Protein on a GLP-1

The short answer: Across studies summarized in a 2024 Lancet commentary, lean mass made up about 25% to 39% of the weight lost on GLP-1 medicines, and the authors link that largely to how much weight is lost. A 2025 expert advisory describes higher protein intake during weight loss, alongside resistance training, but notes evidence gaps and no consensus on exact targets.

This research summary lays out what trials report, what guidelines say, and what hasn’t been tested yet, with every number attributed to its source.

Why does protein come up on a GLP-1?

GLP-1 medicines reduce appetite and, with it, how much people eat. A 2025 joint advisory from four U.S. professional societies (the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society) cites reductions in energy intake of 16% to 39% compared with placebo (Mozaffarian et al., Obesity, May 30, 2025). Eating less can mean less protein.

Weight loss of any kind also involves some loss of lean mass, as the Lancet commentary discussed below notes for calorie restriction without medicines.

This piece summarizes research and guidelines. It isn’t medical or dietary advice, and it doesn’t cover dosing, side effects, or price.

How much of the weight lost is lean mass in trials?

Lean mass (also called lean body mass or fat-free mass) is the non-fat part of the body: muscle plus other tissue. It isn’t the same as muscle. The 2025 advisory notes that muscle is about half of lean body mass.

Two trials used DXA scans (a body-composition imaging method) in a subset of participants. They differed in population, duration, and design, so they shouldn’t be compared with each other. Here is what each reported.

STEP 1 DXA substudy (semaglutide). This is an exploratory analysis of the STEP 1 trial, published as a 2021 conference abstract (Wilding et al., J Endocr Soc). It included 140 participants (95 on semaglutide, 45 on placebo) with a BMI of 40 or below, from 9 sites. At week 68, body weight had changed by -15.0% with semaglutide versus -3.6% with placebo. In the semaglutide group, total fat mass changed by -19.3% and total lean body mass by -9.7%. Lean mass as a share of total body mass rose by 3.0 percentage points. As a conference abstract, it is a preliminary look. The parent STEP 1 trial was funded by Novo Nordisk (Wilding et al., N Engl J Med).

SURMOUNT-1 DXA substudy (tirzepatide). This substudy covered 160 of the 2,539 SURMOUNT-1 participants (124 on pooled tirzepatide doses, 36 on placebo), scanned at baseline and week 72 (Look et al., Diabetes Obes Metab, 2025). The paper reports that the change in body weight, fat mass, and lean mass from baseline to week 72 was “-21.3%, -33.9% and -10.9% with tirzepatide and -5.3%, -8.2% and -2.6% with placebo, respectively.” In absolute terms, average fat mass changed by -15.9 kg with tirzepatide and -3.6 kg with placebo, and average lean mass by -5.6 kg and -1.2 kg. Of the body weight lost, the paper says “approximately 75% was fat mass and 25% was lean mass for both tirzepatide and placebo.” That split is reported alongside post hoc subgroup analyses. The trial was funded by Eli Lilly and Company, and six of the paper’s eight authors are Lilly employees or shareholders.

A commentary that looks across studies. The Lancet piece by Prado and colleagues is a commentary, not a trial. It states that studies suggest fat-free mass decreases with these medications range “from 25% to 39% of the total weight lost over 36-72 weeks,” versus 10-30% in calorie-restriction studies with smaller weight loss. They write that the loss “can be largely attributed to the magnitude of weight loss,” rather than a separate effect of the medicines. They add that this hypothesis must be tested, and that questions about strength and function need further research.

What do guidelines say about protein during weight loss?

The 2025 joint advisory (published May 30, 2025) is an expert-group appraisal, not a systematic review or randomized trial. It gives the general adult Recommended Dietary Allowance (RDA) for protein as 0.8 g/kg/day and says that value is “currently undergoing review for updating by the National Academies of Medicine.” Higher targets, such as 1.2-1.6 g/kg/day, “have also been proposed during active weight reduction.” The advisory also says intake should not fall below 0.4-0.5 g/kg/day, that prolonged intake at or above 2 g/kg/day should be avoided, and that it is unclear whether targets should be based on actual body weight, adjusted body weight, or fat-free mass. It states there is still no consensus on the best approach.

That review status is as of the May 2025 advisory. A National Academies page last updated Feb 3, 2026 says an update to the macronutrient Dietary Reference Intakes, established in 2005, is “under consideration” by the Joint Canada-U.S. Dietary Reference Intakes Working Group (National Academies).

The 2025-2030 Dietary Guidelines for Americans (released Jan 7, 2026) list “protein serving goals: 1.2-1.6 grams of protein per kilogram of body weight per day, adjusting as needed based on your individual caloric requirements” (HHS/USDA). That is general-population guidance. The document text doesn’t mention GLP-1 medicines, and it isn’t a revised RDA.

These ranges are what each source says, not a personal target. The advisory’s protein discussion names foods such as fish, eggs, Greek yogurt, cottage cheese, beans, and lentils in different passages.

Is protein enough on its own?

According to the advisory, no. It says increased protein intake alone is likely inadequate to support muscle mass without structured resistance or strength training, and it describes strength training alongside aerobic activity.

One trial tested exercise next to a GLP-1 medicine. Jensen and colleagues ran a secondary analysis of a randomized trial of 195 adults without diabetes, ages 18 to 65, followed for 52 weeks after an 8-week low-calorie diet (JAMA Network Open, 2024). Liraglutide alone was associated with lower hip and spine bone mineral density than placebo, while exercise plus liraglutide did not differ from placebo at either site. The paper reports that lean mass was preserved in the combination group. The authors report ties to Novo Nordisk, which supplied the study drug; per the paper, the sponsor had no role in the analysis or publication.

