GLP-1 agonists (like Ozempic, Mounjaro, Wegovy etc.) have become a big topic in fitness, weight loss, and health. But for fitness professionals, understanding what they are, how they work, who uses them, and what the effects are, both positive and negative, is essential.
This blog explores: what GLP-1 Agonists do, national use statistics, categories of people who take them, what studies say about weight, muscle gain/loss, and what that means for training, without pushing one side over the other. Here are seven things Fitness Professionals need to know about GLP-1 drugs.
1. What GLP-1 Agonists Are & How They Work
GLP-1 (glucagon-like peptide-1) agonists are medications that mimic a hormone in the gut which helps regulate appetite, slow gastric emptying (making you feel full for longer), and improve insulin secretion.
They were originally developed for type 2 diabetes but are now also prescribed (or used off-label) for obesity and weight management in many countries. It’s essential to recognise that their impact is just as much about lifestyle and diet as it is about the medication itself.
2. National and Population Usage of GLP-1 Drugs & Interest
As of early 2025, approximately 1.6 million adults in Great Britain, about 2.9% of the adult population, reported using a GLP-1 receptor agonist (GLP-1RA) to support weight loss in the past year.
An additional 1.7% (~910,000 adults) used them exclusively for this purpose.
The majority (91.4%) of these users were on medications licensed for weight management, with Mounjaro (tirzepatide) being the most commonly used.
Notably, a substantial minority reported using a GLP-1RA not licensed for weight management, indicating off-label use.
Many more express interest: 6.5% (≈ 3.3 million adults) said they might start in the next year.
3. Who Uses GLP-1 Weight Loss Treatments & Why
Some of the main reasons people take GLP-1s include:
- For medical necessity (type 2 diabetes)
- To manage obesity or reduce excess body fat
- To reduce cardiovascular risk (e.g. approved drug indications)
- Sometimes for aesthetic or weight-loss motivation, though that often depends on access, cost, side effects etc.
Each of these has different implications for training, diet, expectations, and safety.
4. What Studies Show: Effectiveness & Body Composition
In randomised controlled trials (RCTs) of overweight/obese people (with or without diabetes), GLP-1 receptor agonists produce an average weight loss of around −4.57 kg compared with placebo, with decreases in BMI (~−2.07 kg/m²) and waist circumference (~−4.55 cm).
The −4.57 kg figure is a pooled result from a meta-analysis (47 randomised controlled trials), not the outcome of a single RCT.
The trials included ran for different lengths, but related systematic reviews report treatment durations ranging from 16 to 104 weeks, with a median of around 43 weeks.
In other words, the −4.57 kg is the average effect across many trials of varying lengths (most commonly several months to ~1 year), and the meta-analysis also found that longer treatment duration tended to produce larger weight losses.
On the question of muscle vs fat loss: recent genetic evidence suggests GLP-1 use tends to reduce fat mass more than muscle mass, although some lean mass loss is observed.
Clinical trials have raised concerns that weight loss from GLP-1 drugs isn’t just fat. Up to 40% of the loss may come from lean mass, including muscle, particularly in individuals who do not engage in resistance training or consume sufficient protein.
Losing too much muscle can slow metabolism, reduce strength, and increase the risk of regaining weight. In follow-up studies, participants regained around two-thirds of lost weight after stopping semaglutide, with most of it returning as fat rather than lean tissue. This rebound has important implications for health, performance, and long-term quality of life.
However, in higher-risk populations (older adults, sarcopenic etc.), there is evidence that lean body mass loss can be more significant (some studies show 15-40% of the weight lost may come from lean tissue in certain groups) unless countered by resistance training, adequate protein, etc.
5. Common side effects
More than half of users experience side effects, but for most, they’re mild, manageable and tend to improve over time, especially when supported with the right nutrition and lifestyle changes.
Here are some of the most common side effects reported:
- Weight-loss addiction: The dramatic changes can be intoxicating and some people push too far, too fast, leading to over-restriction, over-training or disordered eating patterns.
- Nausea: This is particularly common at higher doses or when meals are skipped or too heavy. Poor nutrition tends to make this worse.
- Egg burps: About one in 10 people get sulphuric, eggy-smelling burps due to changes in gut bacteria and slowed digestion. It’s not dangerous, just unpleasant.
- Constipation: With digestion slowed down and fibre often low (especially in the early days of reduced appetite), constipation is a frequent complaint.
- ‘Ozempic face’ or ‘Ozempic butt’: A term used to describe the loss of volume in the face and glutes due to muscle and fat loss, especially with rapid weight loss and inadequate strength training.
- Gallstones: Rapid weight loss can increase the risk of gallstone formation, a side effect seen in other fast weight-loss methods.
- Pancreatitis: Rare (affecting fewer than one in 500) but serious. Anyone experiencing severe abdominal pain should seek urgent medical attention. And while we’re still learning, there are some unanswered questions.
So far, few studies have looked at how GLP-1–induced weight loss affects bone health. We know fat and muscle are lost, but bone changes remain unclear, a key knowledge gap, especially for women in midlife and older.
6. How This Affects Personal Trainers
As a PT, this trend has several implications:
Clients on GLP-1 may experience reduced appetite and quicker satiety. This is important for meal planning, energy intake, recovery. A more personalised approach may be necessary rather than providing generic information and standardised recipe books.
They may lose weight more easily under medication, but also risk losing muscle unless strength training and protein intake are prioritised. Make sure clients consistently follow their strength training program and consume enough protein to meet their individual needs.
There could be shifts in performance, energy levels, especially when in calorie deficit or adjusting diet. Therefore, it’s important to communicate with and educate clients about what to expect, guiding them through any setbacks to support long-term, sustainable weight loss and healthy lifestyle habits.
Managing expectations is crucial: helping clients understand that medication is a tool, not a magic solution, and that lifestyle (training, sleep, nutrition) remains central.
7. Real World Practice for FITPROs with clients on GLP-1 Agonists
Understanding all of this is best done through ongoing, real-world application. As a personal trainer, you are on the frontline with clients and get to be part of their journey using a GLP-1.
Make sure to read and stay up to date with recent studies.
Ensure to track body composition (not just weight), document strength, fatigue, nutrition, and modify your training accordingly.
This isn’t just textbook stuff, it’s about seeing what happens in the gym, tweaking protocols, getting feedback from clients, and keeping up with the latest research.
The question is do you agree with the use of GLP-1s, and would you work with a client who is using one? Comment below or send a message
Summary
GLP-1 agonists offer a promising tool in the fight against obesity, type 2 diabetes, and weight-related health issues, by helping many people achieve meaningful weight loss, particularly fat mass.
But they aren’t without caveats: muscle loss risk, side effects, access, and sustainability are real considerations.
For fitness professionals, the best approach is to regard GLP-1s as one tool among many. Combining medication (where applicable) with training, nutrition, behaviour, and longer-term planning.
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Hayley “GLP-1 Agonists” Bergman
Parallel Coaching
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Sources
- The Weight-loss Effect of GLP-1 Receptor Agonists in Non-diabetic Individuals with Overweight or Obesity: Systematic Review & Meta-Analysis. PubMed
- Efficacy of GLP-1 Receptor Agonists on Weight Loss, BMI, and Waist Circumference (47 RCTs) PubMed
- Real-World Weight-Loss Effectiveness among Patients with Type 2 Diabetes Wiley Online Library
- Genetic Evidence on Fat vs Muscle Mass Reduction ScienceDaily
- Impact of GLP-1 Therapy in High-Risk for Sarcopenia Populations PubMed
- UK Usage & Interest Statistics about GLP-1 for Weight Loss Society