The Science Behind Appetite Suppression, Anabolic Resistance, and Why Training Is the Variable That Decides the Outcome
August 1, 2026
It’s hard to believe that it’s already August. Coach Erik Castiglione here once again, and since they’re not going anywhere, I’m going to be focusing on GLP-1 medications this month. Specifically, training through the GLP-1 era — what the research shows about muscle loss, why resistance training matters, and how to protect your results. If you’ve been reading along, you already know the headline: these medications get you smaller. Training determines what you’re made of when you get there. In this post, I’m going to explain why that’s true – the actual mechanism explaining what happens in your muscle tissue. No fear-mongering, no oversimplifying. Just the biology, explained the way I’d explain it to a member sitting across from me.
What a GLP-1 Actually Is (Before It Was Ever a Medication)
Here’s something that surprises most people: GLP-1 isn’t a synthetic invention; it’s a hormone your body already makes. In fact, that’s true of peptides in general.
Glucagon-like peptide-1 is released by specialized cells in your small intestine — called L-cells — every time you eat, especially in response to protein, fat, and fiber. Once released, it does three things almost simultaneously: it tells your pancreas to release more insulin, it slows down how fast food empties out of your stomach, and it signals directly to receptors in your hypothalamus that you’re full. This whole system is called the “incretin effect,” and it’s part of why a meal with protein and fiber keeps you satisfied longer than a meal of straight sugar.
Medications like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) are receptor agonists — compounds engineered to bind to the same receptors your natural GLP-1 does, but far more potently and for far longer. Tirzepatide goes a step further and also mimics a second hormone, GIP, which compounds the appetite-suppressing effect. Your body’s natural GLP-1 gets cleared from your bloodstream in a couple of minutes. The medication versions are built to resist that breakdown, so instead of a brief post-meal signal, you get days of sustained appetite suppression and slowed digestion.
That’s the whole mechanism, in plain terms: your body’s own “I’m full, slow down” system, turned up to 11 and left on for days.
Why Turning That System Up Puts Muscle at Risk
Here’s where it connects to what happens on the scale — and in your muscle.
Muscle isn’t a static structure. It’s constantly being broken down and rebuilt in small amounts, a process called muscle protein turnover. Whether you gain, maintain, or lose muscle over time comes down to the balance between muscle protein synthesis (building) and muscle protein breakdown. Synthesis needs two ingredients to happen: available amino acids from protein you’ve eaten, and a mechanical signal — tension on the muscle from resistance training — telling your body those amino acids are actually needed for that specific tissue.
A GLP-1 medication interrupts the first ingredient directly. When appetite is suppressed and food intake drops significantly — which is the entire point of the medication — protein intake usually drops right along with total calories, unless someone is deliberately structuring their meals to prevent it. Fewer circulating amino acids means less raw material available for muscle protein synthesis. This isn’t a flaw in the medication or a reason to avoid it — it’s simply the predictable downstream effect of eating substantially less, whether that reduced intake comes from a medication, a crash diet, or plain old willpower.
What the medication does not do is tell your body which tissue to pull from when it needs energy. Left to its own devices, your body treats a calorie deficit as a calorie deficit — pulling from fat stores and muscle tissue more or less proportionally unless something specifically signals otherwise. That “something” is resistance training, and it’s the second ingredient in the muscle protein synthesis equation — the one no medication provides.
Anabolic Resistance: Why This Hits Harder if You’re Over 40
There’s a wrinkle that makes this especially relevant for a lot of the GLP-1 population: age.
Starting around your 30s and accelerating after 60, your muscle becomes progressively less responsive to the normal signals that trigger protein synthesis — a phenomenon called anabolic resistance. The same amount of protein and the same training stimulus that would have built muscle easily at 25 produces a smaller response at 50. Researchers studying aging muscle have found that older adults typically need a bigger dose of both protein and mechanical tension to get the same anabolic response a younger person gets almost automatically.
Stack anabolic resistance on top of appetite suppression, and you’ve got a population that needs more protein and more of a resistance training stimulus than the general population — at exactly the moment their appetite is working against getting enough food in. To be clear, this isn’t a problem created by GLP-1 medications. It’s the same aging-muscle math that’s always been true, just made more urgent by a medication that reduces the fuel supply on one side of the equation.
Where Resistance Training Actually Intervenes
This is the part that matters most, so it’s worth stating plainly: resistance training doesn’t just “help” in some vague, generally-healthy way. It sends a specific, mechanical signal that directly counteracts what appetite suppression does on its own.
When you load a muscle — squatting, hinging, pressing, pulling, carrying weight — you create mechanical tension and micro-tears in the muscle fibers. That damage triggers a repair-and-rebuild signal, activating a cellular pathway (mTOR, for anyone who wants the specific name) that prioritizes protein synthesis in the tissue that was just loaded. In effect, training tells your body: “this muscle is being used for something important — protect and rebuild it first,” even while the rest of the body is operating in a calorie deficit.
This is exactly why the research on weight loss consistently shows that people who combine a calorie deficit with resistance training lose meaningfully less lean mass than people who diet without training — regardless of whether that deficit is assisted by medication. The training doesn’t cancel out the deficit; it redirects where the deficit pulls from.
And before you as, no, cardio doesn’t send this signal with anywhere near the same specificity. Walking, cycling, and other steady-state cardio modalities are valuable for cardiovascular health, but they don’t create the loaded, progressive mechanical tension that tells your body which tissue to spare. That’s the whole reason “just move more” isn’t sufficient advice for someone on a GLP-1 — the type of movement matters as much as the amount.
The Bottom Line
A GLP-1 medication is your body’s own fullness signal, amplified and extended. That’s a legitimate and often medically appropriate tool for a lot of people — the mechanism is well understood and well studied. But amplifying “eat less” doesn’t come with instructions for your body as to which tissue to protect while eating less. That instruction only comes from resistance training, and it matters more, not less, for anyone over 40 already contending with anabolic resistance.
None of this is an argument against the medication. It’s an argument for treating training as part of the protocol, not an optional add-on — because the biology doesn’t provide that protection on its own.
If you’re on a GLP-1, considering one, or coaching yourself through a rapid weight-loss phase without one, reach out to Coach Erik at Viking Athletics in West Hartford, CT. We build training plans around exactly this mechanism.
FAQ: GLP-1s and Muscle, Quickly
Does a GLP-1 medication directly cause muscle loss? Not directly. It suppresses appetite, which typically reduces total food and protein intake. Without enough protein and a resistance training stimulus, some of the resulting weight loss comes from muscle rather than fat.
Why does resistance training matter more than cardio here? Resistance training creates mechanical tension that signals your body to prioritize rebuilding the muscle being used. Cardio doesn’t send that same specific signal, so it doesn’t protect muscle the way strength training does.
Is this worse for older adults? It can be. Anabolic resistance — the age-related decline in how responsive muscle is to protein and training signals — means people over 40 typically need more of both to get the same muscle-preserving effect.
Does tirzepatide (Mounjaro, Zepbound) work differently than semaglutide (Ozempic, Wegovy)? Tirzepatide mimics both GLP-1 and a second hormone, GIP, which tends to produce a stronger appetite-suppressing effect. The muscle-preservation math is the same for both — more suppression without a training stimulus means more risk to lean mass.
How quickly does resistance training start protecting muscle? The signal is fast — a single well-loaded training session activates the relevant pathway within hours. The benefit compounds with consistency, which is why 2–3 structured sessions a week is the standard recommendation, not an occasional workout.