Bill Campbell's point: weight lost isn't all fat. Here's the coach's job during rapid or GLP-1 weight loss - and where it stops.
Fast weight loss - especially on a GLP-1 medication - takes muscle with the fat unless something protects it. Dr. Bill Campbell, a body-composition researcher, makes the case that resistance training and adequate protein are that protection. This breakdown covers the mechanic in our own words, adds our take for coaching it online, and draws a hard line: the training and protein are yours; the medication is the doctor's.
By Markus Evers · September 2026 · a Coachway video breakdown
the short version
Campbell's case is that weight lost is never purely fat - a share is lean mass - and during rapid or GLP-1-assisted weight loss the two levers that preserve muscle are resistance training and adequate protein, run consistently through the deficit. The one thing a coach should do about it: for any client losing weight fast, program muscle-preserving resistance training and set a protein floor, and track both - because that's precisely the value the medication doesn't provide. The hard line: the training and protein are the coach's job; every medication question - starting, stopping, dosing, side-effects - belongs to the prescribing physician.
Dr. Bill Campbell - Why You Need Resistance Training on GLP-1s
Source: Dr. Bill Campbell on YouTube. Watch on YouTube. We embed the public video and summarise it in our own words; the video, title and channel belong to Dr. Bill Campbell, and nothing here implies his endorsement of Coachway.
Who is Bill Campbell, and why coaches watch
Dr. Bill Campbell is a body-composition researcher and professor who runs a physique and performance research lab, focused on how people lose fat while keeping (or building) muscle. He's known for translating research on protein, resistance training and dieting into practical guidance, and for a measured, evidence-based tone rather than hype. Coaches watch him because body composition - not just scale weight - is what most clients actually want, and because the rise of GLP-1 medications has made muscle preservation during weight loss one of the most important and fastest-growing topics in coaching. This look at why resistance training matters during medication-assisted weight loss is a clear example, which is the lens this page is written for.
What Campbell actually argues (our summary)
This is our summary of the ideas in the video, in our own words - not a transcript, and not medical advice. Watch the original for his exact framing and the research he cites.
- 1
Weight lost isn't all fat. In rapid or medication-assisted weight loss, a meaningful share of what's lost can be lean mass.
- 2
Losing muscle has real costs. It lowers metabolic rate, reduces strength and function, and can worsen the rebound once the diet or medication ends.
- 3
Resistance training is the primary lever. Lifting through the deficit gives the body a reason to keep the muscle it would otherwise shed.
- 4
Protein is the second lever. Hitting an adequate protein intake during the deficit supports muscle retention alongside the training.
- 5
This matters more on a GLP-1, not less. Appetite suppression makes it easy to under-eat protein and lose weight fast - so training and a protein floor become essential, not optional.
a note on scope - read this before you coach it
GLP-1 medications are prescription medicine, and this is the clearest scope line in coaching. A coach owns two things and only two things here: the resistance training and the nutrition habits around the weight loss - programming the lifting, helping the client hit a protein floor, and tracking adherence. A coach does not advise on whether to start or stop a medication, on dose or titration, on side-effects like nausea, or on interactions and contraindications - those are decisions between the client and their prescribing physician, full stop. If a client asks a medication question, the correct answer is "ask your doctor." Watch for red flags like very low overall food intake, dizziness or rapid unexplained changes and refer those out. This page is coaching education, not medical advice.
Coachway's take
We think this is one of the most important coaching topics of the moment, and one of the clearest examples of a coach adding value the medication can't. GLP-1s are helping a lot of people lose weight, but the medication doesn't build or protect muscle - and muscle is exactly what determines whether the client ends up lighter-and-weaker or lighter-and-stronger. Campbell's framing - resistance training and protein as the two levers - is the right one, and it turns a passive "the drug is doing it" into an active coaching plan that protects the outcome.
