The hormone it copies
Your gut makes glucagon-like peptide-1 naturally. Cells in the lower small intestine and colon release it after you eat, and it is part of how your body registers that food has arrived.
Native GLP-1 is almost useless as a drug because it lasts one to two minutes before an enzyme called DPP-4 breaks it down. Semaglutide is that peptide re-engineered: modified to resist DPP-4, and given a fatty acid chain that binds it to albumin in the blood so the kidneys do not clear it quickly. The result is a half-life of about a week, which is the entire reason it can be injected weekly rather than continuously infused.
What happens at the receptor
Binding the GLP-1 receptor does four things at once, and they are worth separating because only one of them is really about weight.
- It prompts insulin release, but only when blood glucose is already high. This is what glucose-dependent means, and it is why semaglutide on its own rarely causes hypoglycaemia.
- It suppresses glucagon, the hormone that tells the liver to release stored glucose.
- It slows how fast the stomach empties, so food sits longer and you feel physically full for longer after a smaller amount.
- It acts on appetite regulation in the brain, in the hypothalamus and in hindbrain regions that sit outside the blood-brain barrier and can be reached by circulating peptide directly.
The brain effect is the one that matters
The gastric effect is real but it is not where most of the weight change comes from. The larger part is central. In controlled feeding studies, people given semaglutide ate substantially less when offered food freely, and rated their hunger lower and their fullness higher across the day.
The effect people describe most often is not feeling stuffed. It is that food stops occupying their attention. The reduction in what gets called food noise is the most consistently reported subjective change and the hardest thing to capture in a trial endpoint.
Why the nausea fades but the appetite effect does not
Both the satiety and much of the nausea come from the same slowed gastric emptying, which is why they arrive together. They do not leave together.
The gastric emptying effect attenuates with continued exposure. The appetite signalling does not, or does so much less. That divergence is the whole logic of titration: you are giving your stomach time to adapt to a drug whose useful effect will persist after the unpleasant one has faded.
It is also why a dose that made you miserable in week one is often unremarkable by week six, and why climbing faster than four-week intervals tends to produce a worse experience without producing faster results.
What it does not do
Semaglutide does not burn fat, raise your metabolic rate, or block absorption. Resting energy expenditure actually falls as you lose weight, exactly as it does with any other method of losing weight.
The entire mechanism is reduced energy intake. That is not a criticism. It is just worth knowing, because it explains why the drug works alongside food rather than around it, and why what you eat still matters while you are on it.
Where the lost weight comes from
It comes from fat and from lean tissue, in a ratio broadly similar to other forms of weight loss. Body composition substudies show a meaningful share of the total is lean mass.
This is the strongest argument for resistance training and adequate protein alongside treatment, and it is the part of the picture that gets least attention in the marketing.
Sources
- Drucker DJ. Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1. Cell Metab. 2018;27(4):740-756.
- Friedrichsen M, et al. The effect of semaglutide 2.4 mg once weekly on energy intake, appetite, control of eating, and gastric emptying in adults with obesity. Diabetes Obes Metab. 2021;23(3):754-762.
- Blundell J, et al. Effects of once-weekly semaglutide on appetite, energy intake, control of eating, food preference and body weight in subjects with obesity. Diabetes Obes Metab. 2017;19(9):1242-1251.
- Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989-1002.
- Wegovy (semaglutide) injection, prescribing information. Novo Nordisk.