Beyond the Scale: Protecting Fitness & Function in the GLP-1 Era
5 mins to read
August 6, 2026
For many people, the goal of starting weight-loss medication is not only to lose weight; it is also to walk farther, exercise longer, and maintain their physical independence. However, emerging evidence suggests that weight loss achieved with GLP-1 therapies may not always be accompanied by parallel improvements in physical activity, cardiovascular fitness, or exercise capacity. Although the weight-loss benefits of GLP-1 therapies are well established, their effects on physical performance have received far less attention.
As the field begins to expand the definition of treatment success beyond the number on the scale, important questions remain regarding the effects of GLP-1 therapies on physical activity, fitness, and functional ability.
Physical Activity
Exercise is a key component of a healthy lifestyle. Many clinicians assume that weight loss naturally makes physical activity easier, leading to increased daily movement and greater participation in exercise. However, emerging real-world data suggests this relationship may be more complex.
In a recent study presented at the Endocrine Society conference, investigators analyzed wearable-device data from adults with obesity who initiated GLP-1 treatment. Among individuals included in the analysis, average daily step counts declined approximately 10% after starting a GLP-1, from 5,000 to 4,500 steps per day. Additionally, time spent in moderate-to-vigorous exercise also decreased 25% to 22 minutes per day on average. Researchers found no evidence that weight loss achieved with GLP-1 therapy was associated with any increase in physical activity.
The reasons for this apparent reduction in activity remain unclear. One potential contributor may be fatigue, which has been reported in clinical studies of obesity medicines, including GLP-1-based treatments. A recent systematic review including nearly 100,000 clinical trial participants identified fatigue as a commonly reported adverse event across several obesity medications. Other research has shown that severe calorie restriction – not dissimilar from the appetite and satiety effects of GLP-1s – is associated with reduced physical activity.
Although these findings need to be confirmed in larger and longer prospective studies, they underscore that reductions in body weight with GLP-1s do not necessarily translate into improvements in activity levels. As obesity treatments become increasingly effective, understanding how these therapies influence daily activity and exercise may prove just as important as understanding their effects on weight.
Cardiovascular Fitness
Improvements in cardiovascular health are among the first benefits expected from successful weight loss. Two commonly used indicators of cardiovascular health and fitness are resting heart rate and heart rate variability (HRV), a measure of the variation in time between individual heartbeats. Both metrics can now be readily monitored using wearable devices. Obesity is associated with higher resting heart rate and reduced HRV, which may contribute to increased cardiovascular risk. Because weight loss achieved through traditional diet and exercise is correlated with improvements in heart rate, HRV, and cardiovascular health generally, similar changes might be expected following GLP-1-induced weight loss.
Interestingly, recent real-world wearable data from the research team at WHOOP showed that initiation of GLP-1 medication was associated with an increase in resting heart rate and a reduction in HRV despite substantial weight loss. While increases in resting heart rate have been consistently observed in clinical studies of GLP-1 therapies, the implications of concurrent changes in HRV remain unclear. These findings raise important questions about whether dropping weight is always accompanied by proportional improvements in cardiovascular fitness.
Unanswered Questions
How do GLP-1 medicines impact exercise physiology?
Although the effects of GLP-1s on CV fitness require further study, these medications have demonstrated clear functional benefits in certain obesity-related comorbidities, including heart failure. In clinical trials of semaglutide, Wegovy improved six-minute walk distance by approximately 20 meters in patients with obesity-related HFpEF, alongside improvements in heart failure symptoms and physical limitations.
However, the mechanisms underlying these improvements remain incompletely understood. The STEP-HFpEF studies did not include detailed muscle assessments or cardiopulmonary exercise testing, leaving unanswered questions regarding the effects of GLP-1 therapies on exercise physiology and fitness itself.
Do GLP-1s decrease the number of calories burned during exercise?
Researchers have also begun to explore whether reductions in lean mass and adaptive decreases in energy expenditure after GLP-1 treatment contribute to the weight-loss plateau that many patients eventually reach and to the substantial weight regain commonly observed after treatment discontinuation. Whether these physiological adaptations alter exercise performance or decrease calories burned during exercise remains an active area of investigation.
The answers to these questions may help clarify who should start and remain on GLP-1 treatment for the long-term. In a recent Kaiser Family Foundation poll, women were nearly twice as likely to report GLP-1 use, even though obesity affects both sexes. The reasons for this difference are not fully established, but it raises an important communication challenge for the field. For patients concerned about strength, stamina, muscle mass, or physical fitness, treatment success may ultimately need to be defined not only by the amount of weight lost, but also by the ability to preserve function, maintain fitness, and support long-term health.