Your physiology doesn't always follow what the textbooks say. In this episode, we sit down with Dr. Marco Altini (ultra runner, endurance coach, data scientist, and thinker at the intersection of physiology, technology, and performance). Marco opens up about the moment he realized that doing the same high-carb diet he recommended to himself for years wasn’t actually working as well as he thought. He walks us through how shifting to a periodized approach - changing fueling based on training demands rather than sticking to a fixed carb ratio - improved his fat oxidation, gut health, and performance markers in ways he hadn’t seen before. Marco grounds the discussion in real data, including how his substrate oxidation changed after adjusting his diet and why better metabolic flexibility matters for endurance athletes. Along the way, we tackle common misconceptions about high-carb diets, the link between GI distress and fueling strategy, and why testing beats guessing when you want to understand your own response to food and training.

What we cover in this episode:

• Why metabolic efficiency is not a single target, but a moving range
• How nutrition periodization can support training without wrecking your gut
• The overlooked connection between GI distress and everyday dietary patterns
• What Marco learned from tracking substrate oxidation instead of guessing
• How fat oxidation and VO₂-related metrics can change with nutrition shifts
• Why weight loss is not the same thing as improved performance
• The role of testing in understanding what your body actually responds to
• How to think critically about fueling without copying elite or influencer strategies

Links:

• Follow Dr. Altini's Substack and find him on Instagram @altini_marco
• Marco's Substack article "Periodized Nutrition and Metabolic Flexbility for Endurance Performance"
• Marco's Substack article "Learnings from a Year of Periodized Nutrition and Metabolic Testing"
• Bob's Metabolic Efficiency Training book (3rd edition)

More about our guest: 

Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. cum laude in human movement sciences and high-performance coaching. He has published more than 50 papers and patents at the intersection between physiology, health, technology and human performance. He is the co-founder of HRV4Training, Endurance Coach at Destination Unknown, advisor at Oura, guest lecturer at VU Amsterdam, and editor of the Wearables department of IEEE Pervasive Computing. He loves running.

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