Muscle, Not Just Kilos: Why Body Composition Matters When You Lose Weight on GLP-1

The three things to remember

• Part of the weight you lose is fat-free mass (including muscle) — in studies, roughly 20–30%. At the same time, the ratio of muscle to fat often improves.
• Preserving your muscle mass is largely in your hands: regular strength training and adequate protein intake make the difference.
• That's why how you lose weight matters—and the only way to see it is through your body composition.

The scale shows you just one number—but it doesn't tell you whether you've lost fat or muscle. And that's the crucial difference when losing weight with GLP-1 medication: losing weight is not the same as losing metabolically active tissue.

This article looks at where things stand with muscle loss on GLP-1, what the research says — and, above all, what you can do yourself so that in the end you lose mainly fat and keep your muscle.

Do you lose muscle when you lose weight on GLP-1?

Yes, to some extent. Whenever you lose weight—whether through dieting, surgery, or GLP-1 medication—part of that weight loss comes from fat-free mass, which includes muscle. Analyses of GLP-1 studies suggest that roughly 20–30% of the weight lost comes from fat-free mass.

Two key points:

First, the overall balance often improves. In the DXA body composition analysis from the STEP 1 trial, fat-free mass decreased by around 10%, but its proportion of total body weight increased slightly because participants lost substantially more body fat. In other words, the body became leaner overall—not proportionally "less muscular."

Second, muscle loss depends more on your behaviour than on the medication itself. And that's where you can make the biggest difference.

Why does muscle mass matter so much in the first place?

Muscle is your body's metabolic engine: the more muscle mass you have, the more energy your body burns—even at rest. If you lose a significant amount of muscle, your resting metabolic rate decreases, making it harder to maintain your weight. This is one of the reasons why crash diets often lead to the yo-yo effect.

Muscle also supports everyday strength, mobility, bone health, and—especially from midlife onwards—helps protect against age-related muscle loss. Low skeletal muscle mass has been associated with an approximately 57% higher risk of all-cause mortality. Preserving your muscle mass is therefore one of the most important factors for achieving lasting weight-loss results.

The good news: keeping muscle is largely in your hands

Here's the part that headlines often leave out: how much muscle mass you preserve is largely up to you—through your nutrition and training. The scientific evidence is clear.

Recent reviews (2025) show that people who combine GLP-1 treatment with strength training and adequate protein intake can significantly reduce the amount of muscle mass they lose compared with simply eating less.

The medication reduces your appetite—but preserving your muscles depends on what you eat and how you move.

How much protein — and what kind of training?

Two key strategies—both strongly supported by scientific evidence:

Protein. During weight loss, your body needs more protein than usual to preserve muscle mass. Current recommendations for people receiving GLP-1 therapy are around 1.2 to 1.6 grams of protein per kilogram of body weight per day—with the higher end of the range being particularly appropriate during weight loss. In practice, this means aiming for 20–40 g of protein per meal, spread throughout the day. Good sources include Greek yogurt or cottage cheese, eggs, legumes, fish, poultry, and tofu.

One challenge is that GLP-1 medications reduce appetite—precisely when many people find it hardest to eat enough protein. This is where a clear nutrition plan (and sometimes targeted supplementation) can make a real difference.

Strength training. A high-protein diet alone is not enough—muscle is best preserved when adequate protein is combined with regular strength training. General recommendations are at least 2–3 strength-training sessions per week, complemented by aerobic exercise (around 150 minutes per week as a guideline). You don't need to become a bodybuilder—even regular training with your own body weight, resistance bands, or dumbbells can make a significant difference.

Metabolically active tissues

If you only focus on the number on the scale, you miss the part that matters most. Losing 6 kilograms can mean 6 kilograms of body fat (great)—or 4 kilograms of fat and 2 kilograms of muscle (not so great). The number is the same, but what lies behind it is completely different.

What really matters is your body composition: how much fat you're losing and how much muscle mass you're preserving. A muscle and fat analysis makes this visible. It turns "I hope I'm doing it right" into "I can see it in black and white." That not only boosts motivation—it also allows you to adjust your approach early if you're losing too much muscle.

How care supports your body composition

That’s exactly why a muscle and fat analysis is an integral part of treatment at care—not just once, but every month:

• Your starting point: An initial assessment establishes your baseline body composition.
• Monthly body composition analysis: Each month, your body composition is measured, distinguishing between fat and muscle—not just your weight.
• Medical video consultation: Your results are reviewed with a physician, including adjustments to your nutrition and exercise plan, management of side effects, and, if needed, changes to your medication dosage.
• Progress tracking in the App: You can follow your body composition over time and see whether you’re moving in the right direction.

This way, the focus isn’t simply on the number on the scale, but on achieving sustainable results—with medical guidance every step of the way.

Want to know where you stand — and whether a supported treatment fits you? Take the no-obligation eligibility check. If you’re eligible, care’s FMH medical team reviews your details — medically checked, no pressure. → Check your eligibility

In brief

When losing weight with GLP-1 medication, you also lose some muscle mass—but how much is largely up to you. Getting enough protein and doing regular strength training are the two most important factors, and tracking your body composition makes all the difference. Measuring it regularly and reviewing the results with a physician helps you preserve muscle and achieve lasting success.

Frequently asked questions

Do you inevitably lose muscle on GLP-1? Part of the weight you lose is fat-free mass, but the muscle loss can be sharply limited with protein and resistance training. In studies that included training and adequate protein, muscle loss was correspondingly small.

How much protein should I eat per day? As a guideline, around 1.2–1.6 g per kilogram of body weight per day — toward the upper end while losing weight — ideally spread across several meals. Your individual amount is something your doctor will discuss with you.

Is protein alone enough to keep muscle? No. Regular strength training (around 2–3 times per week) is essential.

Why does care measure muscle-fat analysis every month? Monthly measurements make your progress visible and allow your treatment to be adjusted when needed.

Medical disclaimer

This article is intended for general information and health education. It does not replace individual medical advice, diagnosis or treatment. Any prescription treatments mentioned are available only on a doctor’s prescription. If you have health concerns, please consult a doctor.