How Ozempic Works: GLP-1 Receptor Agonism, Blood Glucose Control, and Appetite Regulation

Glucagon-like peptide-1 (GLP-1) is an incretin hormone secreted by L-cells in the small intestine within minutes of eating.

In healthy physiology, GLP-1 performs several functions that regulate blood glucose.

Natural GLP-1 actions:

• Stimulates insulin secretion from pancreatic beta-cells in a glucose-dependent manner. This means insulin release increases only when blood glucose is elevated, reducing hypoglycaemia risk
• Suppresses glucagon secretion from pancreatic alpha-cells, which reduces hepatic glucose output
• Slows gastric emptying, moderating the rate at which glucose from food enters the bloodstream
• Acts on hypothalamic receptors to promote satiety and reduce food intake

The incretin defect in type 2 diabetes:

Patients with type 2 diabetes have a diminished incretin effect.

Studies show that the insulin response to oral glucose (which triggers GLP-1 release) is markedly reduced compared to intravenous glucose (which bypasses the gut).

This incretin defect contributes to postprandial hyperglycaemia.

Limitations of native GLP-1:

Natural GLP-1 has a plasma half-life of approximately 2 minutes because it is rapidly degraded by the enzyme dipeptidyl peptidase-4 (DPP-4). This makes native GLP-1 impractical as a therapeutic agent.

Semaglutide was engineered to resist DPP-4 degradation and bind albumin, extending its half-life to approximately 7 days.

Semaglutide is a synthetic GLP-1 analogue with 94% structural homology to native human GLP-1. Three key molecular modifications give it therapeutic viability.

Structural modifications:

• Amino acid substitution at position 8: alanine is replaced with alpha-aminoisobutyric acid, making the molecule resistant to DPP-4 cleavage
• C-18 fatty diacid chain: attached via a linker at position 26 (lysine). This enables non-covalent binding to serum albumin, which dramatically extends the circulating half-life
• Amino acid substitution at position 34: lysine is replaced with arginine to prevent fatty acid attachment at the wrong site

Pharmacokinetic profile:

• Half-life: approximately 7 days (165 hours), supporting once-weekly administration
• Time to peak plasma concentration (Tmax): 1 to 3 days after subcutaneous injection
• Steady-state plasma levels are reached after 4 to 5 weeks of weekly dosing
• Metabolism: semaglutide is broken down by proteolytic cleavage and beta-oxidation of the fatty acid chain. It is not primarily cleared by the kidneys or liver, which is why no dose adjustment is needed in renal or hepatic impairment
• Bioavailability (subcutaneous): approximately 89%

The long half-life means that after stopping Ozempic, semaglutide remains in the circulation for approximately 5 weeks (5 half-lives to near-complete elimination).

This has practical implications for managing side effects and planning pregnancy.

Semaglutide's primary therapeutic target in type 2 diabetes is the pancreatic islet. It acts on both beta-cells and alpha-cells to restore glucose homeostasis.

Beta-cell effects (insulin secretion):

• Semaglutide binds GLP-1 receptors on beta-cells, activating cyclic AMP signalling pathways that enhance glucose-stimulated insulin secretion
• The glucose-dependent mechanism is clinically important: insulin is released only when blood glucose exceeds approximately 4 to 5 mmol/L. Below this threshold, semaglutide does not stimulate further insulin secretion, which explains the low monotherapy hypoglycaemia risk
• First-phase insulin response (the rapid initial insulin spike after eating) is partially restored. This phase is characteristically impaired in type 2 diabetes
• Beta-cell function, measured by HOMA-B, improves with semaglutide treatment. Animal models suggest a protective effect against beta-cell apoptosis, though this has not been confirmed in humans

Alpha-cell effects (glucagon suppression):

• Semaglutide suppresses inappropriate glucagon secretion during hyperglycaemia. Glucagon normally stimulates hepatic glucose production, so its suppression reduces fasting and postprandial glucose levels
• Glucagon secretion is preserved during hypoglycaemia, maintaining the counter-regulatory response. This is a critical safety advantage over sulfonylureas and insulin

Net glycaemic impact:

In SUSTAIN trials, Ozempic 1 mg reduced HbA1c by 16 to 18 mmol/mol (1.5 to 1.8%) from a typical baseline of 64 mmol/mol (8.0%). Fasting plasma glucose fell by approximately 2.0 mmol/L.

Semaglutide's weight loss effect is primarily driven by its action on the central nervous system, specifically the hypothalamus and brainstem nuclei that regulate appetite and energy balance.

