Imagine two groups of people taking the same weight-loss medication. After 40 weeks, both groups have lost nearly 50 pounds per person, on average. Their bathroom scales tell essentially the same story, but when researchers measured their strength, they discovered something remarkable.
On average, one group became weaker while the other became stronger, despite losing almost identical amounts of weight. The difference wasn't the medication. It was resistance training.
A new randomized controlled trial published in Nature Medicine reveals something your bathroom scale simply cannot tell you about your health.
Weight Loss Doesn't Tell the Whole Story
Medications such as semaglutide (Wegovy) and tirzepatide (Zepbound) have changed obesity treatment, producing substantial weight loss and improvements in cardiometabolic health. However, losing weight and improving physical capacity are not necessarily the same thing.
When someone loses considerable weight, the reduction typically includes both fat mass and lean tissue. Some of that lean tissue is skeletal muscle, which plays an important role in glucose metabolism, movement, and maintaining physical independence as we age.
The T-REx trial investigated what happens to muscular strength during medication-assisted weight loss. Researchers enrolled 120 previously sedentary adults receiving tirzepatide, a medication that acts on both GIP and GLP-1 receptors. Participants were randomly assigned to continue their usual physical activities or perform supervised resistance training three times weekly over 40 weeks.
The researchers measured body weight, body composition, and muscular strength. Their findings illustrate why evaluating weight-loss treatment requires more than stepping onto a scale.
Nearly Identical Weight Loss, Very Different Strength Outcomes
After 40 weeks, the medication-only group had lost an average of 49.8 pounds, while those combining medication with resistance training lost 49.1 pounds. Both groups experienced substantial reductions in body fat, but the differences in lean tissue and strength were striking.
View the results as a text table
| Outcome | Tirzepatide Alone | Tirzepatide + Training |
|---|---|---|
| Weight loss | 49.8 lb | 49.1 lb |
| Fat mass loss | 37.9 lb | 39.6 lb |
| Lean soft-tissue loss | 10.3 lb | 7.9 lb |
| Knee extension strength | −9.0% | +17.5% |
| Bench press strength | −8.4% | +10.7% |
| Lat pulldown strength | −6.5% | +15.7% |
Resistance training reduced the loss of lean soft tissue by approximately 2.4 pounds, but I find the strength differences even more compelling. Knee extension strength declined by 9% in the medication-only group while improving by 17.5% in those who trained. Similar differences were observed in upper-body strength.
These exercises may be familiar to gym users, but their relevance extends well beyond the gym.
| Strength Assessment | What It Measures | Connection to Daily Activities |
|---|---|---|
| Knee extension | Quadriceps strength | Standing from a chair, climbing stairs, walking uphill, and controlling the knees when lowering the body |
| Bench press | Upper-body pushing strength | Pushing heavy doors, moving objects, and using the arms to assist when rising from a chair |
| Lat pulldown | Upper-body pulling strength | Pulling objects toward the body, handling loads, and other pulling activities |
These examples illustrate how the muscle groups contribute to everyday tasks; the trial did not directly measure performance in these activities.
Consider someone who has lost considerable weight but also experienced declining quadriceps strength. Climbing stairs might eventually become more demanding, or rising from a low chair may require additional effort. These changes often develop gradually, particularly when someone begins with sufficient strength reserves.
Importantly, becoming lighter can make certain activities easier even when absolute strength declines. That is why researchers also considered strength relative to body weight, which remained approximately stable in the medication-only group but improved substantially among those who trained.
The key lesson is that two people can experience similar weight loss while undergoing very different changes in physical capacity.
You May Be Getting Weaker Without Realizing It
One of the more revealing findings was the difference between what researchers measured and what participants reported experiencing.
Despite measurable strength declines in the medication-only group, relatively few participants reported lethargy or weakness. These symptoms were reported by 14.8% of participants receiving tirzepatide alone and 16.9% of those performing resistance training.
Although these symptom reports are not direct measurements of perceived strength, they illustrate why we should not assume that people will necessarily recognize changes detected through objective testing.
Someone may continue climbing stairs, carrying groceries, and rising from a chair without difficulty despite losing measurable strength. As long as they retain enough capacity to complete those tasks, the changes may go unnoticed.
Over time, however, declining strength reserves can become more consequential, particularly with aging, illness, or additional physical deconditioning. The T-REx trial did not establish that participants who lost strength would eventually develop disability, but it demonstrates why periodic strength assessment can provide information that subjective impressions may miss.
