation signals to the brainstem. GLP-1R in the hypothalamic arcuate nucleus directly suppress appetite-stimulating neuropeptide Y (NPY) and AgRP neurons whilst activating POMC neurons – the hypothalamic circuitry that drives cessation of eating. Gastric emptying is delayed through GLP-1R on gastric smooth muscle, slowing the rate at which nutrients leave the stomach and further prolonging the satiety signal.

Incretin Effect: GLP-1 and Insulin Secretion

GLP-1 is one of two primary incretin hormones – gut-derived signals that potentiate insulin secretion in response to oral nutrient intake beyond what blood glucose alone would stimulate. The “incretin effect” accounts for 50–70% of the total insulin secreted after an oral glucose load in healthy individuals. GLP-1’s insulin-stimulating action is glucose-dependent: it enhances insulin secretion only when blood glucose is elevated, which is why GLP-1 receptor agonists carry minimal hypoglycaemia risk as monotherapy. At normal or low blood glucose concentrations, the GLP-1 signal on β-cells is substantially attenuated. ^[1]

GLP-1 simultaneously suppresses glucagon from pancreatic α-cells – an effect partially mediated through somatostatin from pancreatic δ-cells acting as an inhibitory paracrine signal, rather than direct GLP-1R action on α-cells. Since glucagon raises blood glucose by promoting hepatic glycogenolysis and gluconeogenesis, its suppression during the post-meal period is an important component of GLP-1’s glucose-lowering action, particularly in type 2 diabetes where inappropriate glucagon secretion contributes to hyperglycaemia. ^[1]

GLP-1 and the Ileal Brake: The Food-First Signal

The physiological context for GLP-1 secretion is the ileal brake – the mechanism by which unabsorbed nutrients arriving in the ileum trigger a coordinated satiety response. GLP-1 is the primary hormonal output of that mechanism, secreted alongside PYY (peptide YY 3-36) by the same L-cells. Together, GLP-1 and PYY suppress appetite centrally, delay gastric emptying, and slow intestinal transit – a feedback system that matches the rate of nutrient processing to the rate of nutrient intake.

Not all macronutrients activate this mechanism equally. Fat and protein are the more potent and sustained L-cell stimulants: controlled ileal infusion studies demonstrate that lipid infusion reduces subsequent food intake by approximately 464 kcal and protein by 458 kcal, compared to 399 kcal for carbohydrate – a meaningful difference that reflects both the receptors L-cells express and the more distal delivery of fat and protein relative to rapidly absorbed carbohydrate. Refined carbohydrates are largely absorbed in the proximal small intestine and reach the ileum in diminished quantities, producing a weaker and shorter-lived GLP-1 response than the same calories delivered as fat and protein.

This macronutrient specificity is the physiological basis for protein-first eating strategies and the satiety advantage of dietary fat. It is not that these foods are inherently more filling in a vague sense – it is that they physically deliver more substrate to the ileum, triggering proportionally more L-cell GLP-1 secretion and a more sustained post-meal satiety signal.

GLP-1 Receptor Agonist Medications: Engineered Persistence

GLP-1 receptor agonist (GLP-1 RA) medications – including semaglutide (Ozempic/Wegovy) and tirzepatide (Mounjaro, which also targets GIP receptors) – replicate the actions of endogenous GLP-1 by binding the same GLP-1 receptor, but with two critical differences: DPP-4 resistance and extended half-life. These structural modifications, achieved through amino acid substitutions and in some formulations fatty acid conjugation enabling albumin binding, prevent the rapid N-terminal cleavage that inactivates endogenous GLP-1 within minutes. The result is a continuous, non-pulsatile GLP-1 receptor signal at concentrations substantially above those produced by meal-stimulated L-cell secretion. thelancet

This distinction – endogenous GLP-1 as a short-lived, meal-matched pulse versus pharmacological GLP-1 RA as a sustained, suprathysiological signal – is mechanistically important. Endogenous GLP-1 signals appetite satiation during and immediately after a meal. GLP-1 RAs suppress appetite continuously, including between meals and regardless of what is consumed, which is why they produce the degree of appetite reduction they do. It also means that when GLP-1 RAs are discontinued, the appetite suppression they provided disappears as the medication clears – returning the body to dependence on its endogenous GLP-1 system, which responds only to appropriate meal composition.

Clinical Pearl The practical question for someone coming off GLP-1 RA medication is not whether their appetite will return – it will – but whether their diet still activates the ileal brake sufficiently to provide meaningful endogenous GLP-1 signalling. A post-Mounjaro diet that remains low in rapidly absorbed carbohydrate and prioritises protein and fat at each meal continues to stimulate L-cell GLP-1 secretion through the same mechanism the medication was mimicking. The medication replaced the signal; the dietary strategy sustains it.

GLP-1 Beyond Appetite: Cardiometabolic and Anti-Inflammatory Effects

GLP-1 receptor expression extends beyond the gut, pancreas, and hypothalamus. GLP-1R is expressed in cardiac tissue, vascular endothelium, kidneys, liver, and immune cells – which explains why GLP-1 RA medications have demonstrated benefits across multiple organ systems beyond glucose control and weight. Cardiovascular outcome trials show significant reductions in major adverse cardiovascular events with semaglutide and tirzepatide, including non-fatal myocardial infarction and stroke, through mechanisms that include reduced inflammation, improved endothelial function, and blood pressure lowering independent of weight loss. thelancet

GLP-1 RAs reduce circulating TNF-α and IL-6, block NF-κB signalling, and lower C-reactive protein – systemic anti-inflammatory effects that operate through GLP-1R on macrophages and other immune cells independently of metabolic changes. This anti-inflammatory capacity is directly relevant to skin: chronic low-grade inflammation driven by metabolic dysfunction and insulin resistance is one of the upstream drivers of accelerated dermal ageing, MMP-mediated collagen degradation, and barrier dysfunction. The reduction in systemic inflammatory burden with GLP-1 RA treatment – or with dietary strategies that sustain natural GLP-1 signalling and reduce post-prandial glucose spikes – works in the same direction as the inflammatory-removal treatments in the aesthetics context. ^[2]

GLP-1’s glucose-lowering and insulin-sensitising effects also reduce the rate of advanced glycation end-product (AGE) formation in dermal collagen. As established in the Glycation entity, persistent hyperglycaemia drives glucosepane cross-link formation in collagen fibrils, impairing mechanical properties and making glycated collagen resistant to normal MMP-mediated remodelling. Stable post-prandial glucose – achieved either through GLP-1 RA medication or through dietary patterns that maximise natural GLP-1 and minimise glucose spikes – reduces this formation rate, even if it does not reverse existing accumulated cross-links.