Losing Weight on a GLP-1? Don't Lose Your Muscle With It

By C. Erick Kaufman, MD — Medical Director, Holistically Rx

GLP-1 medications like semaglutide and tirzepatide have transformed medical weight loss — average losses of 15–20% are now routine, as I covered in my recent article on the new ACP obesity guidelines. But 2026 research has put a spotlight on something every patient on these medications needs to understand: not all the weight you lose is fat.

A 2026 systematic review and meta-analysis in the International Journal of Obesity confirmed what clinicians have observed: a meaningful share of the weight lost on GLP-1 therapy can come from lean tissue — muscle — not just fat. Rapid, large-scale weight loss of any kind carries this risk, and losing muscle matters: it slows your metabolism, weakens strength and balance, and in older adults raises the risk of sarcopenia — the age-related muscle loss that steals independence.

The good news? Muscle loss during weight loss is largely preventable. Here is exactly what I tell my patients.

Rule One: Eat More Protein Than You Think You Need

Anyone in a significant calorie deficit — and GLP-1 medications create one by quieting appetite — needs deliberately high protein intake to give the body a reason to keep its muscle. The research on lean-mass preservation supports roughly 1.2–1.6 grams of protein per kilogram of body weight per day during active weight loss, with older adults aiming for the higher end (review of preservation strategies, 2026).

Do the math on yourself: for a 220-pound (100 kg) person, that's 120–160 grams of protein daily. Almost nobody hits that number by accident — especially when a GLP-1 has shrunk your appetite. It takes planning: protein at every meal, protein-forward snacks, and sometimes supplementation. Smart eating strategies matter more on these medications, not less.

Rule Two: Resistance Training Is Not Optional

Protein gives your body the raw material to preserve muscle; resistance training gives it the instruction to. Walking is wonderful, but it is not enough. The evidence is consistent: patients who combine GLP-1 therapy with weight-bearing and resistance exercise — two to three sessions weekly of bands, weights, machines, or bodyweight work — preserve dramatically more lean mass than those who rely on the medication alone.

If you take one sentence from this article, make it this one: get enough daily protein, and do weight-bearing — ideally resistance — training. That is the counsel I give every single patient who starts a GLP-1.

Rule Three: Track What You're Losing, Not Just How Much

The bathroom scale can't tell fat from muscle — and this is where most weight-loss programs fall short. The number going down feels like winning, but if too much of that number is muscle, you're trading long-term metabolic health for short-term scale victories.

That's why monitoring matters: body-composition assessment at baseline and at intervals during therapy changes the question from "how much have you lost?" to "what have you lost?" — and lets your physician adjust protein targets and training before muscle loss becomes a problem. It's the same philosophy behind our use of tools like continuous glucose monitoring: measure what actually matters, then personalize.

The Bigger Picture: Medication Is One Pillar, Not the Whole House

None of this is a reason to fear GLP-1 therapy — the health benefits of treating obesity are profound and well-proven, whether with semaglutide or tirzepatide. It's a reason to treat obesity the way chronic disease deserves to be treated: with medication and nutrition and movement and monitoring working together, supervised by a physician who watches all four.

That's how we practice at Holistically Rx. If you're on a GLP-1 — or considering one — and no one has talked to you about protein targets, resistance training, or body composition, let's have that conversation. Schedule a free consultation and we'll build a plan that protects your strength while you lose the weight.

This article is for educational purposes only and is not medical advice. Medical weight-loss treatment requires evaluation by a licensed physician. Always consult your healthcare provider about your individual situation before starting or stopping any medication.