You are crushing PRs in the gym, your marathon time is finally where you want it, and you feel strong. Then you notice stubborn fat that will not budge despite your training and nutrition being dialed in. A friend mentions they are using a GLP-1 medication and dropping weight fast. You start wondering: could this work for athletes? Would it help with body composition without wrecking your performance? Or would you end up weaker, slower, and watching your hard-earned muscle disappear?

Here is the complicated truth: we do not really know. GLP-1 medications were developed and tested in people with obesity and diabetes, not in athletes training five or six days a week. The clinical trials measuring weight loss and metabolic improvements never asked whether someone could still deadlift their max or maintain their 5K pace. Performance outcomes were not measured. Athletic populations were not studied.

What we do have is data on body composition changes, some concerning findings about cardiorespiratory fitness, emerging research on muscle preservation, and a lot of athletes now using these medications off-label based on incomplete evidence. Some are seeing the body composition changes they want. Others are reporting that their muscle feels like it is "slipping away" despite training hard. The experience varies dramatically between individuals.

This article examines what we actually know about GLP-1 medications and athletic performance, the muscle mass question that has everyone concerned, body composition changes backed by real data, practical strategies if you decide to use these medications while training, and honest acknowledgment of the significant gaps in research that leave athletes making decisions without complete information.

What Happens to Muscle Mass on GLP-1 Medications

The most studied aspect of GLP-1 medications relevant to athletes is their effect on lean body mass (which includes muscle but also bone, water, and organ tissue).

How Much Muscle Loss Occurs?

Clinical trials show variable results, but lean body mass typically accounts for 15% to 40% of total weight loss with GLP-1 therapy. In the STEP trials of semaglutide, approximately 25% to 40% of weight lost came from lean tissue. With tirzepatide in SURMOUNT trials, lean mass loss represented approximately 25% of total weight reduction.

To put this in perspective, if you lose 30 pounds on a GLP-1 medication, somewhere between 7.5 and 12 pounds might come from lean tissue (not all of which is skeletal muscle). The remaining 18 to 22.5 pounds would be fat mass. This ratio is actually similar to or slightly better than what occurs with diet-only weight loss, where 20% to 35% of weight lost typically comes from lean tissue.

A 2024 review in Diabetes, Obesity and Metabolism examined contemporary evidence including MRI-based studies and concluded that skeletal muscle changes with GLP-1 treatment "appear to be adaptive" rather than pathological. The muscle volume reductions were commensurate with what would be expected given aging, disease status, and amount of weight lost. The review noted improvements in insulin sensitivity and reduction in muscle fat infiltration, suggesting improved muscle quality even when quantity decreased.

Is Muscle Loss Different From Regular Weight Loss?

Any weight loss involves some lean tissue loss. Your body reduces overall tissue mass as total body mass decreases. This is a normal physiological adaptation, not unique to GLP-1 medications. When you weigh less, you need less muscle to move your body around.

The question is whether GLP-1 medications cause excessive muscle loss beyond what weight loss itself would produce. Current evidence suggests they do not cause dramatically worse muscle loss than equivalent weight loss through other methods, though the rapid pace of weight loss with these medications may contribute to more lean mass loss in some individuals.

However, it is important to distinguish between lean body mass (measured by DEXA scans in most trials) and actual skeletal muscle mass (which requires MRI to measure accurately). Lean body mass includes water, which can fluctuate significantly. Some of the "lean mass loss" reported in trials may reflect water loss rather than actual muscle tissue loss.

Cardiorespiratory Fitness: The VO2max Question

VO2max measures how efficiently your body uses oxygen during maximum effort exercise. It is considered one of the most important markers of cardiorespiratory fitness and a strong predictor of overall health and mortality risk.

What Research Shows

A 2025 review by researchers at the University of Virginia examined available data on GLP-1 drugs and cardiorespiratory fitness. Their findings raised important concerns for athletes and active individuals.

The researchers found that despite substantial weight loss and some improvements in heart function markers, GLP-1 drugs showed "no clear evidence of CRF (cardiorespiratory fitness) enhancement." VO2max did not significantly improve in most studies despite the weight loss that would normally be expected to improve this measure.

