Evidence explainerWomen's GLP-1 & Weight
GLP-1s and Muscle: How Much Lean Mass Women Lose, and the Protein and Strength Plan Experts Recommend
About a quarter of the weight lost on tirzepatide in its main trial was lean mass, the same share as on placebo. A 2025 joint advisory says protein alone will not protect muscle without strength training.
Key takeaways
- In a SURMOUNT-1 body-scan substudy of 160 adults (73% women), tirzepatide cut body weight by 21.3%, fat mass by 33.9% and lean mass by 10.9% at 72 weeks; about 75% of the weight lost was fat and 25% lean, the same split seen with placebo [1].
- Across GLP-1 trials, lean mass has ranged from about 15% or less to 40% to 60% of the weight lost, and lean mass counts organs, bone and water as well as muscle [2].
- A 2025 joint advisory from four obesity and nutrition societies says more protein alone is likely inadequate to preserve muscle without structured strength training [3].
- The advisory recommends strength training at least 3 times a week plus at least 150 minutes of moderate aerobic exercise while on a GLP-1 [3].
- Protein targets of 1.2 to 1.6 g per kg of body weight a day have been proposed during active weight loss; for people with obesity it is unclear whether to use actual or adjusted body weight [3].
What the body scans show
The clearest data come from a substudy of SURMOUNT-1 that measured body composition with DXA scans at the start and at week 72. With tirzepatide, weight fell 21.3%, fat mass 33.9% and lean mass 10.9%; with placebo the figures were 5.3%, 8.2% and 2.6% [1].
Of the weight lost, roughly 75% was fat and 25% lean in both groups, and the split held across most subgroups, including by sex and age [1]. Several authors of that analysis are employed by the drug's maker, Eli Lilly [1].
Lean mass is not the same as muscle
Reported lean-mass losses vary widely between GLP-1 studies, from about 15% or less of total weight lost to 40% to 60% [2]. Part of the reason is that lean mass also includes organs, bone, fluids and the water inside fat tissue [2].
A 2024 review using MRI data concluded that muscle changes on GLP-1 drugs appear adaptive: muscle volume falls roughly in line with age, disease and the weight lost, and improvements in insulin sensitivity and muscle fat likely help preserve muscle quality [2]. The same review notes that older age and more severe disease raise the risk of sarcopenia, so these factors may matter when choosing who takes these drugs [2].
How much protein
The general adult recommendation is 0.8 g of protein per kg of body weight a day; targets of 1.2 to 1.7 g/kg have been proposed during strength training, and 1.2 to 1.6 g/kg during active weight loss [3]. For a woman weighing 70 kg (154 lb), 1.2 g/kg works out to 84 g a day.
The advisory adds two limits: protein should not fall below 0.4 to 0.5 g/kg a day, and prolonged intake above 2 g/kg a day should be avoided [3]. It also flags women who are perimenopausal or menopausal, and anyone older or not strength training, as more exposed to muscle loss when appetite drops [3]. Our protein guide for women over 50 covers food sources.
Strength training does the heavy lifting
The advisory's central point is that protein works alongside resistance exercise, not instead of it: GLP-1s should be prescribed together with a structured exercise program of strength training at least 3 times a week plus at least 150 minutes of moderate aerobic activity [3].
Its modelling estimates that, without structured strength training, muscle loss is about 10% to 15% of total weight lost in women and 20% to 25% in men [3]. Our strength training after menopause guide has a starter plan; see also our creatine monograph.
Partners we link to
- GalaPaid linkWomen-focused online weight care: compounded semaglutide or tirzepatide (not FDA-approved) at a published flat $179/mo on a yearly subscription, visits and shipping included (checked Oct 8, 2026). LegitScript certified.See Gala's GLP-1 plans
- FoundPaid linkWeight care with named board-certified clinicians: brand-name GLP-1 prescriptions (medication billed separately, insurance accepted) or compounded semaglutide and tirzepatide, which are not FDA-approved. LegitScript certified.See Found's weight plans
Quick answers
Do women lose less muscle than men on a GLP-1?
The 2025 joint advisory cites modelling that puts muscle loss at about 10% to 15% of weight lost in women and 20% to 25% in men without structured strength training [3].
Is more protein always better on a GLP-1?
No. The advisory says prolonged intake above 2 g per kg of body weight a day should be avoided, and that protein alone is likely inadequate without strength training [3].
More in Women's GLP-1 & Weight
- Women's GLP-1 & Weight hub
- Semaglutide vs Tirzepatide: How Much Weight the Trials Showed, and What Happens When You Stop
- Compounded Semaglutide and Tirzepatide: What the FDA Actually Says, and the Red Flags to Check
- GLP-1s, the Pill and Pregnancy: The Label Warnings Women Need Before Starting
- Hair Shedding on Semaglutide or Tirzepatide: How Common It Is in Women, and Whether Iron Matters
- GLP-1s With PCOS or After Menopause: What the Evidence Shows
References
- [1]Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight. Diabetes Obes Metab, 2025.
- [2]Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes Obes Metab, 2024.
- [3]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. Obes Pillars, 2025.