The single question we get asked most often: how much of the weight I am losing is muscle? The honest answer is, more than most people realize, and the reason is not the medication. It is the calorie deficit and the training stimulus around it.
The STEP-1 numbers
The STEP-1 trial, published in the New England Journal of Medicine in 2021, was the first large randomized trial to give us a body composition read on people losing meaningful weight with semaglutide.
Is 39 percent unusually high?
It is roughly in line with what the research shows for calorie-restricted weight loss in general. In a 2017 review of dietary weight loss trials, lean-mass loss commonly landed in the 20 to 30 percent range of total weight lost, and it climbed toward 40 percent when protein intake was low and resistance training was absent.
What actually moves this
The research points to three levers you can actually pull:
- Resistance training two to four times a week. This is the single strongest signal in the data.
- Protein from food at roughly 1.4 to 1.6 g per kg of goal body weight per day. Hitting this target on a suppressed appetite is the hard part.
- Muscle-supportive nutrients: creatine, HMB, adequate vitamin D. These are the ingredients where the evidence for lean-mass support on a deficit is the strongest.
The takeaway
You do not get to skip resistance training on a GLP-1. The research is clear that if you do not challenge your muscles while eating less, your body will let a meaningful chunk of them go. Daily Foundation covers the muscle-support nutrient side, so you can focus on food and training.