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In this "Ask Me Anything" (AMA) episode, Peter explores the effects of GLP-1 receptor agonists—including drugs such as Ozempic, Wegovy, and Zepbound—with a particular focus on their impact on muscle. He examines how much lean mass people actually lose during treatment and how those changes compare to weight loss achieved through other methods, while explaining why measurements of lean mass on DEXA scans can sometimes be misleading. Peter discusses the effects of these medications on bone mass, fracture risk, strength, and physical function, highlighting why functional outcomes may be more important than body composition metrics alone. He also explores how GLP-1–based therapies affect different fat depots throughout the body, identifies who may be most vulnerable to muscle loss, and outlines practical strategies for preserving muscle and bone health while using these medications. Finally, Peter reviews early insights into retatrutide, a next-generation weight-loss therapy, including what current evidence suggests about its effects on weight loss and muscle mass.

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We discuss:

• The evolution of GLP-1 receptor agonists from diabetes drugs to breakthrough weight-loss therapies [1:45];
• Early concerns about lean mass loss with GLP-1 receptor agonists and the limitations of clinical trial data [3:45];
• How newer research has changed the understanding of lean mass loss on GLP-1 receptor agonists and why DEXA measurements can misrepresent muscle loss [6:15];
• Comparing lean mass loss across semaglutide, tirzepatide, and traditional weight-loss interventions [10:30];
• Comparing lean mass loss from GLP-1 receptor agonists with bariatric surgery, and whether these drugs cause muscle loss beyond normal expectations from substantial weight loss [13:15];
• The limited evidence regarding the timing of lean mass loss during GLP-1 therapy and the implications for exercise and nutrition strategies [16:00];
• Body composition changes after stopping GLP-1 receptor agonists: weight regain, fat regain, and lean mass recovery [17:45];
• Why lean mass measurements are an imperfect proxy for muscle health and function [21:45];
• The effects of GLP-1 receptor agonists on bone mineral density, fracture risk, and the importance of resistance training [23:00];
• Do GLP-1 receptor agonists directly cause muscle loss or simply mimic the effects of calorie restriction? [26:00];
• Why strength and physical function often improve despite lean mass loss on GLP-1 receptor agonists [28:00];
• Who is most at risk for lean mass loss during GLP-1–induced weight loss? [34:45];
• Intramuscular fat (IMAT), DEXA limitations, and the challenge of measuring true muscle loss [37:00];
• Preserving muscle while losing weight: resistance training, protein intake, and emerging research on preserving muscle during GLP-1–induced weight loss [39:00];
• Resistance-training principles for preserving lean mass during GLP-1–induced weight loss [43:45];
• Managing side effects and prioritizing protein intake while training on GLP-1 receptor agonists [46:15];
• Retatrutide: early evidence on its effects on weight loss, lean mass, and muscle function, as well as the limitations of the data being collected in ongoing clinical trials [48:00];
• The risks of using gray-market retatrutide before FDA approval [52:15];
• Key takeaways [54:30]; and
• More.

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