Muscle loss with the weight-loss injection: why strength training matters now

For many people, the weight-loss injection is a turning point. After years of dieting, the scale is finally moving. But when you lose weight quickly, your body doesn't only lose fat. A considerable part of the weight is muscle. In this article you'll learn what the studies show, why it matters and what you can do about it.

What the studies show

In the large approval studies for GLP-1 medications, body composition was measured precisely in a subgroup of participants using DXA scans. The result: a quarter to almost 40% of the weight lost was lean mass, which consists mainly of muscle.¹ ²

In other words: if you lose 10 kg, around 2.5 to 4 kg of it is lean mass. These are averages and differ from person to person. But the direction is clear.

Why muscle matters when you lose weight

Muscles are much more than looks. They carry you through everyday life, protect your joints and bones and contribute to your energy expenditure. If you lose a lot of muscle while losing weight, you often feel weaker, even though the number on the scale goes down.

• Everyday strength: stairs, groceries, getting up from the floor. You should feel stronger after losing weight, not weaker.

• Bones and joints: rapid weight loss also puts strain on your bones. Strength training provides a targeted stimulus and stabilises your joints.

• The time afterwards: in one study, around two thirds of the lost weight returned within a year of stopping the medication.⁴ If you have built muscle and solid training habits, you are better prepared for this phase.

Strength training makes the difference

Muscles only stay if they are used. Strength training sends exactly this signal, even when you eat less. In a study of older adults who were losing weight, the strength training group lost about 2% of their muscle mass, while the group doing only endurance training lost about 5%.⁵

The combination of medication and exercise has also been studied: the group with both lost almost twice as much body fat as the group with medication alone and maintained their weight best. In this study, participants mainly did endurance training, and an older GLP-1 medication was used.³

What you can do

In 2025, four US professional societies for lifestyle medicine, nutrition and obesity jointly stated: adequate protein and strength training are an integral part of every GLP-1 therapy.⁶ For you, this means:

1. Strength training at least twice a week, ideally three times, each time for the whole body. Two sessions are the minimum to maintain or build muscle.

2. Progress gradually. Clean technique first, then more weight or repetitions. Only a growing stimulus keeps your muscles engaged.

3. Enough protein. How much you need is determined by your doctor as part of your treatment. With little appetite, it pays to plan this consciously.

4. Measure instead of guessing. The scale doesn't show whether you are losing fat or muscle. Strength values and a measurement of your resting metabolic rate show whether your body is keeping up.

What if you feel nauseous on a given day?

Nausea, fatigue or dizziness are common in the first weeks and after a dose increase. Then the rule is: less intensity, more breaks, calmer exercises. Training should do you good, not overwhelm you. A short, light session is better than a skipped week.

Discuss your training with your doctor, especially if you have cardiovascular disease or also take insulin or other blood sugar medication.

Coaching in Zurich and Lucerne

I am a personal trainer and sports therapist and work with people who are losing weight with the weight-loss injection and want to stay strong: at EVO Fitness in Zurich and Lucerne, outdoors or at your home. You can find out what this looks like, what it costs and what the studies show in detail here (in German): Strength training with the weight-loss injection.

The initial consultation is free and also possible in English: Book a consultation.

Sources

1. Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med 2021;384:989–1002. PubMed

2. Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med 2022;387:205–216. PubMed

3. Lundgren JR et al. Healthy Weight Loss Maintenance with Exercise, Liraglutide, or Both Combined. N Engl J Med 2021;384:1719–1730. PubMed

4. Wilding JPH et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: The STEP 1 trial extension. Diabetes Obes Metab 2022;24:1553–1564. PubMed

5. Villareal DT et al. Aerobic or Resistance Exercise, or Both, in Dieting Obese Older Adults. N Engl J Med 2017;376:1943–1955. DOI

6. Mozaffarian D et al. Nutritional Priorities to Support GLP-1 Therapy for Obesity. Joint Advisory by ACLM, ASN, OMA and TOS, 2025. Press release

Note: This article provides information about training and does not replace medical advice. I do not prescribe medication and give no recommendations on its use or dosage. The study figures are averages and do not apply to every individual.