GLP-1 Muscle Loss Calculator
Not all weight loss is fat loss. In the body composition substudies behind these medications, a meaningful share of the weight people lost was lean mass. This calculator estimates how that split could look for you, using the trial average weight loss and the lean mass share those substudies reported, and then shows the two habits that protect muscle.
How this estimate works
The first step reuses the same model as the GLP-1 Weight Loss Calculator. It starts from the average total weight loss in the pivotal trial for your medication, about 14.9 percent of body weight over 68 weeks for semaglutide in STEP 1, and about 20.9 percent over 72 weeks for tirzepatide in SURMOUNT-1, then takes the share of that average that matches the number of months you choose.
The second step applies the lean mass share reported in the body composition substudies of those trials, where participants had DXA scans to separate fat mass from lean mass. Across that reporting, roughly 25 to 40 percent of the total weight lost was lean mass. The calculator applies that band to your estimated total loss to give a lean mass range, and treats the rest as fat loss.
Two cautions belong with these numbers. Lean mass is not the same as muscle: it includes water, organs and other tissue, and some early lean mass change is simply water. And the band is a population average, so your own split can fall outside it in either direction. Protein intake and resistance training are the main levers that push an individual result toward the better end of the band.
Frequently asked questions
How much muscle do people actually lose on GLP-1 medication?
In the body composition substudies of the large trials, roughly a quarter to two fifths of the total weight lost was lean mass rather than fat, although the share varies widely between individuals. Some of that lean mass is water and other non muscle tissue, so it is not all muscle, but the share is large enough to be worth protecting.
Does losing lean mass matter if the scale is going down?
Yes. Muscle supports strength, balance, bone health and metabolic rate. Keeping more of it makes long term weight maintenance easier and helps you feel stronger, not just lighter. It matters more with age, because lost muscle becomes harder to rebuild.
What actually protects muscle during GLP-1 weight loss?
Two habits have the most support: eating enough protein, in the range of 1.2 to 1.6 grams per kilogram of body weight per day, and regular resistance training, such as weights, resistance bands or bodyweight exercise, two to three times a week. Getting enough sleep and avoiding an overly aggressive rate of loss also help.