In recent years, we’ve seen a huge rise in the use of medications designed to support weight loss, most of which work by mimicking a gut hormone called GLP-1 (glucagon-like peptide-1). GLP-1 is naturally released after eating and slows stomach emptying, sends satiety signals to the brain, and helps regulate blood sugar levels. When used in higher, pharmaceutical doses, these drugs can profoundly reduce appetite and make it far easier to stick to a calorie deficit — the essential driver of weight loss. 

The main prescription options currently include: 

• Semaglutide : Wegovy (licensed for weight loss) and Ozempic (licensed for type 2 diabetes, often used off-label for weight loss). 
• Liraglutide : Saxenda (daily injection, shorter-acting). 
• Tirzepatide : Mounjaro (also marketed as Zepbound in the US), which combines GLP-1 action with another hormone called GIP for even greater appetite suppression and weight loss potential. 
• Retatrutide : not yet available but currently in advanced research. This “triple agonist” acts on GLP-1, GIP, and glucagon receptors, and early trials have shown average weight loss of over 24% in less than a year — approaching results usually only seen with bariatric surgery. 

These drugs don’t directly “burn fat” or speed up your metabolism — their main effect is to make you feel fuller, for longer, with smaller amounts of food. This makes maintaining a calorie deficit more achievable for many people who have struggled with other methods. 

The short answer is yes, and the evidence is now strong. 

• STEP-1 trial (NEJM, 2021) : Weekly semaglutide led to an average 14.9% weight loss over 68 weeks, compared with 2.4% in the placebo group. 
• SURMOUNT-1 trial (NEJM, 2022) : Tirzepatide produced 15–21% average weight loss over 72 weeks, depending on dose. 
• Retatrutide phase 2 trial (NEJM, 2023) : Average weight loss exceeded 24% at 48 weeks in people with obesity. 

This degree of weight loss is clinically significant — improvements are seen in blood pressure, cholesterol, blood sugar control, sleep apnoea, fatty liver disease, and even systemic inflammation. 

The most common side effects are gastrointestinal – nausea, bloating, constipation, diarrhoea, and reflux – especially when starting or increasing the dose. Most settle with time and careful dose escalation. 

More serious but less common risks include gallbladder disease, pancreatitis, and bowel obstruction. These require close monitoring by your prescribing clinician. 

However, one major issue that gets far less attention is the impact on body composition. 

GLP-1 drugs can lead to significant loss of lean body mass which includes muscle and bone, especially if you’re not doing enough resistance training or eating enough protein. This matters because: 

• Losing muscle slows your metabolism and makes weight regain more likely once you stop the drug. 
• Loss of muscle strength reduces functional capacity and quality of life. 
• Loss of bone mineral density increases your risk of fractures in later life. 

For this reason, we recommend that anyone taking these medications should combine them with structured strength training and a protein-rich diet (around 80-120g per day for most active adults). 

It is imperative for anyone prescribed a course of weight loss medication to begin or continue with strength training, as resistance training is the most effective way to build and maintain strength, muscle mass, and bone density whilst losing weight. 

Equally important is tracking what type of weight you’re losing. Without accurate measurement, you won’t know if the changes on the scale are mostly fat or if you’re also losing excessive muscle and bone. We recommend having an initial baseline measurement, ideally with a low-cost DEXA scan before starting the medication and repeating it periodically. This allows you and your healthcare team to track changes in fat mass, lean mass, and bone density, so you can adjust your training and nutrition strategies accordingly. 

Preserving strong bones is essential for everyone as we age, because reduced bone mineral density (BMD) increases the risk of fractures and serious fractures in older age are linked to reduced mobility, independence, and even higher mortality risk. The concern with rapid weight loss, particularly when driven by GLP-1 or similar medications, is that the loss of lean body mass can extend to bone tissue, increasing the risk of osteopenia or osteoporosis. 

From a nutritional standpoint, individuals using GLP-1 are at an increased risk of vitamin D and calcium deficiency which contribute significantly to the loss of bone mass. Consequently, it’s crucial to get adequate calcium into your diet by including foods such as yoghurt and milk (or fortified alternatives), or by considering a supplement if intake is low. A daily vitamin D supplement is also recommended, ideally taken with a meal that contains some fat to support absorption. 

