EP 63: GLP-1 Drugs and Muscle Loss with Dr. Stuart Phillips

Episode Summary GLP-1 medications like semaglutide are everywhere, from your doctor's office to the Super Bowl, and the conversation has moved well beyond clinical use. In this episode, Dr. Jeremy Bettle sits down with Dr. Stuart Phillips, one of the world's leading researchers in muscle metabolism and aging, to cut through the noise on weight loss drugs. They cover what these medications actually do in the body, why losing weight without resistance training and proper nutrition is a serious long-term risk, and how to protect your muscle mass, bone density, and metabolic health if you are going to use them. This is not a conversation about whether GLP-1s are good or bad. It is a conversation about using them responsibly, with the full picture.   Guest Bio Dr. Stuart Phillips is a Distinguished Professor of Kinesiology at McMaster University and director of the Physical Activity Centre of Excellence (PACE). His groundbreaking research explores muscle metabolism, protein needs, and aging, revealing the transformative power of strength training and nutrition. With 28 years of pioneering work in the field, Dr. Phillips is an evidence-based advocate for accessible, impactful interventions to improve healthspan and mobility.   Links Instagram: @mackinprof X: @mackinprof LinkedIn: search Stuart Phillips   Three Actionable Takeaways Be honest with yourself about why you want to take these drugs, and if it is aesthetic, commit to learning how to lift weights. Simply taking the drug without building the habits around strength training is ultimately not going to be helpful. Resistance exercise is the foundational piece that protects your muscle and bone while on a low-calorie budget, and starting that practice now is an investment that pays you back for decades. Pair any weight loss effort with nutrient-dense protein and consider working with a dietitian. When your appetite is suppressed, every bite counts more. Prioritizing protein-rich, micronutrient-dense foods over energy-dense, nutrient-poor choices is how you lose fat rather than muscle. Dietetic support is not a luxury here, it is one of the most practical things you can do to protect your long-term health. Treat this as a teachable moment, not a finish line. Whether you are taking GLP-1s for aesthetic reasons or genuine clinical need, the goal is to use the reduced food noise as a reset. Build the habits now, including the exercise, the nutrition, the hydration, and the monitoring, so that if and when you come off the drug, your body is stronger than when you started.   Key Insights GLP-1 is a naturally occurring hormone. Semaglutide mimics it, suppressing appetite centrally and slowing gastric emptying. Receptors for GLP-1 are found throughout the body including the brain, gut, heart, and vasculature, which is why the drug has wide-ranging effects beyond just appetite reduction. Up to 40% of the weight lost on GLP-1 trials is lean mass, not fat. Roughly half of that lean mass loss is muscle. In practical terms, some trial data suggests a year on these drugs can produce muscle loss equivalent to approximately a decade of normal aging. When you stop the drug and regain weight, you gain fat, not muscle. Weight regain after stopping GLP-1s is almost entirely body fat, which means you can end up in a worse metabolic position than when you started if you haven't built a protective base of muscle. Micronutrient deficiencies are a real and underappreciated risk. Cutting food intake dramatically means calcium, vitamin D, iron, vitamin B12, and magnesium can all drop to concerning levels. A food-first approach to nutrient density is preferable to relying on supplements alone. Bone density goes in the wrong direction without the right interventions. One drug-company-run trial looking at bone density showed it declined, not improved. Resistance exercise, calcium, vitamin D, and adequate protein are the three nutrients and the stimulus that bone requires. Dehydration is a common and overlooked side effect. These drugs suppress the desire to drink as well as eat. Monitoring hydration is important, particularly for those experiencing elevated resting heart rate while on the medication. Real-world discontinuation rates are high. One study of 700,000 people found that 50% of users were off the drugs within a year, primarily due to cost and side effects. Trial data does not reflect what actually happens in the general population. For resistance training, the rep range matters less than the intensity. Anywhere from 3 to 25 repetitions can build muscle and bone effectively, provided the final reps are genuinely hard, approximately 1 to 2 reps shy of failure. Consistency over time matters more than any specific protocol. Protein targets should be roughly double the standard RDA. The RDA of 0.8g per kg of body weight is too low for most adults, and especially for those in a caloric deficit. Dr. Phillips points to approximately 1.6g per kg (0.7g per pound) as a more appropriate target, combined with resistance exercise. The case for GLP-1s is strongest where the clinical need is clearest. For people with a BMI over 30 and additional metabolic risk factors, the benefits, including reductions in major cardiovascular events, are substantial and well supported. The risk-benefit picture looks very different for someone who simply wants to lose 10 to 15 pounds.