What Is GLP-1? The Gut Hormone the Weight-Loss Medicines Copy

GLP-1 is a hormone your gut makes every time you eat. Cells in the lining of the small intestine and colon release it when food arrives, and it does three things: tells the pancreas to release insulin in proportion to the sugar coming in, slows the stomach, and signals to the brain that a meal is under way.^{1, 2} It lasts about two minutes in the blood before an enzyme breaks it down, which is one reason fullness fades a few hours after eating.³ The medicines prescribed in the UK for weight management and type 2 diabetes, tirzepatide (Mounjaro), semaglutide (Wegovy, Ozempic) and liraglutide (Saxenda), are engineered copies of this hormone, built to resist that enzyme and hold the signal on for a day or a week rather than minutes.⁴ One hormone, three jobs, and a class of medicines that copies it at a level the body never produces on its own.

• What it is: glucagon-like peptide-1, a peptide hormone of 30 amino acids cut from the same precursor as glucagon
• Where it comes from: L cells in the lining of the small intestine and colon, released by protein, fat, carbohydrate and fermented fibre
• Food against medicine: a meal roughly doubles it for an hour; the medicines hold the receptor active all week

What does GLP-1 stand for, and is it a peptide?

GLP-1 stands for glucagon-like peptide-1. It is a peptide: a short chain of amino acids, 30 of them in the main active form, which makes it a small protein rather than a steroid or a fat-based hormone.¹ The "glucagon-like" part is a family name. Your body makes one long precursor protein, proglucagon, and cuts it differently in different tissues. In the pancreas it becomes glucagon, the hormone that raises blood sugar between meals; in the gut the same precursor is cut into GLP-1 and GLP-2.¹ So GLP-1 resembles glucagon in its sequence and does close to the opposite job once a meal arrives. When people say "GLP-1" about a medicine, they mean a GLP-1 receptor agonist: a molecule shaped to fit the hormone's receptor and switch it on.

Where is GLP-1 made, and why does food release it?

By L cells, specialised hormone-producing cells scattered along the lining of the small intestine and colon, most densely towards the far end.^{1, 5} They face into the gut and carry sensors for amino acids, fatty acids and glucose, and release GLP-1 into the blood in proportion to what they sense. Protein and fat are the strongest triggers per calorie, refined carbohydrate the weakest. Fibre works later and lower down: bacteria in the colon ferment it into short-chain fatty acids, which the same cells detect, so a high-fibre day produces a longer, gentler release over hours.⁵ A little is also made by neurons in the brainstem.¹ It was identified as a human incretin, a gut hormone that boosts insulin after a meal, in 1987.⁶

What does GLP-1 do in the body?

Three things, all of them about managing a meal. It tells the pancreas to release insulin in proportion to the sugar arriving, and only while sugar is arriving, which is why GLP-1 on its own rarely causes low blood sugar.¹ It slows the stomach's emptying so food reaches the intestine at a rate the body can handle, which is partly why a meal keeps you full. And it acts on the appetite centres of the brain: when researchers infused GLP-1 into healthy volunteers around mealtimes, they reported more fullness and ate about 12% less at lunch.⁷ Then it is gone. The enzyme DPP-4 breaks it down within minutes of release, and a few hours after eating the signal has faded and hunger returns.³ That short life is the problem the medicines were designed around.

Which UK medicines copy GLP-1?

Three. Semaglutide (sold as Wegovy for weight management and as Ozempic for type 2 diabetes) and liraglutide (Saxenda) are modified versions of the hormone that resist DPP-4 and bind to proteins in the blood, so one injection keeps the receptor active for a day with liraglutide or a week with semaglutide.⁴ Tirzepatide (Mounjaro) is a single molecule that activates the GLP-1 receptor and the receptor for a second gut hormone, GIP, also for a week at a time.⁴ At that level and duration the three effects above run continuously: in a trial that measured it, people on semaglutide ate about a quarter to a third less at test meals without any dietary instruction, and in STEP 1 they reduced body weight by 14.9% over 68 weeks against 2.4% on placebo.^{8, 9} All three are prescription medicines; whether one suits you is your prescriber's decision, and the NHS pages for tirzepatide, semaglutide and liraglutide cover the rest.^{10, 11, 12} How Mounjaro works takes the tirzepatide mechanism further, and GLP-1 side effects covers what a continuous signal does to the stomach.

Is GLP-1 the same as Ozempic?

No. GLP-1 is the hormone; Ozempic is a brand of semaglutide, a medicine that copies it. The class is called "GLP-1s" in conversation and "GLP-1 receptor agonists" on paper, which is where the mix-up starts. It matters in both directions. Everyone makes GLP-1, with or without a prescription, so "taking GLP-1" is not something a food or a capsule can offer you. And the medicine is not the hormone: it holds the receptor active at a level and for a duration no meal produces, which is why its effects on appetite and the stomach are stronger and longer than any meal's.

What is the difference between GLP-1 and GLP-2?