This is surprising because weight loss typically improves VO2max. Carrying less weight should make it easier for your cardiovascular system to deliver oxygen during exercise. The fact that this expected improvement does not occur suggests that the loss of fat-free mass (which includes muscle) may be offsetting the benefits of weighing less.

The researchers noted that cardiorespiratory fitness is "a potent predictor of all-cause and cardiovascular mortality risk" and found in a study of nearly 400,000 individuals that "CRF was far superior to overweight or obesity status for predicting the risk of death." This highlights why the lack of VO2max improvement with GLP-1 therapy deserves attention, even when weight loss is substantial.

Some small studies suggested that exercise training during GLP-1 therapy might improve VO2max, but these studies had methodological limitations and larger, well-controlled trials are needed to confirm whether structured exercise programs can restore the expected cardiorespiratory fitness improvements.

Practical Strategies for Athletes Using GLP-1 Medications

If you are an athlete or highly active person using GLP-1 medications, specific strategies can help preserve muscle mass and minimize performance decrements.

Prioritize Protein Intake

Despite reduced appetite, meeting protein targets is essential for muscle preservation. Research and clinical recommendations suggest aiming for 1.2 to 2.0 grams of protein per kilogram of body weight daily, depending on training intensity and goals. For a 150-pound (68 kg) athlete, this translates to roughly 82 to 136 grams of protein per day.

Distribute protein evenly across meals rather than consuming it all at once. Research suggests 20 to 35 grams of high-quality protein per meal optimally stimulates muscle protein synthesis. When appetite is low, protein shakes, Greek yogurt, cottage cheese, and other easily consumed protein sources help meet targets without requiring large meal volumes.

Some case reports of individuals preserving or even gaining lean tissue while on GLP-1 medications showed protein intakes of 1.6 to 2.3 grams per kilogram of fat-free mass (a measurement that accounts for body composition rather than total weight). These higher intakes may be necessary for athletes prioritizing muscle preservation.

Resistance Training Is Non-Negotiable

Resistance training is considered "the most potent nonpharmacological stimulus" for preserving muscle during weight loss according to research on muscle preservation strategies. For athletes on GLP-1 medications, resistance training becomes even more critical than usual.

Aim for at least two to three resistance training sessions weekly, focusing on compound movements that work multiple muscle groups (squats, deadlifts, presses, rows). Maintain training intensity even if volume needs to decrease due to energy constraints. Progressive overload (gradually increasing weight, reps, or difficulty) signals your body to preserve muscle as essential tissue.

Some athletes may need to adjust training volume during active weight loss phases, prioritizing recovery between sessions. Listen to your body regarding fatigue and recovery needs, as rapid weight loss can affect energy availability for high-intensity training.

Strategic Fueling Around Training

Even with reduced appetite, strategic nutrition around training sessions supports performance and recovery. Eat a light meal or snack one to two hours before training (protein plus some carbohydrate). Liquid nutrition like smoothies may be better tolerated than solid food given slowed gastric emptying from GLP-1 medications.

Refuel within 30 to 60 minutes after resistance training with protein and carbohydrate. This timing window is when muscles are most receptive to nutrients for repair and growth. If appetite is low, protein shakes with added carbohydrate can provide necessary nutrients in easily consumed form.

Monitor Body Composition, Not Just Weight

The scale cannot distinguish fat loss from muscle loss. For athletes, tracking body composition through methods like DEXA scans, bioelectrical impedance, or even simple measurements and progress photos provides better feedback than weight alone.

If you are losing weight but also losing significant strength in the gym, this may indicate excessive muscle loss that requires intervention (more protein, more resistance training, possibly slower rate of weight loss). Strength metrics matter. Track your performance on key lifts or exercises to assess whether muscle function is being preserved.

What We Don't Know: Research Gaps

Honesty about limitations in current evidence is important for athletes making informed decisions.

We do not have large, well-controlled studies specifically in athletes or highly active populations. Most GLP-1 research enrolled sedentary or moderately active people with obesity. How elite athletes or serious recreational athletes respond may differ.

