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

Get the Daily Fitness Insider newsletter (free): https://www.miketnelson.com/newsletter