What does it take to finish one of the most extreme endurance races on the planet?

In this episode of the World Extreme Medicine Podcast, host Eoin Walker is joined by ultra-endurance athlete Sébastien Marlier, who recently completed the Bretzel Ultra Triathlon — a continuous event five times longer than a standard Ironman.

With 19km of swimming, 900km of cycling, and 211km of running, this race demands everything from the human body — and mind. Sébastien unpacks his physical and psychological preparation, from training his digestion system to handle real food, to using mental conditioning tools like the “thermometer” and “VACOG” techniques to stay grounded under pressure.

We explore:

• How he adapted his nutrition and hydration to avoid fatigue and GI distress

• Why mental resilience tools were key to finishing strong

• The role of ego, emotion, and community in ultra-endurance performance

• What he’d do differently next time (hint: sunscreen and compression socks!)

• Lessons from pain, recovery, and the post-race crash

  
  

Sébastien also shares a heartfelt thank-you to the two fellow athletes who helped him during the race:

• Shanda Hill (Canada), who offered advice and anaesthetic cream during a critical moment
• Goulwen Tristan (France), who shared essential hygiene tips on the bike course

Both went on to win this year’s Bretzel Ultra Triathlon, and their kindness helped Sébastien reach the finish line.

Since the recording, Sébastien has also completed the Double Ultratriathlon World Championships in Emsdetten, Germany, placing 7th overall with a time of 25h28min, and he’s just getting started.

His future plans include taking on the Deca Ironman in 2026 (10x the Ironman distance) and applying for the legendary Enduroman Arch to Arc challenge. Both will raise money to support children and teenagers who can’t afford access to sport.

Whether you’re an endurance athlete, medic, or just curious about the limits of human performance — this conversation is packed with practical insights and powerful reflection.

📌 Find out more about the Bretzel Ultra Triathlon here