Do you lose muscle mass when losing weight with a GLP-1 medication?

Yes, GLP-1 medications such as semaglutide or tirzepatide also cause you to lose muscle mass. Roughly one quarter of the weight you shed comes not from fat but from lean mass. On average this amounts to about 0.86 kg in randomised studies, but with large weight losses of 15 to 25 percent it can rise to approximately 6 kg. That is comparable to what you lose over more than ten years of ageing.

Not every GLP-1 medication behaves the same way. Liraglutide, the less potent drug, was the only one in an analysis of 22 studies to achieve weight loss without measurable loss of muscle mass. Semaglutide and tirzepatide are far more effective for weight and fat loss, but they are also associated with the greatest muscle loss. Whether muscle loss directly means you function worse is not certain: there are indications that muscle quality actually improves, because less fat is stored within the muscle tissue and insulin sensitivity increases. There is still too little evidence on long-term muscle strength.

The most concrete thing you can do right now is resistance training. In studies, supervised resistance training lasting more than ten weeks increased lean mass by an average of approximately 3 kg and muscle strength by approximately 25 percent. This is also relevant if you later stop taking the medication: people who retain more muscle mass are less likely to regain weight rapidly afterwards.

Pay attention to what you eat as well. GLP-1 medications strongly suppress appetite, which makes deficiencies in protein, calcium, vitamin D and iron a real risk. After one year, more than 13 percent of users had a vitamin D deficiency, and more than 60 percent were consuming too little calcium and iron. In a comparative study, GLP-1 users also had 26 to 30 percent lower iron stores than people taking a different medication. Monitoring your diet and consciously eating enough protein are therefore not excessive measures.

Older people and those who already have little muscle mass face an extra risk of sarcopenia when using GLP-1 medications. Several reviews cite this as a reason to exercise additional caution with this group when choosing these drugs. New agents that directly inhibit muscle breakdown are being investigated, but they are not yet available to regular patients.