28th May 2024

Fact vs Fiction: what experts say about weight loss medications

Whether you’re considering GLP-1 weight loss drugs, already using them or are simply curious as to what the interest is in them, one thing is for sure: there is a myriad of information and it can be tricky to know what to trust. At The Fast 800, we have been exploring the science behind these weight loss medications and asked our panel of medical and health experts to provide some insights into this new wave of drugs taking the world by storm.

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The Fast 800 Experts

Dr Michael Mosley

Dr Michael Mosley, co-founder of The Fast 800 and a doctor, author and presenter who has spent more than a decade researching in the area of type 2 diabetes, rapid weight loss and metabolic health

Dr Patrick Garratt

Dr Patrick Garratt, a general practitioner with a special interest in diabetes and weight loss, in practice for more than 20 years

Dr Tanya Smith

Dr Tanya Smith, a general practitioner with special interest in weight management, in practice for more than 35 years

Professor Roy Taylor

Professor Roy Taylor, The Fast 800 Scientific Advisor and a professor of medicine who has completed groundbreaking research in diabetes, rapid weight loss and metabolic health


Gabi - The Fast 800 Nutritionist & Recipe Developer, a degree-qualified nutritionist with particular interest in weight loss and metabolic and hormonal health


Pascal - The Fast 800 Exercise Manager, a personal trainer and exercise scientist with particular interest in helping people improve mobility, fitness and strength while losing weight

What are GLP-1 weight loss medications?

“The phrase ‘weight loss medications’ these days is most commonly referring to GLP-1 agonists. These include liraglutide (Saxenda), semaglutide (Wegovy, Ozempic) and tirzepatide (Mounjaro). They were all originally developed to treat diabetes, and some have been approved for weight loss as the main indication, depending on where you live. They are suitable for patients with obesity (a BMI of 30, though lower thresholds apply in some ethnic groups), and particularly useful for patients with a lot of weight to lose.” – Dr Tanya

“These drugs were primarily intended for people with type 2 diabetes and we know that people with type 2 diabetes benefit from significant weight loss.” – Michael Mosley

“They are currently injectable medications. That means that people have to inject themselves either once a day or once a week. The needles are very small, pretty easy to use and it’s one of those things that patients discuss with their doctor or chemist prior to using.” – Dr Patrick

Do GLP-1 weight loss medications work, and if so, how?

“They work by reducing appetite and delaying stomach emptying therefore giving a feeling of fullness after eating. The studies seem to suggest that patients can lose around 15-20% of their body weight in about a year which is significant and far outstrips any other weight loss medication. However it is likely that they need to be combined with long-term dietary and lifestyle changes to maintain this loss.” – Dr Tanya

“The way these new drugs work is primarily on the brain, and that makes it different to many of the other drugs. You also get some local effects, because they also have an impact on the gut and the stomach as well. But it seems to be the brain effect that makes these drugs so effective.” – Michael Mosley

Are GLP-1 weight loss medications safe?

“The reason that these medications are more widely available is because there have been large, randomised controlled trials showing they’re effective.” – Michael Mosley

“They have been around for some years and their safety issues have been explored. It was found that patients with diabetes were losing weight as an incidental side effect when prescribed these medicines. Some medical studies confirmed this side effect and they have therefore been marketed as weight loss medication. The new weight loss specific products now appear to be effective and safe with prescription safeguarding, and lifestyle changes.” – Dr Tanya

What are the side effects of these medications?

“Well established side effects of GLP-1 agonists include nausea, vomiting, diarrhoea, constipation and fatigue. Over time, secondary side effects could include things like muscle loss and nutrient deficiency related issues such as hair loss. By choosing to consume a well-balanced, nutrient dense diet such as a Mediterranean-style diet, some of these side effects, particularly the secondary effects, can be minimised.” – Gabi

“Another side effect that I do see is that people sometimes describe a bit of fatigue, tiredness, a sort of cloudiness, particularly when they start the medications and particularly when they’re increasing their dose.” – Dr Patrick

“Nausea and appetite suppression can make it difficult to reach your calorie target for the day, which can worsen side effects if you can’t get adequate protein or fibre in. Making meals smaller can help, and using shakes can be a way to get some nutrition in when appetite is poor. Severe recorded side effects include pancreatitis, gallstones, thyroid cancer, low blood sugar in diabetics, deterioration in vision in diabetics, kidney failure, increased heart rate, serious allergic reactions, depression and suicidal thoughts. Aside from consulting your doctor about these medications, it is also important to obtain your medicine from a reliable source as there have been counterfeit products to purchase online.” – Dr Tanya

“What you really want to do, obviously, is lose fat without losing muscle and unfortunately that doesn’t seem to be the case with these. Particularly as you get older, it’s really important to preserve your muscle mass. One of the interesting side effects of the drugs is they also seem to help curb cravings and indeed there is research currently going on showing they might be effective in dealing with addictions, like alcohol and also drugs.” – Michael Mosley

“While exercise is an important part of any healthy lifestyle, it has the potential to play a particularly crucial role in minimising one of the least desirable side effects of all GLP-1 weight loss medications – muscle loss. During any period of rapid weight loss, it’s common for a lot of the weight lost to be muscle. This can have significant health impacts particularly as you age, where mobility, fall prevention and strength are increasingly important.” – Pascal

Are GLP-1 weight loss medications miracle drugs?