Two caveats: the trial did not test protein intake, so it says nothing about protein targets, and the analysis was exploratory, without adjustment for multiple comparisons.

For background on how physical activity relates to insulin sensitivity, see Insulin Resistance, Explained.

What do people on a GLP-1 actually eat?

Johnson and colleagues ran a cross-sectional study of 69 U.S. adults who had used a GLP-1 medicine for at least one month, using 3-day food records (Frontiers in Nutrition, April 25, 2025). Mean protein intake was 77.3 g/day. Using adjusted body weight, 43% consumed at least 1.2 g/kg of protein, 10% at least 1.6 g/kg, and 5% at least 2.0 g/kg.

Read that with care. It was a small convenience sample, the intake was self-reported, and participants used a mix of GLP-1 medicines. The study was funded by GNC Holdings, LLC, and all of its authors are employed full time by GNC Holdings. In this observational study, these were the results; they can’t show what people on GLP-1 medicines eat in general.

The advisory describes reduced appetite and energy intake on GLP-1 medicines. GI side effects aren’t covered in this piece.

What are the limits of the evidence?

• Scans measure amount, not function. The DXA substudies were small subsets of larger trials. DXA measures lean mass, not muscle quality or strength. The advisory says current technologies monitor muscle mass but not muscle health, quality, or function.
• Strength and function need more research. The commentary’s authors add that questions about strength and function need further research.
• No protein-target trial here. None of the studies cited tested a specific protein target in people taking a GLP-1 medicine. The advisory’s ranges come from weight-loss literature and expert consensus.
• Funding matters. SURMOUNT-1 was funded by Eli Lilly and Company, and most of the substudy paper’s authors are Lilly employees or shareholders. The parent STEP 1 trial was funded by Novo Nordisk. The Johnson study was industry-funded, as noted above.
• People differ. Needs can vary with age, kidney health, activity, and medical history. A clinician or registered dietitian can interpret this for you.

Stopping a GLP-1 medicine is outside the scope of this piece.

What questions can you bring to your care team?

Questions that can start the conversation:

• Has anyone checked my strength or body composition?
• Would a registered dietitian help with my protein and nutrient intake?
• Does my kidney health, or any other condition, change what applies to me?
• What kind of strength or resistance activity fits my health?

A clinician who knows your history can help answer them.

Frequently asked questions

Do GLP-1 medicines cause muscle loss?

In trials, the weight lost included lean mass: 25%-39% across studies, per a 2024 Lancet commentary. The authors link this largely to how much weight is lost, and they note that idea still must be tested. Lean mass is not the same as muscle.

Is there an official protein target for people on a GLP-1?

The sources reviewed don’t set a GLP-1-specific target. A 2025 expert advisory notes that 1.2-1.6 g/kg/day has been proposed during weight loss and that there is no consensus on how to calculate it. The 2025-2030 Dietary Guidelines list the same range as a general-population goal. A clinician or dietitian can interpret this for an individual.

Is protein alone enough to support muscle during weight loss?

The 2025 advisory says protein alone is likely inadequate without structured resistance training. The trials cited here did not test protein.

Do people on GLP-1s eat enough protein?

One small observational study (69 adults, funded by GNC Holdings) found 43% reached at least 1.2 g/kg/day. It can’t show what all users eat.

Is more protein always better?

Not necessarily. The advisory cautions against prolonged intake at or above 2 g/kg/day, and individual needs vary.

Looking for more? Browse more metabolic health explainers on the Read hub.

Sources

1. Mozaffarian D, et al. Nutritional priorities to support GLP-1 therapy for obesity: A joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. Obesity 2025 (published May 30, 2025);33(8):1475-1503. doi:10.1002/oby.24336. (PubMed)
2. U.S. Department of Health and Human Services and U.S. Department of Agriculture. Dietary Guidelines for Americans, 2025-2030. Released Jan 7, 2026.
3. Wilding JPH, et al. Impact of Semaglutide on Body Composition in Adults With Overweight or Obesity: Exploratory Analysis of the STEP 1 Study. J Endocr Soc 2021;5(Suppl 1):A16. doi:10.1210/jendso/bvab048.030 (conference abstract).
4. Look M, et al. Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab 2025;27(5):2720-2729. doi:10.1111/dom.16275. Funded by Eli Lilly and Company. An erratum (2025;27(11):6823) corrects a figure axis label and does not affect the text figures cited here.
5. Prado CM, Phillips SM, Gonzalez MC, Heymsfield SB. Muscle matters: the effects of medically induced weight loss on skeletal muscle. Lancet Diabetes Endocrinol 2024;12(11):785-787 (commentary). doi:10.1016/S2213-8587(24)00272-9
6. Jensen SBK, et al. Bone Health After Exercise Alone, GLP-1 Receptor Agonist Treatment, or Combination Treatment: A Secondary Analysis of a Randomized Clinical Trial. JAMA Netw Open 2024 (published Jun 25, 2024). doi:10.1001/jamanetworkopen.2024.16775
7. Johnson B, et al. Investigating nutrient intake during use of glucagon-like peptide-1 receptor agonist: a cross-sectional study. Front Nutr 2025 (Apr 25, 2025). doi:10.3389/fnut.2025.1566498. Funded by GNC Holdings, LLC; all authors employed full time by GNC Holdings, LLC.
8. National Academies of Sciences, Engineering, and Medicine. Updates and Innovation Opportunities for the Dietary Reference Intakes Process. Last update Feb 3, 2026.
9. Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med 2021. doi:10.1056/NEJMoa2032183. Funded by Novo Nordisk (parent trial of the DXA substudy in source 3).

This article is for general education. It isn’t a substitute for personal medical advice. If you have questions about protein, weight loss medicines, or your own health, talk with a licensed clinician or registered dietitian who knows your health history.

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