Episode 387: Nathan T. Jenkins on Metabolic Flexibility, Aerobic Base, and First Principles

In this episode of the Flex Diet Podcast, I sit down with Dr. Nathan Jenkins to discuss how to improve performance, body composition, and long-term health through the lens of aerobic development and metabolic flexibility. Nathan shares his journey from exercise physiology and coaching CrossFit athletes to his new role teaching physiology at the University of Georgia School of Medicine. We discuss why metabolic flexibility is such a powerful framework for understanding everything from biochemistry and exercise performance to metabolic disease. We also dive into interpreting maximal exercise testing data, including VO₂ measurements, EKG analysis, and near-infrared spectroscopy (NIRS), along with how different energy systems work together during training and competition. Nathan explains why building an aerobic base is critical for repeated high-intensity performance, how pacing impacts outcomes, and why the ability to "suffer" is actually a trainable skill. To wrap up, we cover practical strategies you can apply right away, including prioritizing protein and carbohydrates, improving sleep quality, and addressing common micronutrient deficiencies through blood work, such as magnesium, vitamin D, and omega-3 status. 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: 02:47 Meet Dr Nathan Jenkins 04:38 CrossFit Nutrition Reality Check 07:09 Leaving Tenure for Family 09:21 Back to Academia and Med Ed 11:29 Why Exercise Physiology Matters 14:36 Stress Tests Reveal Pathology 18:19 Max Testing and NIRS Setup 22:42 Raw Data Over Machine Outputs 24:18 Bohr Effect and Oxygen Delivery 28:09 Energy Systems in the Real World 33:05 Coaching Basics That Matter 35:40 Teaching Cardio From First Principles 36:36 Aerobic Base Reality Check 37:19 Cardio for Meatheads Pitch 39:13 CrossFit Endurance Breakthroughs 41:49 Fixing Time Domain Weakness 45:46 Pacing Like Froning Fraser 48:42 RAAM Gamesmanship Story 51:45 Pain Management Truths 53:23 VO2 Max Feels the Same 55:54 Suffering as a Skill 59:09 Cold Plunge Mindset Study 01:00:52 Caffeine Placebo and Belief 01:03:22 Heat Acclimation and 10K Prep 01:04:36 Metabolic Flexibility Lens 01:08:16 Teaching Diabetes Integration 01:10:56 Bloodwork Meets Flexibility 01:12:37 Where Fat Goes Wrong 01:15:07 Sustainable Deficit Strategy 01:16:34 Four Priorities Blueprint 01:17:21 Protein and Carb Targets 01:21:08 Sleep and Micronutrients 01:27:06 Omega-3 Testing Nuance 01:29:36 Wrap Up and Next Steps 01:31:52 Newsletter and Flex Diet Cert 01:33:20 Final Thanks and Subscribe Flex Diet Podcasts you may enjoy: Episode 344: Metabolic Adaptations, Lactate, and Training Smarter with Dr. Phil Batterson YouTube: https://youtu.be/PPZyO1nxSPA Episode 383: Body Composition, Strength Training, and Sustainable Habits with Martin Silva YouTube: https://youtu.be/p8oM0gW488U Connect with Dr. Jenkins: Website: https://www.drnathanjenkins.com/ Instagram: https://www.instagram.com/drnathanjenkins 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