What is GLP-1? How it works for weight loss

You've probably heard of Ozempic, Wegovy or Mounjaro. But beyond the headlines, what's actually happening in your body when you take these medications? It all starts with GLP-1, a hormone your body already produces naturally. So, what does it do and how can medications that mimic its effects help with weight management?

From how GLP-1 affects your appetite to its role in weight loss, here's what you need to know about the science behind these medications and how to support your health while taking them.

Your body actually already makes GLP-1. Short for glucagon-like peptide-1, GLP-1 is a hormone released by your gut after you eat. It sends signals to your gut, brain and pancreas, helping regulate your appetite and blood sugar levels.¹ GLP-1 medications, also known as GLP-1 receptor agonists, are designed to mimic the effects of this natural hormone.

You might have heard them called 'weight loss injections', but not all GLP-1 medications are prescribed for weight loss; some are used to manage type 2 diabetes. Most are given as injections, although some are also available as tablets.² Plus, there's another important difference: while the GLP-1 your body naturally produces only lasts a few minutes, GLP-1 medications are designed to work for much longer – sometimes for days.³

GLP-1 might be best known for its role in appetite, but it does more than help you feel full. One of its main jobs is helping to regulate your blood sugar levels. It does this by encouraging your pancreas to release insulin when your blood sugar rises, while reducing the release of another hormone called glucagon.

These two hormones work together to help keep your blood sugar levels balanced. Insulin helps lower blood sugar, while glucagon helps raise it when needed.^4,5 GLP-1 also slows how quickly food leaves your stomach and sends signals to your brain that help you feel satisfied after eating. Together, these effects can help you feel fuller for longer and reduce feelings of hunger.⁴

GLP-1 medications work by mimicking the effects of the GLP-1 hormone your body naturally produces. By activating GLP-1 receptors, they help your body respond in similar ways to its own GLP-1, but their effects last much longer.² Depending on the medication, they're prescribed to help manage type 2 diabetes, support weight loss, or both. For people with type 2 diabetes, GLP-1 receptor agonists can help lower blood sugar by increasing insulin release when needed, reducing glucagon and slowing how quickly food leaves the stomach.⁶

When used for weight management, they also act on areas of the brain involved in appetite and fullness. This can help reduce hunger, cravings and what's sometimes called 'food noise' – those persistent thoughts about food, eating or what you're going to eat next. As a result, some people find they eat less and lose weight.^6,7 It's worth knowing that not all medications work in exactly the same way. For example, Mounjaro (tirzepatide) is known as a 'dual agonist' because it acts on both GLP-1 and GIP receptors, rather than GLP-1 receptors alone.

Researchers are also exploring medications that act on three receptors. One example is retatrutide, a 'triple agonist' that targets GLP-1, GIP and glucagon receptors. In a 2023 clinical trial, people taking the highest dose lost an average of 24.2% of their body weight over 48 weeks.⁸ However, retatrutide remains an investigational treatment and isn't currently approved for use in the UK.

So, how do weight loss injections work in practice? Rather than directly burning fat, GLP-1 medications help regulate appetite and increase feelings of fullness. This can make it easier to eat less, which may lead to weight loss over time.^6,7 But GLP-1 for weight loss isn't intended to work in isolation. A healthy, balanced diet, regular movement and ongoing support from your prescribing team are all important parts of weight management. And when your appetite is lower, making sure you're getting enough nutrients becomes particularly important. Even if you're eating smaller portions, your body still needs protein, fibre, vitamins, minerals and healthy fats.²

Gradual weight loss can also help you focus on maintaining your nutritional intake and muscle strength along the way. Visible changes in weight may take weeks or months, as everyone's experience is different.⁹ If you're losing weight more quickly than expected or finding it difficult to eat enough, speak to your prescribing team for advice.¹⁰ Weight loss isn't guaranteed, and results vary from person to person.¹¹ However, research has also found improvements in quality of life and mobility among some people living with obesity who take GLP-1 medications.¹¹

GLP-1 medications are mainly prescribed for people living with type 2 diabetes or, where the relevant eligibility criteria are met, people living with obesity or are overweight alongside weight-related health conditions. In the UK, different medications have different approved uses and prescribing criteria. They're generally recommended alongside changes to diet and physical activity, rather than as a replacement for them.² Research is also exploring other potential benefits of GLP-1 medications. Some studies have found a reduced risk of cardiovascular events, such as heart attack and stroke, in certain groups of people. However, these benefits depend on the medication and the person taking it.^6,13

