GLP-1 agonists reliably produce 15 to 22% weight loss over roughly 68 weeks. Roughly a third of that mass is lean tissue unless you protect it with protein and resistance training. Plan for regain on cessation. STEP 1 (Wilding 2021, n=1,961) delivered 14.9% mean weight loss at 68 weeks on semaglutide 2.4 mg/wk, against 2.4% on placebo. SURMOUNT-1 hit 20.9% on tirzepatide 15 mg/wk. These are the largest pharmacological weight-loss signals in the trial literature outside of bariatric surgery.
The body-composition catch is rarely stated plainly. In a deficit of any kind, lean mass loss tracks proportional to fat loss unless you actively defend it. The DEXA substudies from STEP and SURMOUNT show that ballpark 25 to 40% of the lost mass on a GLP-1 is lean tissue, depending on baseline composition, protein intake, and training status. For a 200-lb person losing 30 lb on semaglutide, that is roughly 8 to 12 lb of lean tissue gone unless you intervene.
The intervention is not exotic. It is the same protein plus resistance training stack that defends lean mass in any deficit. The complication is that GLP-1 agonists suppress appetite specifically, which makes hitting protein targets harder, which is the wrong direction for the lean-mass problem.
Cessation is the other underpriced fact. STEP 4 showed that patients randomized off semaglutide regained roughly two-thirds of lost weight within a year. The trial data does not support short cycles of GLP-1 use as a fat-loss tool that you graduate from. The honest framing is that this is a chronic appetite-modulation medication, like a statin is for ApoB. Plan accordingly, including the cost.
- Trial-grade weight loss: semaglutide 2.4 mg/wk gives ~15% over 68 weeks; tirzepatide 15 mg/wk gives ~21%.
- Lean-mass cost: roughly a third of lost mass is lean tissue without protein plus training defense.
- Protein target on a GLP-1: 2.0 to 2.2 g/kg/day, with explicit per-meal targets so reduced appetite does not push intake under floor.
- Resistance training: 2 to 3 sessions/week is the floor; the medication does not change the dose-response curve.
- Cessation: plan for regain. Two-thirds of lost weight returns within a year off-medication in trial data.
What to actually do
- Wire the protein and resistance protocol BEFORE the first injection. Two weeks at 2.0 g/kg/day plus two strength sessions per week. If you can't hit this at full appetite, you definitely won't on a GLP-1.
- Track lean mass, not just scale weight. A bioimpedance device every 4 weeks, or a DEXA every 6 months. If lean mass is dropping faster than fat, your protein and training are the issue, not the medication.
- Decide your exit plan upfront. Short course (12 to 24 weeks) for a defined weight target, with a protein and training program that survives cessation. Or open-ended chronic use, on the same logic as a statin. Avoid the middle path of stopping and "seeing what happens."
GLP-1 medication is a real tool with real trade-offs. The hype overstates the magic and understates the lean-mass problem. The dismissal underrates what 15 to 20% durable weight loss does to cardiometabolic risk. For the full breakdown of trial data, mechanism, and the protein protocol, see the full article.