Foundayo, as a newly launched medicine, doesn't yet have its own published body-composition substudy data (the kind of detailed DXA scan research available for Mounjaro), but it shares the same core GLP-1 mechanism and produces weight loss through the same calorie-deficit pathway as other medicines in this class, where the evidence consistently shows around 20–25% of weight lost comes from lean mass and 75–80% from fat — a favourable ratio broadly consistent across successful weight loss by any method. The two most evidence-backed ways to protect muscle during treatment are adequate protein intake (around 1.2 to 1.6g per kilogram of body weight daily) and regular resistance training, ideally at least twice a week.
Foundayo only launched in the UK in August 2026, and unlike Mounjaro, which has a published DXA (dual-energy X-ray absorptiometry) substudy specifically measuring fat versus lean mass loss, Foundayo doesn't yet have this same level of detailed body-composition data publicly available. This is a genuine, honest gap worth acknowledging rather than glossing over — it reflects how new the medicine is, not a specific concern about its effect on muscle.
What we do know is that Foundayo works through the same broad mechanism as other GLP-1 medicines — appetite suppression leading to a calorie deficit, which is the actual driver of both fat loss and any accompanying muscle loss. Since this underlying mechanism is shared across the class, and the pattern of proportional fat-to-muscle loss has been consistently documented in related medicines including Mounjaro and Wegovy — see our dedicated Wegovy and muscle preservation guide for that medicine's specific evidence — there's reasonable basis to expect Foundayo will show a broadly similar pattern, even without dedicated Foundayo-specific imaging data confirming this yet.
Worth knowing
A body composition substudy of Mounjaro's SURMOUNT-1 trial found that when participants lost around 21% of total body weight, roughly 25% of that came from lean mass and 75% from fat. As more real-world and trial data on Foundayo accumulates over time, this guide will be updated to reflect any Foundayo-specific findings as they become available.
When your body is in a significant calorie deficit — which is exactly what Foundayo's appetite-suppressing effect creates — it draws on stored energy from multiple sources, not exclusively fat. Muscle tissue is metabolically active and requires energy to maintain, so during periods of reduced calorie intake, particularly if protein intake and physical activity aren't actively supporting muscle retention, some lean mass loss is a normal and expected part of the process. This isn't specific to Foundayo, or even to GLP-1 medicines generally — it happens with any method that produces significant, sustained weight loss, including diet alone or bariatric surgery.
Prioritise Protein Intake
Protein provides the building blocks your muscles need to repair and maintain themselves, and requirements increase — not decrease — during a calorie deficit. Clinical guidance for GLP-1 treatment generally recommends 1.2 to 1.6g of protein per kilogram of body weight daily, notably higher than the standard 0.8g/kg general recommendation. Since Foundayo's appetite suppression often means eating less overall, deliberately prioritising a protein source at each meal becomes more important, not less. See our dedicated nutrition guide for a full breakdown of protein targets and food sources.
Resistance Training, Not Just Cardio
- Aim for at least two resistance training sessions a week, focusing on compound exercises that work multiple muscle groups (squats, presses, rows) rather than isolated single-muscle exercises
- If you're new to strength training, starting with bodyweight exercises is a reasonable and effective entry point — you don't need a gym or equipment to begin
- Resistance training sends your body a genuine signal to retain and build muscle, even in a calorie deficit, which cardio alone doesn't provide to the same degree
Avoid an Overly Aggressive Calorie Deficit
A calorie deficit of roughly 10–25% is generally considered sustainable without disproportionately compromising lean mass. Since Foundayo can suppress appetite quite significantly once you reach an effective dose, some people end up in a much larger deficit than intended simply because they're not hungry — eating deliberately even when appetite is low, rather than skipping meals, helps avoid an unnecessarily steep deficit.
Don't Under-eat on Purpose
It might seem logical that eating even less would speed up weight loss, but severely restricting calories on top of Foundayo's own appetite suppression tends to accelerate muscle loss disproportionately, without meaningfully accelerating fat loss. If you're struggling to eat enough due to reduced appetite, a protein shake or nutritionally complete drink between meals can help you reach your protein target without needing to eat a large volume of food.
| Strategy | Practical action |
|---|---|
| Protein | Include a protein source at every meal; consider a protein shake if appetite is very limited |
| Resistance training | At least 2 sessions weekly, focusing on major muscle groups |
| Calorie deficit | Avoid restricting further than your appetite naturally suggests |
| Titration pace | Progress through Foundayo's schedule steadily rather than rushing, giving your body time to adjust |
| Hydration | Stay well hydrated, since this supports both muscle function and recovery |
Important
If you're new to resistance training or have any existing health conditions, particularly joint or cardiovascular issues, it's worth speaking to your GP or a qualified trainer before starting a new exercise programme. Building up gradually is safer and more sustainable than starting too intensively.
Muscle is metabolically active tissue, meaning it burns calories even at rest, and maintaining more of it supports your metabolic rate as you lose weight. Muscle also plays a central role in mobility, balance, and long-term functional independence, particularly as people age. For older adults especially, unintentional muscle loss during rapid weight loss carries genuine long-term risks around frailty and reduced physical function, which is exactly why the preservation strategies in this guide matter as much as the weight loss itself.
Timing Protein and Exercise Around Your TitrationSide effects tend to be most noticeable in the days following a Foundayo dose increase — see our dedicated Foundayo side effects guide for the full picture — which can make both eating enough protein and exercising feel harder during that window. Rather than abandoning your routine entirely during a difficult few days, many people find it helpful to focus on smaller, more frequent protein-rich meals, and to keep exercise lighter (a walk rather than a heavy session) until symptoms ease, then return to your normal resistance training routine as you start to feel better. Consistency over the following weeks matters more than any single session. For the full titration timeline, see our dedicated Foundayo dosage and titration guide.
Tracking Progress Beyond the ScaleSince the scale can't distinguish between fat loss and muscle loss, relying on weight alone can be misleading. If protecting muscle matters to you, consider tracking additional measures alongside weight: how your clothes fit, strength progress in the gym (are you maintaining or improving your lifts, even as weight drops?), and if accessible, periodic body composition measurements. Strength that holds steady or improves during weight loss is a genuinely reassuring sign that muscle is being protected, regardless of what the scale alone shows.
Protect Your Muscle, Not Just Your Weight
Our GPhC-registered pharmacy team can talk through practical nutrition and lifestyle strategies to help you protect muscle mass while taking Foundayo®.
Start Your Free Online Consultation → Happy Pharmacy is a GPhC-registered pharmacy (No. 9012585) — Superintendent Pharmacist: Palvinder Deol — Rated 4.8★ Excellent on Trustpilot- Foundayo® (orforglipron) Summary of Product Characteristics, MHRA Products website.
- Does Tirzepatide Cause Muscle Loss? SURMOUNT-1 body composition substudy analysis. Diabetes, Obesity and Metabolism.
- Effects of Tirzepatide on Skeletal Muscle Mass in Adults: A Systematic Review. Cureus, 2025.
- GLP-1 and Muscle Loss: Semaglutide, Tirzepatide and Lean Mass. Clinical review, 2026.