On a GLP-1? How to Keep Your Muscle While You Lose Weight

Medications like semaglutide (Wegovy, Ozempic) and tirzepatide (Zepbound, Mounjaro) have changed weight loss. In clinical trials they produce average weight reductions of 5 to 18% beyond placebo, along with real metabolic benefits. But the scale only tells you how much you lost. It does not tell you what you lost.

Weight is not all fat

Whenever anyone loses weight, by any method, part of it is lean mass: muscle, along with water, organ tissue and bone. In the STEP 1 trial of semaglutide, a body-composition substudy found that of an average 13.6 kg lost, 8.3 kg (62%) was fat and 5.3 kg (38%) was lean mass. Not all lean mass is muscle, and a 2025 joint advisory from four US nutrition and obesity medicine organizations estimates the true muscle share at around 20% of the total.

The picture has become more reassuring. A 2026 meta-analysis in the International Journal of Obesity found that over 12 months, GLP-1 medications produce weight loss mainly through fat reduction, with relative preservation of lean tissue. Even so, its authors recommend pairing the medication with nutrition and exercise to protect long-term results.

Why muscle matters more after 40

Muscle is not just about how you look. It is the tissue that lets you climb stairs, carry groceries, catch yourself when you trip and manage blood sugar. From midlife onward we lose it gradually anyway (see Why Strength Training After 40 Is Non-Negotiable). Rapid weight loss on top of that natural decline, with nothing in place to protect muscle, can leave you lighter but weaker. And muscle is far harder to rebuild than it is to keep.

What the experts recommend

In 2025, the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association and The Obesity Society published a joint advisory on nutrition during GLP-1 therapy. Of its eight priorities, two stand out for anyone who wants to stay strong:

• Enough protein. During active weight loss, the advisory suggests about 1.2 to 1.6 grams of protein per kilogram of body weight per day, or a practical target of roughly 80 to 120 grams a day. That is well above the standard 0.8 g/kg.
• Regular strength training. Strength work at least three times a week, alongside at least 150 minutes of moderate aerobic activity.

The American College of Sports Medicine made the same point in its 2026 fitness trends report: people who keep exercising during medication treatment maintain greater fat loss and preserve more lean mass.

Eating well when you are not hungry

On a GLP-1, the challenge is rarely willpower. It is appetite. Many people simply do not feel like eating, and some deal with nausea or early fullness. That makes the quality of each meal matter more. Some practical ideas:

• Protein first. Start each meal with the protein on your plate: eggs, Greek yogurt, fish, chicken, tofu, beans or lentils.
• Spread it out. Several moderate servings of protein through the day are easier than one large one.
• Keep plants in. Vegetables, fruit, whole grains and legumes provide fiber and the vitamins and minerals a smaller appetite can miss.
• Drink enough. Eating less often means drinking less too.
• Choose minimally processed foods. When you eat less, every bite should count.

Where Pilates fits

Strength training does not have to mean a crowded gym. Classical Pilates on the apparatus is resistance training: springs load the legs, hips, back and arms through a full range of motion, with the control and alignment that make it sustainable for people who are new to exercise, returning after a break or carrying extra weight. In private sessions, the load can be progressed gradually as you get stronger, and adjusted on days when side effects leave you low on energy.

For clients who add Nourishment, the same sessions include practical, evidence-based guidance on protein, meals and energy, built around your routine rather than a meal plan.

The bottom line

A GLP-1 can help you lose weight. What you keep is up to you: enough protein, regular strength work and time. The goal is to come out of treatment not just lighter, but stronger.

This article is general information, not medical or nutrition advice. GLP-1 medications are prescribed and monitored by your doctor. Talk to your prescriber or a registered dietitian before changing how you eat or exercise, especially if you have kidney disease or another medical condition.