The 30-second summary
- Roughly a third to 40% of the weight lost on a GLP-1 can be lean mass, not fat.
- A protein floor of 1.2–1.6 grams per kilogram of goal body weight — most women land near 120 grams a day — is the working target to preserve muscle.
- Resistance training twice a week multiplies the effect of protein. Cardio alone does not.
The thing your scale won't tell you
The scale is a liar of omission. It tells you the weight is leaving. It does not tell you what kind.
In the STEP 1 trial of semaglutide — the registration study that put Wegovy on the map — participants lost an average of 14.9% of their starting body weight over 68 weeks. In the subset who had DXA body-composition scans, roughly 40% of that weight was lean mass. The finding was repeated in the tirzepatide SURMOUNT trials, and echoed in every smaller study that has looked. The drugs work. They also do not discriminate.
This is the uncomfortable math that most weight-loss apps built before the GLP-1 era are not equipped to handle. They congratulate you for the number going down. They do not ask what you lost.
Why muscle loss happens on a GLP-1
The mechanism is not mysterious. It is the mechanism of every aggressive weight-loss protocol, with one amplifier.
When you are in a meaningful caloric deficit, your body meets its energy needs partly from fat and partly from protein. Which ratio depends on three things: how aggressive the deficit is, how much protein you are eating, and whether the muscles are being asked to do anything.
GLP-1s make the deficit aggressive and near-automatic. They reduce appetite so effectively that many women find themselves eating 30–50% less than they used to — without any conscious effort to restrict. If the protein proportion of that smaller intake is not deliberately high, the math is unforgiving. A woman who was eating 2,000 calories and 70 grams of protein before the medication, and is now eating 1,300 calories and 50 grams of protein, has cut her protein below any reasonable preservation threshold.
The body does what any well-run biological system does under scarcity: it trims what is not in active use. And for most women, most muscle is not in active use at a given hour of the day.
What the research actually supports
There is no single perfect trial in GLP-1 users that prescribes an exact protein number. What there is, is a convergent body of evidence from adjacent populations.
A systematic review of weight-loss interventions in older adults concluded that higher-protein diets during caloric restriction consistently preserved more lean mass than standard-protein diets, with effects visible in 12- to 24-week timeframes. (Kim et al., Obesity Reviews, 2016)
A meta-analysis of resistance-training plus protein interventions in caloric deficit found that combining the two — training and elevated protein — produced substantially better lean-mass retention than either alone. (Cermak et al., AJCN, 2012)
In trials of very-low-calorie diets, protein intakes of at least 1.2 grams per kilogram of body weight per day were associated with significantly less lean-mass loss than intakes below that threshold.
Most clinical nutrition guidance for weight loss, including the International Society of Sports Nutrition position stand, converges on 1.2 to 1.6 grams of protein per kilogram of goal body weight per day — with 1.6 g/kg leaning toward people who are strength training and younger, and 1.2 g/kg more typical for sedentary older adults.
For a woman whose goal weight is 75 kilograms (165 pounds), that math looks like 90 to 120 grams of protein per day. Most of the women Steady is built with land near the upper end of that range, for two reasons: they are losing weight faster than the populations the guidelines were written for, and they want a margin of safety.
What 120 grams looks like
The number sounds abstract until you translate it.
- 3 ounces of chicken breast — 26 grams
- 1 cup of Greek yogurt (plain, 2%) — 20 grams
- 1 scoop of whey protein in water — 24 grams
- 2 large eggs — 12 grams
- ½ cup of cottage cheese — 13 grams
- 4 ounces of salmon — 25 grams
- 1 cup of cooked lentils — 18 grams
Hit any four of those in a day and you are past 80 grams. Add a protein shake and a dinner and you are at 120. On a GLP-1, the challenge is rarely wanting that much food — it is making room for it in a shrunken appetite. The practical answer is protein first on the plate, then fiber, then the rest.
Resistance training multiplies the effect
Protein without training is a flooring, not a ceiling. The studies that show the strongest lean-mass preservation combine protein with two or three sessions per week of resistance work.
The work does not have to be dramatic. Bodyweight squats, push-ups, rows with a resistance band, a 30-minute strength circuit twice a week — the stimulus is the point, not the intensity. Muscle that is used is muscle that the body is reluctant to give up.
Cardio alone does not have the same effect. Walking, while enormously valuable for cardiovascular health and mood, does not create a strong signal for lean-mass retention. A well-designed GLP-1 regimen, for most women, looks like: daily walking, twice-weekly resistance work, and a protein floor held every day regardless of appetite.
What Steady does with this
In Steady, protein is the first number on the home screen. It is set by your goal weight, your activity level, and your dose — not by a generic macro wheel. It is logged in under ten seconds from the plus button. It is reviewed weekly in your progress summary, so a bad week shows up as a pattern before it becomes a month.
The coach knows your protein target, your recent trend, and your cycle phase. It will not congratulate a week where you lost three pounds and ate 50 grams of protein. It will ask, politely, whether you want to talk about it.
Sources
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. NEJM 2021; 384:989-1002. NEJM
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity. NEJM 2022; 387:205-216. NEJM
- Kim JE, O'Connor LE, Sands LP, et al. Effects of dietary protein intake on body composition changes after weight loss in older adults: a systematic review and meta-analysis. Obesity Reviews 2016. PubMed
- Cermak NM, Res PT, de Groot LC, et al. Protein supplementation augments the adaptive response of skeletal muscle to resistance-type exercise training: a meta-analysis. AJCN 2012. PubMed
- Jäger R, Kerksick CM, Campbell BI, et al. ISSN Position Stand: Protein and Exercise. JISSN 2017;14:20. PubMed
Medical disclaimer: Articles in the Steady research hub are educational, not medical advice. Do not change your dose, diet, or training program based on anything you read here without talking to your prescriber or a registered dietitian. See our full medical disclaimer.