When it comes to exercise, bones (like muscles) adapt to the demands placed on them. When the skeleton is exposed to mechanical load – whether that’s the impact of landing from a jump, or the compressive forces from lifting a challenging weight – microscopic strain occurs in the bone tissue. This strain sends a signal to specialised bone cells to remodel and strengthen the bone, making it denser and more resilient. 

The degree of bone adaptation depends on both the type and intensity of the load. In general, higher strain (within safe limits) produces greater stimulation for bone growth. Importantly, this doesn’t mean endless light repetitions; bones respond best to relatively short bursts of high-intensity, high-impact, or heavy-load exercise. 

It’s also worth noting that strong muscle contractions themselves exert significant forces on the bone. For example, a well-executed squat doesn’t just strengthen the leg muscles, the pulling and compressive forces from the contracting muscles also stimulate the thigh and hip bones to remodel and strengthen. 

In practice, the most effective bone-strengthening programs combine: 

• High-intensity resistance training: lifting challenging weights for lower repetitions, progressing over time. 
• Impact-loading activities: such as hopping, bounding, or jumping (where appropriate and safe for the individual). 
• Multi-directional movements: changing direction, twisting, and loading in different planes to stimulate bone in a variety of ways. 

As with muscle training, it’s important not to overdo it: excessive or poorly structured loading can increase the risk of stress fractures or other injuries. A tailored, progressive program designed by a qualified Strength & Conditioning Coach or Exercise Physiologist ensures you achieve the right balance of stimulus and recovery, optimising bone health whilst minimising injury risk. 

Preserving strong bones is essential for everyone as we age, because reduced bone mineral density (BMD) increases the risk of fractures and serious fractures in older age are linked to reduced mobility, independence, and even higher mortality risk. The concern with rapid weight loss, particularly when driven by GLP-1 or similar medications, is that the loss of lean body mass can extend to bone tissue, increasing the risk of osteopenia or osteoporosis. 

From a nutritional standpoint, individuals using GLP-1 are at an increased risk of vitamin D and calcium deficiency which contribute significantly to the loss of bone mass. Consequently, it’s crucial to get adequate calcium into your diet by including foods such as yoghurt and milk (or fortified alternatives), or by considering a supplement if intake is low. A daily vitamin D supplement is also recommended, ideally taken with a meal that contains some fat to support absorption. 

When it comes to exercise, bones (like muscles) adapt to the demands placed on them. When the skeleton is exposed to mechanical load – whether that’s the impact of landing from a jump, or the compressive forces from lifting a challenging weight – microscopic strain occurs in the bone tissue. This strain sends a signal to specialised bone cells to remodel and strengthen the bone, making it denser and more resilient. 

The degree of bone adaptation depends on both the type and intensity of the load. In general, higher strain (within safe limits) produces greater stimulation for bone growth. Importantly, this doesn’t mean endless light repetitions; bones respond best to relatively short bursts of high-intensity, high-impact, or heavy-load exercise. 

It’s also worth noting that strong muscle contractions themselves exert significant forces on the bone. For example, a well-executed squat doesn’t just strengthen the leg muscles, the pulling and compressive forces from the contracting muscles also stimulate the thigh and hip bones to remodel and strengthen. 

In practice, the most effective bone-strengthening programs combine: 

• High-intensity resistance training: lifting challenging weights for lower repetitions, progressing over time. 
• Impact-loading activities: such as hopping, bounding, or jumping (where appropriate and safe for the individual). 
• Multi-directional movements: changing direction, twisting, and loading in different planes to stimulate bone in a variety of ways. 

As with muscle training, it’s important not to overdo it: excessive or poorly structured loading can increase the risk of stress fractures or other injuries. A tailored, progressive program designed by a qualified Strength & Conditioning Coach or Exercise Physiologist ensures you achieve the right balance of stimulus and recovery, optimising bone health whilst minimising injury risk. 

GLP-1 and related weight loss drugs can be game-changing tools for people struggling with obesity and its health consequences. But they’re not magic on their own and without a proactive plan for muscle and bone health, the weight you lose could come at the expense of strength, function, and long-term resilience. 

Pairing these medications with structured strength training, adequate protein, regular body composition tracking, and expert guidance from medical, dietetic, and S&C professionals gives you the best chance of not just losing weight, but keeping it off, feeling strong, and staying healthy for years to come. 

Interested in expert advice on muscle and bone health? Get in touch today.