Does Ozempic Cause Muscle Loss? What the Body Scan Studies Show

Ozempic® and Wegovy® (semaglutide) and Mounjaro® and Zepbound® (tirzepatide) lead to large weight loss, and any large weight loss includes some lean mass. Body scan studies found that most of the weight people lost on these medicines was fat, and a smaller share was lean mass.

In the STEP 1 body scan sub-study of semaglutide, about 5.3 kg (12 lb) of the 13.6 kg (30 lb) lost was lean mass, roughly 38–40% ^[2]. In the SURMOUNT-1 body scan sub-study of tirzepatide, about 25% of the weight lost was lean mass, the same share seen in the placebo group ^[3].

That is not the same as saying 40% of the weight lost was muscle. The scans measure lean mass, which is a wider category. The practical message is simple: the more weight you lose, the more it is worth protecting the muscle you have, and two habits help most. Eat enough protein, and do regular strength training.

STEP 1 was a large trial of semaglutide for weight management, the medicine sold as Wegovy®. Ozempic® contains the same active ingredient. In the full trial, people on semaglutide lost 14.9% of their body weight on average, compared with 2.4% on placebo ^[4].

A subgroup of 140 adults also had DXA scans at the start and at week 68 ^[1]. DXA (dual-energy X-ray absorptiometry) is a low-dose body scan that estimates fat mass, lean mass and bone separately. In the semaglutide group ^[1]:

• Body weight: down 15.0%, compared with 3.6% on placebo ^[1]
• Total fat mass: down 19.3% ^[1]
• Visceral fat (the fat stored around the organs): down 27.4% ^[1]
• Total lean mass: down 9.7% ^[1]

In kilograms, a 2025 joint advisory on nutrition for GLP-1 therapy reports that of the 13.6 kg (30 lb) lost on average, 8.3 kg (18 lb) was fat mass, about 62%, and 5.3 kg (12 lb) was lean mass, about 38% ^[2].

One detail often gets lost. Because fat fell faster than lean mass, the proportion of the body made up of lean mass actually went up, by 3.0 percentage points ^[1]. People finished with less lean mass in absolute terms, but a higher share of lean mass overall.

Tirzepatide was tested for weight management in the SURMOUNT-1 trial. A body composition sub-study scanned 160 participants with DXA at the start and at week 72 ^[3]. Here is how the two sub-studies compare:

Two points matter when you read this table. First, these were separate trials with different people, so this is not a head-to-head comparison. It does not show that one medicine is better for muscle than the other.

Second, in SURMOUNT-1 the placebo group lost much less weight, yet about 25% of what they lost was also lean mass ^[3]. That share was similar across most subgroups, including men and women and different age groups ^[3]. Visceral fat fell by 40.1% on tirzepatide ^[3].

This fits a long-standing rule of thumb: with calorie restriction, roughly a quarter of the weight lost is fat-free mass ^[6]. The researchers who reviewed that rule stress it is only an approximation that varies between people ^[6]. Put simply, these medicines produce a lot of weight loss, and a lot of weight loss usually brings some lean mass with it.

No. On a DXA scan, lean mass is everything that is not fat or bone. It includes muscle, but also water, organs and connective tissue. A drop in lean mass is therefore not all muscle, and the trials above did not measure muscle on its own.

This is why headlines claiming that 40% of the weight lost on Ozempic® is muscle go further than the evidence. The accurate version is that roughly that share was lean mass in one sub-study ^[2].

Still, it would be a mistake to shrug it off. Muscle supports strength, balance and everyday tasks like climbing stairs, carrying shopping and getting up from a chair. It matters even more as you get older, which we cover in muscle loss after 40. If you are losing a meaningful amount of weight, it makes sense to protect as much muscle as you reasonably can.

The body scan sub-studies were not designed to test ways of protecting muscle. For that, the evidence comes from wider research on weight loss, and it points to two habits.

1. Eat enough protein

Appetite often drops on these medicines, and protein intake can drop with it. The 2025 joint advisory notes that the basic recommended intake is 0.8 g of protein per kg of body weight a day, and that higher targets of 1.2–1.6 g/kg a day have been proposed during active weight loss ^[2]. As a simpler alternative, it suggests an absolute target of 80–120 g a day ^[2].

If your body weight is high, multiplying by your full weight can give a target that is unrealistic. Our guide on how much protein to eat on a GLP-1 medication explains a more practical approach, and the free protein calculator works out a range for you.

2. Do strength training

The World Health Organization recommends muscle-strengthening activities that work all major muscle groups on 2 or more days a week for adults ^[5]. The joint advisory suggests strength training at least 3 times a week for people on GLP-1 medication ^[2]. A meta-analysis found that exercise programs including resistance training had a unique lean-mass-preserving effect compared with aerobic exercise alone ^[7].

A realistic target is at least 2 sessions a week, ideally 3 ^{[5] [2]}. Sessions do not need to be long. A handful of movements, such as a squat or sit-to-stand, a push, a pull and a hip hinge, done with steady effort and progressed gradually, covers the major muscle groups.

If side effects make eating or exercising hard, or you have other health conditions, talk to your prescriber before making changes. Do not change how you take your medication on your own.

You cannot get a DXA scan every week, but you can watch three signals at home:

1. Weight trend. Weigh yourself under the same conditions a few times a week and compare weekly averages, not single days.
2. Strength. Log a few key exercises. If you lift the same or more while your weight comes down, that is a reassuring sign.
3. Body composition trend (optional). A smart scale's body fat reading is an estimate. A study comparing consumer smart scales with DXA found their body composition estimates were not accurate enough to rely on for individuals ^[8]. Used the same way each week, the trend can still add context.

If your strength keeps dropping for several weeks while your weight falls quickly, look at your protein and training, and mention it to your prescriber or clinician. For more detail, see signs you're losing muscle, not fat and are smart scales accurate?.

Putting those three signals side by side once a week is the weekly check Dyadfit is being built around.