GLP-1 Agonists and the Musculoskeletal System: What Every Active Patient Needs to Know

GLP-1 receptor agonists — semaglutide (Ozempic, Wegovy), tirzepatide (Mounjaro, Zepbound), and related agents — have become among the most widely prescribed medications in the United States, driven by compelling efficacy data for weight reduction and cardiovascular risk. As an orthopaedic surgeon with a patient population that is increasingly on these medications, I want to discuss what the musculoskeletal literature says about what GLP-1 agonists do to bone and muscle — because the full picture is more nuanced than the weight-loss headline suggests.

The Weight Loss Composition Problem

Weight loss from GLP-1 agonists is real, substantial, and associated with meaningful cardiovascular benefit. But the composition of that weight loss matters clinically. Studies examining body composition during GLP-1 treatment consistently show that approximately 25–40% of total weight lost comes from lean mass — predominantly skeletal muscle — rather than fat. This is not unique to GLP-1 agonists; caloric restriction by any mechanism produces some lean mass loss. But the magnitude and rapidity of weight loss with these agents amplifies this effect, and the consequences for functional capacity and long-term metabolic health are real.

Muscle mass is the primary determinant of resting metabolic rate, of functional capacity, and of the reserve against the sarcopenia that drives frailty in older adults. Losing 25–30 lbs over 6 months when 7–10 lbs of that is muscle mass is a different metabolic event than losing the same weight primarily from fat. The former accelerates the very process — sarcopenia — that the musculoskeletal medicine community is most concerned about in aging adults.

Bone Density: The Emerging Signal

The bone density data on GLP-1 agonists is emerging and, at this point, signals a concern worth monitoring rather than a definitive risk. Several mechanisms are plausible: rapid weight loss reduces mechanical loading on bone — and bone density is exquisitely responsive to mechanical load. Reduced nutritional intake during GLP-1 treatment may impair calcium and protein intake. And some animal data suggests GLP-1 receptors on osteoblasts may play a direct role in bone turnover.

Observational data from semaglutide trials show modest reductions in bone mineral density at the hip and lumbar spine, within the range of what is typically seen with other rapid weight loss interventions. The clinical significance over 5–10 years of use is not yet established, but it is a conversation worth initiating — especially in women over 50 who are already in the higher-risk window for osteoporosis.

Tendon Health: A Signal Worth Watching

Emerging post-marketing data has raised questions about tendon health during GLP-1 use, with a pharmacovigilance analysis from 2024 identifying a possible association between GLP-1 agonist use and tendinopathy, particularly Achilles tendinopathy. The proposed mechanism involves the rapid change in mechanical loading as body weight decreases, combined with potential direct effects of GLP-1 receptors on tendon fibroblasts. This signal is preliminary and requires prospective study, but it is consistent with what is known about the sensitivity of tendons to rapid changes in the load environment.

What This Means in Clinical Practice

For patients on GLP-1 agonists who are also physically active, the practical recommendations are:

• Resistance training is not optional — it is essential: Progressive resistance training during GLP-1 therapy is the most effective mitigation strategy for lean mass loss. Clinical trial data suggests that active resistance training reduces the proportion of lean mass in total weight loss from ~40% to ~25% and preserves strength more effectively than aerobic activity alone.
• Protein intake needs deliberate attention: GLP-1 agonists significantly reduce appetite, and patients on these medications frequently undereat protein. A minimum of 1.2–1.6 g/kg/day of protein is the current recommendation for preserving lean mass during weight loss — a target that requires intentional planning when caloric intake is substantially reduced.
• Bone density baseline: For patients over 45 starting GLP-1 therapy, particularly women, a baseline DEXA scan before treatment and at 12–24 months provides a clinically useful reference point.
• Tendon symptoms get taken seriously: New tendon pain during GLP-1 therapy — particularly Achilles, patellar, or rotator cuff — warrants clinical evaluation rather than the "keep exercising and it will resolve" advice appropriate for pure mechanical tendinopathy.