GLP-1 medications are prescription-only, so a healthcare professional will need to assess whether they're suitable for you. Your prescribing team can also help you manage side effects, support your nutritional needs and plan for longer-term weight management, including what happens if you stop treatment.²

Like any medication, GLP-1s can cause side effects, although not everyone experiences them. Because these medications affect appetite and digestion, some of the most common side effects are gut-related. These can include:

• feeling sick or nauseous

• vomiting

• diarrhoea

• constipation

• stomach discomfort

Other possible side effects include reduced appetite, tiredness, hair loss and irritation around the injection site.² Less common but potentially serious side effects can include inflammation of the pancreas (pancreatitis), severe allergic reactions and certain eye problems. Low blood sugar can also occur, particularly when GLP-1 medications are taken alongside some diabetes treatments.² Severe, persistent stomach pain, especially if it spreads to your back, needs urgent medical assessment.

Another consideration is muscle loss. Some research suggests that people taking GLP-1 medications may lose muscle as well as body fat during weight loss.¹⁴ That's why getting enough protein and including strength-based exercise in your routine can be particularly important. If you experience side effects, speak to your prescribing team for advice. They can help you decide whether your treatment needs reviewing.

Taking a GLP-1 medication is just one part of looking after your health. If you're eating less because your appetite has changed, it's especially important to make the food you do eat count. There's no single 'GLP-1 diet' you need to follow. Instead, focus on building everyday habits that support your nutritional needs, movement and overall wellbeing.²

1. Make the most of your meals

When you're feeling fuller more quickly, getting enough nutrients can take a little more planning. Aim for a balanced diet that includes protein, fibre, healthy fats and a variety of vitamins and minerals. Protein is particularly important for maintaining muscle during weight loss, so include foods like eggs, Greek yoghurt, beans and lentils. Not sure how much to aim for? Find out how much protein you really need.

Fibre matters too. From oats and wholegrains to fruit, vegetables and pulses, there are plenty of everyday foods high in fibre to choose from. And while you might have heard of fibremaxxing, there's no need to dramatically increase your intake overnight. Small, gradual changes are a good place to start, and if you're finding it difficult to eat enough, speak to your prescribing team or a registered dietitian for support.¹¹

2. Keep hydrated

Staying hydrated is important for everyday health, especially if you're experiencing digestive side effects. Keep water within easy reach and sip regularly throughout the day. Not sure if you're drinking enough? Find out how much water you should drink.

3. Make movement part of your routine

Regular movement supports your overall health, while strength training can help maintain muscle during weight loss. And you don't need to spend hours in the gym. Simple exercises like squats, push-ups or lifting weights can all help, especially when combined with aerobic activities you enjoy, whether that's walking, cycling or swimming.

4. Don't overlook sleep and self-care

Eating well and staying active are important, but so is giving your body time to rest. Prioritising a good night's sleep and making time for self-care can help support your overall wellbeing.

5. Ask for support when you need it

Finding it difficult to eat enough or meet your nutritional needs? Your prescribing team can help you work out what's right for you. Supplements may be useful in some circumstances, but they're no substitute for a balanced diet, so always check with a healthcare professional before taking them alongside prescribed medication.

GLP-1 might be making headlines, but it's also a hormone that's been working behind the scenes in your body all along. And remember medication is only part of the picture – eating a balanced diet, staying active and looking after your overall wellbeing still matter.

Looking for practical ways to get started? Explore our GLP-1 friendly food shop for easy additions to your shopping basket, or discover how to support your body while taking a GLP-1.

Disclaimer – This article is intended to support general wellness and weight management only and does not provide advice for the use of GLP-1 medications. If you are prescribed a GLP-1 medicine, your dietary intake and overall health should be monitored by your prescriber or another qualified healthcare professional.

Any symptoms or side effects, including changes to appetite, digestion or bowel habits, should be discussed with your prescriber or a healthcare professional, as these may indicate that your current treatment or dosage needs review. If you're finding it difficult to eat regular, balanced meals, you should also tell your prescriber.

Always check with a prescriber or healthcare professional before introducing new foods, supplements or products alongside medication, as there may be interactions or contraindications.