Same precursor, same cells, released together, different jobs. GLP-2 acts on the gut lining itself: it increases blood flow to the intestine, helps the lining grow and repair, and improves nutrient absorption.¹³ It has little effect on appetite or insulin, and the one GLP-2 medicine in use is for people with short bowel syndrome, not for weight. When a forum post says "GLP-2" about a weight-loss injection, it nearly always means the GIP part of tirzepatide, a different hormone again, released higher up the small intestine.⁴

Can you increase GLP-1 naturally?

Yes, within limits worth stating plainly. Protein at a meal raises it most reliably; soluble fibre from oats, beans and lentils raises it over hours through fermentation; unsaturated fats from olive oil, nuts and avocado help; and eating the vegetables and protein before the carbohydrate on the same plate produced a higher GLP-1 response to the same food in a controlled trial.^{14, 15} In a four-day feeding study, a high-protein diet raised GLP-1 after dinner and 24-hour fullness compared with a normal-protein one.¹⁴ The limit is size and duration. A meal roughly doubles GLP-1 for an hour or so and then DPP-4 clears it; the medicines hold the receptor active at pharmacological levels for a week. Food makes each meal more filling. It does not do what the injection does, and nothing sold as a GLP-1 booster has shown otherwise in a trial. Which foods increase GLP-1 has the UK food list with the studies behind it, do GLP-1 supplements work goes through the ingredients, and what is the GLP-1 diet turns it into meals. On the medicine, these foods matter because a small appetite still has to carry the same protein and fibre, the subject of our guide to living well on Mounjaro. After the medicine, your own GLP-1 is what you have, and what happens when you stop explains why that matters.

Frequently asked questions

Is GLP-1 bad for you?

The hormone is not: you have made it after every meal of your life. The question usually means the medicines, whose common side effects are the hormone's jobs overdone, a slowed stomach above all. They are set out with the trial figures and the MHRA's safety signposts on GLP-1 side effects: what is common and what helps.

Which GLP-1 medicines are available for weight loss in the UK?

Semaglutide (Wegovy), liraglutide (Saxenda) and tirzepatide (Mounjaro) are licensed for weight management in the UK; Ozempic is semaglutide licensed for type 2 diabetes. Who may be prescribed them is for the NHS and your prescriber.

How does GLP-1 work?

It is released from the gut lining when food arrives and acts on receptors in the pancreas, the stomach and the brain: insulin goes up while sugar is arriving, the stomach slows, and you feel full.^{1, 7}

How long does GLP-1 last in the body?

Your own: about two minutes in the blood before DPP-4 breaks it down, with a meal's release tailing off over a few hours.³ The medicines last a day (liraglutide) or a week (semaglutide, tirzepatide), because they were built to.

Does GLP-1 burn fat?

No. It reduces how much you want to eat and steadies blood sugar; any fat loss follows from eating less. The same is true of the medicines, which is why protein and resistance training matter on them. What supplements to take on a GLP-1 medicine covers that side.

Sources

1. Holst JJ. The physiology of glucagon-like peptide 1. Physiological Reviews, 2007. PubMed 17928588.
2. Müller TD, et al. Glucagon-like peptide 1 (GLP-1). Molecular Metabolism, 2019. PubMed 31767182.
3. Deacon CF, et al. Both subcutaneously and intravenously administered glucagon-like peptide I are rapidly degraded from the NH2-terminus in type II diabetic patients and in healthy subjects. Diabetes, 1995. PubMed 7657039.
4. Drucker DJ. Mechanisms of action and therapeutic application of glucagon-like peptide-1. Cell Metabolism, 2018. PubMed 29617641.
5. Gribble FM, Reimann F. Function and mechanisms of enteroendocrine cells and gut hormones in metabolism. Nature Reviews Endocrinology, 2019. PubMed 30760847.
6. Kreymann B, et al. Glucagon-like peptide-1 7-36: a physiological incretin in man. Lancet, 1987. PubMed 2890903.
7. Flint A, et al. Glucagon-like peptide 1 promotes satiety and suppresses energy intake in humans. Journal of Clinical Investigation, 1998. PubMed 9449682.
8. 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, Obesity and Metabolism, 2017. PubMed 28266779.
9. Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). New England Journal of Medicine, 2021. PubMed 33567185.
10. NHS. Tirzepatide. nhs.uk/medicines/tirzepatide.
11. NHS. Semaglutide. nhs.uk/medicines/semaglutide.
12. NHS. Liraglutide. nhs.uk/medicines/liraglutide.
13. Drucker DJ, Yusta B. Physiology and pharmacology of the enteroendocrine hormone glucagon-like peptide-2. Annual Review of Physiology, 2014. PubMed 24161075.
14. Lejeune MP, et al. Ghrelin and glucagon-like peptide 1 concentrations, 24-h satiety, and energy and substrate metabolism during a high-protein diet. American Journal of Clinical Nutrition, 2006. PubMed 16400055.
15. Shukla AP, et al. Carbohydrate-last meal pattern lowers postprandial glucose and insulin excursions in type 2 diabetes. BMJ Open Diabetes Research and Care, 2017. PubMed 28989726.

This article is general information, not medical advice. Tirzepatide, semaglutide and liraglutide are prescription medicines: whether one is suitable for you is a decision for your prescriber, and nothing here replaces their advice.