Hypothalamic signalling:

• GLP-1 receptors are expressed in the arcuate nucleus and paraventricular nucleus of the hypothalamus
• Semaglutide activates pro-opiomelanocortin (POMC) neurons, which produce alpha-melanocyte-stimulating hormone, a potent appetite suppressant
• Simultaneously, it inhibits neuropeptide Y (NPY) and agouti-related peptide (AgRP) neurons, which drive hunger
• The net result is reduced hunger, earlier satiety, and decreased interest in food

Brainstem effects:

• GLP-1 receptors in the area postrema and nucleus tractus solitarius modulate nausea and satiety. This overlap explains why nausea is the most common side effect: the appetite-suppressing and nausea-inducing pathways share receptors
• Vagal afferent signalling is also modulated, contributing to the delayed gastric emptying effect

Reward pathway modification:

• Functional MRI studies in patients on semaglutide show reduced activation in the insula and putamen (brain regions associated with food reward) when viewing images of high-calorie foods
• Patients report reduced cravings, particularly for fatty and sugary foods
• Some patients report reduced interest in alcohol, though this is not a licensed indication and controlled trial data are limited

These central effects explain why semaglutide produces more weight loss than purely peripheral GLP-1 actions would predict.

Semaglutide has demonstrated cardiovascular benefit beyond what can be explained by glucose lowering and weight loss alone.

This is one of the key reasons NICE NG 28 positions GLP-1 agonists favourably.

SUSTAIN 6 cardiovascular outcomes trial:

• 3,297 patients with type 2 diabetes at high cardiovascular risk
• Median follow-up: 2.1 years
• Primary endpoint (MACE: cardiovascular death, non-fatal MI, non-fatal stroke): 26% relative risk reduction with semaglutide versus placebo (HR 0.74, 95% CI 0.58 to 0.95)
• Driven primarily by a 39% reduction in non-fatal stroke
• Cardiovascular death was numerically lower but did not reach statistical significance individually

SELECT trial (semaglutide 2.4 mg in obesity without diabetes):

• 17,604 patients with BMI of 27 or greater and established cardiovascular disease but without diabetes
• 20% reduction in MACE events (HR 0.80, 95% CI 0.72 to 0.90)
• This trial established cardiovascular benefit in a non-diabetic population for the first time with a GLP-1 agonist

Proposed mechanisms of cardiovascular protection:

• Anti-inflammatory effects: semaglutide reduces C-reactive protein and other inflammatory markers
• Improvement in lipid profile: modest reductions in LDL cholesterol and triglycerides
• Blood pressure reduction: approximately 2 to 5 mmHg systolic, independent of weight loss
• Direct vascular effects: GLP-1 receptors on endothelial cells may mediate improved endothelial function
• Reduction in visceral adiposity, which is strongly linked to cardiovascular risk

Understanding where Ozempic sits relative to other glucose-lowering therapies helps contextualise its mechanism and clinical role.

Versus metformin:

Metformin primarily reduces hepatic glucose output and improves peripheral insulin sensitivity. It does not act on the incretin system and produces minimal weight loss (1 to 2 kg).

Ozempic and metformin target different pathways and are commonly combined. NICE NG 28 recommends metformin as first-line, with GLP-1 agonists as an option when additional control is needed.

Versus sulfonylureas (gliclazide, glimepiride):

Sulfonylureas stimulate insulin secretion regardless of blood glucose level, producing a glucose-independent effect. This creates a significant hypoglycaemia risk.

Ozempic's glucose-dependent mechanism avoids this problem. Sulfonylureas also promote weight gain.

Versus DPP-4 inhibitors (sitagliptin, linagliptin):

DPP-4 inhibitors prevent the breakdown of endogenous GLP-1, raising its plasma levels modestly (2 to 3 fold).

Ozempic achieves much higher GLP-1 receptor activation because it is administered at pharmacological doses. This explains the superior HbA1c reduction and weight loss with GLP-1 agonists.

Versus SGLT2 inhibitors (dapagliflozin, empagliflozin):

SGLT2 inhibitors block glucose reabsorption in the kidney, causing glycosuria. They offer complementary benefits: heart failure and renal protection.

NICE supports combining GLP-1 agonists with SGLT2 inhibitors in patients with cardiovascular or renal disease.

Versus insulin:

Insulin is the most potent glucose-lowering therapy but causes weight gain and hypoglycaemia. Ozempic can delay or reduce insulin requirements.

SUSTAIN 5 demonstrated that adding semaglutide to basal insulin reduced HbA1c by 13 mmol/mol and body weight by 3.7 kg.