“I’ve heard many people and the media talk about these new medications as wonder drugs. I don’t think there’s any drug that I’ve come across that is actually a wonder drug that is revolutionary. What I see with patients is sometimes they expect a little bit too much from the medications.” – Dr Patrick

“While they can help you achieve weight loss, with portion sizes reduced, so is your intake, which can often result in nutrient deficiencies.” – Gabi

“Historically, I have been very sceptical about weight loss potions and pills because, frankly, they come with a long history of quackery and most of them have either been ineffective, or they have come with some really nasty side effects. What makes these drugs so different is they have been shown in randomised controlled trials to be both safe and effective. While I think these drugs are remarkable, I certainly wouldn’t describe them as miracles because they do come with significant side effects and there are drawbacks. If they were miracle drugs, you would be preserving 100% of your muscle and they would basically help you lose weight, then you’d stop taking the drug, and you’d just keep the weight off. That’s not what these drugs do.” – Michael Mosley

Will GLP-1 weight loss medications help me to lose weight for good?

“When somebody stops taking these medications, unfortunately the effects also wear off instantly. That means that for the treatment of diabetes, blood sugars can rise once again. In terms of weight loss management, it does mean that the weight can go back on.” – Dr Patrick

“Exercise isn’t just necessary for maximising weight loss, it is a key factor in building and maintaining a healthy body. Incorporating an exercise routine, like that on The Fast 800 Programme, into your weekly routines is very important for glucose control, your metabolism and maintaining muscle mass.” – Pascal

“One of the significant downsides of using GLP-1 drugs is that you lose muscle when you lose weight, but when you regain the weight, a lot of it is fat. So you may end up in a slightly worse position than you were when you started. What they will do is suppress your appetite, but what they will not do is teach you healthy ways of eating, or indeed introduce you to something like an exercise regime or stress management, which is why it is so important to have a long-term plan – The Fast 800 absolutely does that.” – Michael Mosley

“Recognise the portion size and frequency of your eating whilst taking the drugs, which is allowing the weight loss and avoidance of weight regain. Then try to replicate this when the drugs are stopped.” – Prof Taylor

What advice would you give someone considering using GLP-1 drugs?

“Weight control is a long-term game and if no action is taken, weight regain is certain. Ask yourself ‘do I really want to rely on drugs or could I manage the supportive dietary route shown to be successful by so many people? Can I cope with the likely nausea for the early days or weeks, and the possibility of vomiting?’ and make a long-term plan for when the drug will be stopped. A drug-free life is generally better, no drug comes without side effects, and the whole family will benefit by seeing the outcomes of the eating pattern for a healthy life.” – Prof Taylor

“It is very important that this medicine is not used in isolation and new habits with diet and exercise must be formed while taking the medicines. Don’t just use the medicines to starve yourself and lose weight as quickly as possible. Use this opportunity with a smaller appetite to reset your whole approach to your diet so that it is not difficult to continue with it long term.” – Dr Tanya

“As a GP, I have limited time with my patients, therefore I’m unable to go into detail about their nutrition, how they can maintain their appropriate amount of protein and vitamins, alongside exercise. That is why I’ve found The Fast 800 has been very useful to both me and my patients in helping them with these aspects of their new regime.” – Dr Patrick

“For anyone using these medications my first recommendation is to go Mediterranean. There are countless studies on the benefits of a Med-style diet for cardiovascular health, weight loss and more, so this is where I recommend starting – think adequate protein, a variety of seasonal vegetables providing fibre, healthy fats, and small amounts of complex carbs. Setting up good habits around food and eating a Mediterranean-style diet can foster a healthier relationship with food when weaning off these medications to help maintain weight loss.” – Gabi

“While many people on GLP-1 medications may not experience significant limitations in their ability to exercise, unfortunately, this does not apply to everyone. Many people at the beginning of a weight loss journey are also re-introducing themselves to exercise, and even seasoned exercisers may need to take things down a notch when hit with common side effects of weight loss medications, including fatigue and nausea. Start with low-to-moderate intensity exercise and gradually increase the intensity and duration over time, prioritising resistance and strength building exercises. It is most important that you prioritise consistency across the week, with a schedule that you can maintain and build on over the long-term.” – Pascal

“I don’t believe these drugs are sustainable, in the sense that once you stop taking them, then the weight will come back on. One of the advantages of combining the GLP-1s with The Fast 800 programme is that you get the benefits of changing how you eat, but also reducing stress and working on your exercise. If you do that, then you get multiple benefits, ranging from improvements in your cholesterol, blood pressure and blood sugars to things like mood and ‘get up and go’ energy. All the things that we commonly hear from people who have done the programme.” – Michael Mosley

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