The practical nuance we'd stress is that protein is genuinely hard on a GLP-1. Appetite suppression is the whole point of the medication, and it makes hitting a protein target far harder than it is on a normal diet - so this is where a coach earns their keep: protein-first meals, convenient sources, shakes when whole food won't fit, and a realistic floor rather than an idealised number the client will never reach. Training has to be sustainable too - a structured, progressible resistance routine the client actually does beats an ambitious one they abandon when energy is low.
And we can't say the scope line strongly enough, because this topic sits right on it. The training and the protein are the coach's job. The medication is not - not whether to take it, not the dose, not the side-effects, not the interactions. That's the prescribing physician's domain, and the honest, safe answer to any medication question is to send the client to their doctor. Staying on the training-and-protein side of that line isn't a limitation; it's the whole job, done responsibly.
So our version of his message is: for any client losing weight fast - GLP-1 or otherwise - program muscle-preserving resistance training, set a realistic protein floor and give them practical ways to hit it, track both, watch for red flags, and keep every medication question with their doctor. The goal is to lose the fat and keep the muscle, so the client comes out the other side stronger.
Program the training, track the protein floor
Muscle preservation is a training-and-nutrition job, and both live in the platform. You build the muscle-preserving resistance program in the workout builder - structured, progressible, sustainable through the deficit - so the client has a real stimulus to keep muscle, not just a scale dropping. Then the protein floor and training adherence come back through check-ins and progress tracking, so you can see whether protein is actually being hit despite reduced appetite and whether the muscle is being protected. The mechanic is Campbell's; the platform is how you deliver and monitor the two levers that are squarely in your scope - while every medication question stays with the client's doctor.
For the nutrition side, the companion breakdowns on Layne Norton on diet adherence and Sohee Lee on adherence cover the protein-and-adherence habits that make this work.
The muscle-preservation checklist (rapid / GLP-1 weight loss)
A coach-scope checklist for protecting muscle while a client loses weight fast. Everything here is training and nutrition - the medication stays with their physician.
Muscle-preservation checklist
| Lever | What to do | In / out of coach scope |
|---|---|---|
| Resistance training | Structured, progressible, a few times a week | In scope |
| Protein floor | Realistic daily target; protein-first meals, shakes | In scope |
| Track adherence | Log training + protein at check-ins; watch the trend | In scope |
| Red flags | Very low intake, dizziness, rapid changes | Refer to physician |
| The medication | Start/stop, dose, side-effects, interactions | OUT - physician only |
How to use it: for any client losing weight fast, program resistance training, set a realistic protein floor and help them hit it despite reduced appetite, and track both. Keep every medication question with the prescribing physician, and refer red flags out. The goal is to lose fat and keep muscle. Coaching education, not medical advice.
MUSCLE-PRESERVATION CHECKLIST - rapid / GLP-1 weight loss (via Coachway) - weight lost isn't all fat; resistance training + protein protect the muscle - RESISTANCE TRAINING | structured, progressible, a few times a week | IN coach scope - PROTEIN FLOOR | realistic daily target; protein-first meals, shakes | IN coach scope - TRACK ADHERENCE | log training + protein at check-ins; watch the trend | IN coach scope - RED FLAGS | very low intake, dizziness, rapid changes | REFER to physician - THE MEDICATION | start/stop, dose, side-effects, interactions | OUT of scope - PHYSICIAN ONLY HOW TO USE: For any client losing weight fast (GLP-1 or otherwise), program resistance training, set a realistic protein floor and help them hit it despite reduced appetite (protein-first meals, convenient sources, shakes), and track both training and protein at check-ins. The medication doesn't build or protect muscle - that's the coach's job, and it's the difference between lighter-and-weaker and lighter-and-STRONGER. SCOPE (hard line): A coach owns the TRAINING and the PROTEIN/nutrition habits - nothing else. A coach does NOT advise on whether to start/stop a medication, on dose/titration, on side-effects, or on interactions/contraindications. Those belong to the PRESCRIBING PHYSICIAN. Any medication question -> "ask your doctor." Refer red flags (very low intake, dizziness, rapid unexplained changes) out. Coaching education, NOT medical advice.
Preserving muscle during weight loss - FAQ
Why do people lose muscle on GLP-1 medications or rapid weight loss?