We do not have clear data on whether GLP-1 medications affect training adaptation, meaning whether you build muscle and improve fitness as effectively when taking these medications compared to when not taking them. Small studies suggest exercise may help, but rigorous trials are lacking.

We do not fully understand the long-term effects on muscle health, bone density, and functional capacity in athletic populations. Most trials followed participants for 68 to 176 weeks. Effects beyond this timeframe remain unknown.

We do not know optimal strategies for athletes who want to use these medications specifically for body composition changes rather than overall weight loss. The dosing, duration, and combination with training protocols have not been studied in athletic populations.

The Bottom Line for Athletes

GLP-1 medications offer powerful tools for weight and metabolic health management. For athletes who genuinely need weight loss for health reasons, these medications can be effective when combined with appropriate exercise and nutrition strategies.

However, athletes considering these medications primarily for body composition changes (rather than health-related weight loss) should understand that muscle preservation requires deliberate, intensive effort. Without prioritizing protein intake and resistance training, muscle loss will occur alongside fat loss.

The lack of VO2max improvement despite weight loss is concerning for endurance athletes whose performance depends heavily on cardiorespiratory fitness. Whether structured training programs can overcome this limitation remains to be definitively proven.

For strength and power athletes, the muscle mass loss (even if partially adaptive) may affect performance in sports where absolute strength, power output, or muscle mass itself matters for success.

Athletes using these medications should work closely with sports medicine providers, consider body composition tracking beyond just weight, maintain high protein intake even with reduced appetite, prioritize resistance training, and monitor performance metrics to catch excessive muscle loss early.

At Mochi Health, we provide comprehensive weight management care focused on overall health rather than athletic performance enhancement. Our registered dietitian nutritionists can help ensure you are meeting protein and nutritional needs during treatment. While our primary focus is health-related weight management, we understand that active individuals have specific concerns about preserving lean tissue. You can explore our approach to weight management at https://joinmochi.com/medications.

Check Your Eligibility

If you want to learn whether GLP-1 treatment is appropriate for your health needs and receive personalized guidance on nutrition and exercise during treatment, you can start by completing Mochi's eligibility questionnaire. Check your eligibility here: https://app.joinmochi.com/eligibility.

References

Jastreboff, A. M., Aronne, L. J., Ahmad, N. N., Wharton, S., Connery, L., Alves, B., Kiyosue, A., Zhang, S., Liu, B., Bunck, M. C., & Stefanski, A. (2022). Tirzepatide once weekly for the treatment of obesity. New England Journal of Medicine, 387(3), 205-216. https://doi.org/10.1056/NEJMoa2206038

Neeland, I. J., Linge, J., Tinsley, G. M., Yoshimi, K., Stennett, D., Mancio, J., Dahlqvist Leinhard, O., & Sattar, N. (2024). Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. Diabetes, Obesity and Metabolism, 26(9), 3751-3766. https://doi.org/10.1111/dom.15728

Tinsley, G. M., Heymsfield, S. B., Xia, Z., Lofton, H., Huang, S. J., Voils, J., & Austin, G. L. (2024). Preservation of lean soft tissue during weight loss induced by GLP-1 and GLP-1/GIP receptor agonists: A case series. Obesity Science & Practice, 10(5), e70022. https://doi.org/10.1002/osp4.70022

Weeldreyer, N. R., Liu, Z., & Angadi, S. S. (2025). Cardiorespiratory fitness and weight loss with glucagon-like peptide-1 receptor agonist therapy: Challenges and opportunities. Obesity Reviews, 26(1), e13839. https://doi.org/10.1111/obr.13839

Wilding, J. P. H., Batterham, R. L., Calanna, S., Davies, M., Van Gaal, L. F., Lingvay, I., McGowan, B. M., Rosenstock, J., Tran, M. T. D., Wadden, T. A., Wharton, S., Yokote, K., Zeuthen, N., & Kushner, R. F. (2021). Once-weekly semaglutide in adults with overweight or obesity. New England Journal of Medicine, 384(11), 989-1002. https://doi.org/10.1056/NEJMoa2032183

This article is for educational purposes only and should not be considered medical advice. GLP-1 medications are not approved for athletic performance enhancement. Consult with healthcare providers about whether these medications are appropriate for your individual health needs.