Strength, Balance, and Bone Health
The importance of resistance training extends beyond this particular trial.
Muscular strength supports mobility and contributes to our ability to respond to unexpected physical challenges. Consider tripping on an uneven sidewalk. Preventing a fall may require generating force quickly, taking a corrective step, and recovering balance.
Strength contributes to this response, although coordination, reaction time, balance, and muscular power also matter. Research supports appropriately designed exercise programs combining resistance, balance, and functional training to reduce falls in older adults.
Bone health is another consideration. Bones respond to mechanical loading, and progressive resistance training can help maintain or improve bone mineral density when the exercise stimulus is appropriate. Stronger bones may reduce susceptibility to fractures when falls occur, although fracture risk depends on several factors.
The T-REx investigators did not measure bone density, falls, or fractures, so these are broader benefits supported by other research rather than findings from this trial.
Taken together, these considerations make a compelling case for preserving strength and physical resilience, particularly during periods of substantial weight loss.
Exercise Advice Is Not an Exercise Prescription
An important feature of T-REx was the training program itself. Participants weren't simply encouraged to exercise more. They completed supervised resistance training three times weekly, progressing through five eight-week phases that gradually increased training demands.
The program included familiar movements such as squats, leg presses, knee extensions, lat pulldowns, and chest presses, using resistance bands, machines, free weights, and body-weight exercises. Participants attended approximately 91% of their scheduled sessions.
This is an important distinction because effective training requires more than choosing a few exercises. The prescription should consider current physical capacity, appropriate exercise selection, intensity, progression, and recovery.
For someone who has been sedentary for years, the starting point may involve relatively simple movements and modest resistance. As strength and movement confidence improve, the training stimulus can progressively increase.
Resistance training should not be viewed primarily as another way to expend calories. Its value lies in the physiological adaptations that help maintain and improve muscular strength and physical capability.
Looking Beyond Weight Loss: A Health Ballistics Perspective
At Plan2Peak, a central principle of our Health Ballistics framework is that health cannot be represented adequately by a single measurement.
Body weight, insulin resistance, muscular strength, and cardiorespiratory fitness provide different information about an individual's physiological condition. Someone may experience substantial weight loss while remaining physically deconditioned, while another person may make meaningful improvements in metabolic health and physical capacity with relatively modest changes in weight.
For individuals using weight-loss medications, this means considering how resistance training, aerobic conditioning, and appropriate nutrition can complement pharmacological treatment. The objective is not to choose between medication and lifestyle interventions when both are appropriate, but to develop a comprehensive strategy that improves overall health and long-term physical capabilities.
At Plan2Peak, we approach this through individualized assessment, exercise prescription, nutritional guidance, and ongoing evaluation of meaningful health and performance outcomes.
The Bottom Line
The T-REx trial reminds us that weight loss alone cannot tell us whether someone is becoming physically healthier. Both groups lost nearly identical amounts of weight, yet one experienced declining muscular strength while the other became substantially stronger. Had researchers relied exclusively on body weight, they would have missed one of the most important findings of the study.
Perhaps equally important, relatively few participants reported symptoms of weakness despite objectively measured differences in strength. That finding reinforces the value of periodically assessing physical capacity rather than assuming that feeling well necessarily means strength is being maintained.
Weight-loss medications can provide meaningful benefits, but those benefits should not lead us to overlook muscular strength, functional capacity, bone health, and cardiorespiratory fitness.
Weight loss changes what you weigh. Training changes what you can do. Good health care should consider both.
Take the Next Step
If you're taking a weight-loss medication or working to improve your metabolic health, consider whether you're measuring the outcomes that matter most.
At Plan2Peak, our Health Ballistics assessments and personalized coaching programs help you understand your health and physical capacity, identify opportunities for improvement, and develop an individualized strategy for making meaningful progress.
Explore free assessments, learn how the Health Ballistics framework connects the signals, or consider professionally guided coaching for your next steps.
References
1. Green DJ, et al. Resistance exercise to attenuate lean mass loss during tirzepatide therapy for obesity: a randomized controlled trial. Nature Medicine. Published November 9, 2026. https://doi.org/10.1038/s41591-026-04752-5
2. Exercise for fracture prevention: evidence and consensus-based practice guideline of the osteology umbrella association (DVO). 2026. https://doi.org/10.1186/s12902-026-02194-1
3. Physical Therapy Management of Fall Risk in Community-Dwelling Older Adults: An Evidence-Based Clinical Practice Guideline From the American Physical Therapy Association Geriatrics. 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12533762/