Because weight lost is never purely fat - some of it is lean mass, and the faster and less-supported the loss, the larger that lean-mass share tends to be. GLP-1 medications work largely by suppressing appetite, so people eat much less and lose weight quickly; without a deliberate stimulus to keep muscle, the body sheds some of it along with the fat. Appetite suppression also makes it easy to under-eat protein specifically, which compounds the problem. None of this is an argument against the medication - it's a prescribing decision between a patient and their doctor - it's the reason the training-and-nutrition side, which is the coach's job, matters more during medication-assisted or rapid weight loss, not less.
How much resistance training and protein does it take to preserve muscle?
The two levers are resistance training and adequate protein, run consistently through the deficit. On training: a structured resistance program a few times a week, taken reasonably close to effort, provides the signal to retain muscle the body would otherwise let go. On protein: a sufficient daily intake - commonly discussed in the region of roughly 1.6 g or more per kg of body weight, individualised - supports retention alongside the training. The practical challenge on a GLP-1 is that appetite suppression makes hitting protein genuinely hard, so protein-first meals and shakes become tools rather than nice-to-haves. Exact targets should be individualised, and anything touching a medical condition or the medication itself belongs with the client's physician or dietitian.
Can a coach give advice about GLP-1 medications?
No - and this is the hard line. A coach does not advise on whether to start or stop a GLP-1, on dosing or titration, on managing side-effects like nausea, or on interactions and contraindications. Those are medical decisions that belong solely to the prescribing physician. What a coach owns is the training and the nutrition habits around the weight loss: programming resistance training, helping the client hit a protein floor, and tracking adherence. If a client asks a medication question, the honest and correct answer is to send them to their doctor. Staying on the training-and-protein side of that line is not a limitation - it's exactly the value a coach adds that the medication doesn't provide, delivered safely.
How do you coach a client who is losing weight on a GLP-1?
You focus entirely on the two things that protect the outcome and are squarely in your scope: muscle-preserving training and adequate protein. Program a structured resistance routine and progress it; set a realistic protein floor and give the client practical ways to hit it despite reduced appetite (protein-first meals, shakes, convenient sources); and track both training adherence and protein intake at check-ins so you can see whether muscle is being protected. Keep every medication question with their doctor, watch for red flags like very low overall intake and refer those out, and frame the goal as losing fat while keeping the muscle - so that when the client eventually comes off the medication, they keep a stronger, more metabolically healthy body.
More fat-loss and nutrition breakdowns
- · Layne Norton on diet adherence - the protein and adherence habits that hold up.
- · Sohee Lee on sustainable fat loss and adherence - keeping a client consistent through a deficit.
- · The full library: fitness creator coaching breakdowns.
Coach the fat off and the muscle on
all guidesLayne Norton on Why Adherence Beats Diet Type, Broken Down for Coaches
Dr. Layne Norton argues the best diet for fat loss is the one a client can actually stick to. Here is how a coach turns adherence into an accountability system with habits and weekly check-ins - plus a copy-paste check-in template and habit scorecard.
Read the guideSohee Lee: Sustainable Fat Loss and Client Adherence (Coach Breakdown)
Sohee Lee's video on setting up fat-loss dieting numbers, broken down for coaches: why adherence beats the perfect plan, and the habit-tracking and weekly check-in systems that make it stick.
Read the guideDr Stacy Sims on Female Physiology: "Women Are Not Small Men" (Coach Breakdown)
Dr Stacy Sims' TEDx argument that women are not small men, broken down for coaches: her documented positions on fueling, the menstrual cycle and midlife training, and how to program female clients within scope.
Read the guideFree tools for this: DOTS Calculator · Plate Loading Calculator · Wilks & DOTS Calculator
Protect the muscle the medication can't
A GLP-1 drops the weight; it doesn't keep the muscle - that's your job. In Coachway you program the resistance training, set and track a protein floor, and monitor adherence at check-ins, all while medication questions stay with the doctor. Start your 14-day free trial and coach fat off, muscle on.
