Episode 380: Breathing, CO2 Regulation, and Performance: Deep Dive with Respiratory & Sleep Scientist Martin McPhilimey

In this episode, I sit down with respiratory and sleep scientist Martin McPhilimey to take a deep dive into breathing, CO₂ regulation, and how they actually impact performance, stress, and sleep. We get into why respiratory physiology is way more complex than most people think, how his clinical and research background shaped his approach, and why breathwork exploded during COVID with names like Wim Hof and James Nestor. From there, we dig into allostatic load and why piling on high-stress practices—like aggressive hyperventilation or cold exposure—can backfire if you’re already running hot. We also talk about the difference between actually relaxing your system versus just dissociating from it. We cover how inspiratory muscle training can improve performance and recovery, why respiratory rate is one of the most sensitive markers of stress, and question some common ideas, such as “CO₂ tolerance.” We also touch on the risks and limited data around inhaled CO₂ therapies and what actually makes sense to apply in the real world. Sponsors: Daily Fitness Insider Newsletter: https://flex-diet.kit.com/bfa1510fa8 Available now: Grab a copy of the Triphasic Training II book I co-wrote with Cal Deitz here. Episode Chapters: 04:22 Why Breathing Is Complex 07:52 Martin’s Origin Story 11:20 From Clinics to Online 14:57 Breathwork Goes Mainstream 20:13 Wim Hof Risks and Load 24:06 When Intense Breathwork Fits 29:18 Relaxation vs Dissociation 31:30 Respiratory Muscle Training 35:12 Who Should Train Breathing 36:35 Simple Device Protocol 37:35 Why Teams Ignore Breathing 38:27 Respiratory Rate as Stress 39:52 Feedforward Breathing Control 42:22 Rethinking CO2 Tolerance 46:03 Nasal Breathing Tradeoffs 48:28 Testing Limits and HRV 50:48 Overnight Rate and Calm 54:07 CO2 Inhalation and Risks 56:45 State Encoding and Fear 01:00:35 Wrap Up and Disclaimers Flex Diet Podcasts you may enjoy: Episode 326: Row Hard, Breathe Easy: Boosting Aerobic Capacity with Dr. Kenneth Jay YouTube: https://youtu.be/hpvKokMBRv4 Episode 215: Breathing, Heart Rate, and HRV: An Interview with Dr. Scotty Butcher, The Strength Jedi YouTube: https://www.youtube.com/watch?v=nyVEmi7VHbg Connect with Martin: Instagram: https://www.instagram.com/martinmcphilimey/ Website: https://martinmcphilimey.com/ Get In Touch with Dr Mike: Instagram: Drmiketnelson YouTube: @flexdietcert Email: Miketnelson.com/contact-us