How GLP-1 Receptor Agonists Work
GLP-1 drugs work in two places simultaneously: in the brain (reducing appetite and food cravings) and in the gut (slowing digestion). Together, these effects make it significantly easier to eat less.
Quick read · 4 min
- •GLP-1 is a natural hormone your gut releases after eating — these drugs mimic it
- •In the brain: appetite and food cravings are reduced
- •In the gut: food moves through the stomach more slowly, so you feel full for longer
- •GI side effects (nausea, diarrhoea) come from the same gut-slowing effect — they usually improve with time
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Last reviewed: March 2026
What GLP-1 is naturally
GLP-1 (glucagon-like peptide-1) is a hormone naturally produced in the gut — specifically by L-cells in the small intestine — after you eat. Its job is to signal to the body that food has arrived.
Naturally, GLP-1 lasts only 1–2 minutes before being broken down by an enzyme called DPP-4. GLP-1 receptor agonist drugs mimic this hormone but are engineered to resist DPP-4 breakdown — lasting for hours (liraglutide) or a full week (semaglutide).
How it works in the brain
Plain English:
GLP-1 drugs reach appetite-control centres in the brain and turn down the hunger signal — food becomes less appealing, not just less needed. Food still tastes the same, but the urgency around eating is significantly reduced.
GLP-1 receptors are found throughout the hypothalamus (the brain's primary hunger control centre), as well as in the amygdala, insula, and areas involved in food reward. When activated:
- •Appetite signals in the arcuate nucleus of the hypothalamus are suppressed
- •Ghrelin (the hunger hormone) is reduced
- •Fullness hormones (peptide YY, cholecystokinin) are enhanced
- •The reward value of food — the urgency and craving — is reduced via the mesolimbic system
- •Leptin sensitivity improves — the brain becomes more responsive to existing fullness signals
How it works in the gut
Plain English:
Food stays in the stomach longer — so you feel full from smaller portions, and feel full for longer after eating. This slowing of digestion reinforces the appetite reduction happening in the brain.
GLP-1 receptors in the gut and on the vagus nerve (the gut-brain highway) produce several peripheral effects:
- •Gastric emptying is slowed — food moves out of the stomach more slowly, prolonging satiety
- •The vagus nerve relays fullness signals from the stomach to the brainstem
- •Insulin secretion is stimulated (in a glucose-dependent manner, reducing hypoglycaemia risk)
- •Glucagon is suppressed — this reduces blood glucose between meals
Why GI side effects occur
Nausea, vomiting, and diarrhoea are the most common side effects of GLP-1 drugs — and they are directly caused by the same gut-slowing mechanism that makes the drug effective. Slowing gastric emptying can cause nausea, particularly when the dose is increased. This is why dose escalation schedules are gradual: starting low and increasing slowly gives the gut time to adapt and minimises the severity of GI side effects.