Most runners assume that logging more miles automatically makes them better fat burners. The science says otherwise.

In this episode, Joost sits down with New Zealand-based sports dietitian and researcher Mikki Williden (PhD, public health) to unpack one of the most misunderstood topics in endurance sports: metabolic flexibility. Mikki has a 2:55 marathon PB, has raced the Boston Marathon, the Southern Lakes Ultra, and the Grand to Grand Ultra — and she's spent 15+ years working with athletes on exactly this.

What you'll learn:

• Why diet — not training volume — is the #1 driver of fat oxidation
• The difference between low-carb and ketogenic for endurance athletes
• How to time carbohydrates across your training week for better metabolic flexibility
• What the research by Dr. Prins, Noakes & Kutnick on 10g of carbs/hour means for your race fueling
• What "brain bonking" actually is — and how a small amount of glucose prevents it
• Why endurance athletes need more protein than they think (and how to get it in)
• How to lose body fat without wrecking your training
• Electrolytes on low-carb: why sodium needs spike and what to do about it

Key takeaway: Metabolic flexibility isn't about going keto. It's about shifting your daily diet to train your body to access fat at low intensities — so you can spare carbohydrates for when it really counts.

About Mikki Williden:Mikki is a registered sports dietitian and PhD researcher based in New Zealand. She hosts the podcast Mikkipedia, where she interviews researchers and practitioners on nutrition, performance, and metabolic health. Find her at mikkiwilliden.com or on Instagram @mikkiwilliden.

Free coaching call:Want to apply these principles to your own marathon training? Book a free call with Joost at cal.eu/pacebuddies/freecall.