GLP-1 Protein & Muscle Calculator

Why the target isn't scaled to your scale weight

The usual advice is 1.2–2.0 g of protein per kg of body weight. Applied literally to a 240 lb body that's up to 218 g a day, which nobody eats and no guideline intends — protein needs scale with lean tissue, and lean tissue doesn't grow proportionally with fat. Clinical nutrition handles this with adjusted body weight: take the weight at the top of the healthy BMI band, then add a quarter of the excess.

reference = IBW + 0.25 × (actual − IBW), where IBW = 25 × height² in metres and kg

The daily target is then 1.5 g/kg of that reference (2.0 g/kg if you're lifting), with 1.2 g/kg as the floor you shouldn't go under. If you're already at or below a BMI of 25, the calculator uses your actual weight — no adjustment needed.

Where the lean-mass range comes from

SURMOUNT-1's DXA substudy measured it directly: tirzepatide cut fat mass 33.9% and lean mass 10.9% — about 3:1, which is the ordinary ratio for any weight loss, drug or not. Across trials and recent systematic reviews the lean share of total loss lands between 20% and 40%. This calculator shows 25–40%, because that's the range worth planning against rather than the one worth hoping for. Adequate protein plus resistance training moves you toward the bottom of it; neither one alone does much.

Worked example

240 lb at 5'9". IBW at BMI 25 is 169 lb; the excess is 71 lb, so the reference weight is 169 + 0.25 × 71 = 187 lb (84.7 kg). At 1.5 g/kg that's 127 g of protein a day, or 32 g across four eating occasions — comfortably over the ~25 g per meal below which muscle protein synthesis blunts. Planning to lose 50 lb means 12.5–20 lb of that is lean mass. Two resistance sessions a week and hitting the protein number is what decides which end of that range you land on.

The practical problem

None of this is hard arithmetic; it's hard eating. A GLP-1 works by making food unappealing, and protein is the macro that goes first — meat becomes off-putting for a lot of people well before carbohydrate does. That's why the per-meal number matters more than the daily one: if you're down to two small meals, hitting 127 g means 64 g per sitting, which is roughly a full chicken breast and a scoop of whey, at a moment when you don't want either. Spreading over four occasions is usually the difference between hitting the target and missing it by 40%.

FAQ

Is losing muscle on a GLP-1 worse than losing it any other way?

Proportionally, no — the fat-to-lean ratio is about the same as any comparable calorie deficit, and the same as bariatric surgery. What's different is the magnitude and speed: these drugs routinely produce 15–20% total body weight loss, far more than diets sustain, so the absolute muscle loss is larger than most people have ever experienced. And unlike a diet, the appetite suppression makes it harder to eat your way out of it.

Does the muscle come back if I stop?

Partly, and this is the uncomfortable part. In the STEP 1 extension, weight regained after stopping came back disproportionately as fat. So a cycle of on-then-off can leave you at your original weight with a worse body composition than you started with — the strongest argument in the literature for either staying on a maintenance dose or building the resistance-training habit before you stop.

Do I need protein powder?

Not necessarily, but it's the pragmatic answer when appetite is suppressed. 30 g of whey in water goes down when a chicken breast won't. Greek yogurt, cottage cheese, and skyr do the same job at 15–25 g a serving. The goal is grams, not any particular source.

How much resistance training is actually enough?

Two full-body sessions a week is the floor with real evidence behind it, and it's the number this calculator shows — not because more isn't better, but because two is achievable and the gap between zero and two is far larger than the gap between two and four. Cardio is good for you and does very little for this specific problem.

Should I get a DXA scan?

If you're planning a 40+ lb loss, a scan at the start and one a year in tells you what a bathroom scale can't — whether the weight coming off is the weight you meant to lose. They run $50–150 in most US metros. Failing that, track your lifts: if strength is holding, lean mass mostly is too.