The traditional sports nutrition doctrine says endurance athletes need carbohydrates. Claire, a 38-year-old runner from Bristol, decided to test this assumption at the 2025 Bristol Half Marathon — and then the London Marathon.
Claire had been keto for 8 months before entering the London Marathon. Her training was entirely fat-fuelled. “Every long run, every tempo session — no gels, no energy drinks. Just water and electrolytes.”
“The first 6 weeks of keto running were hard. After that, something shifted. I could run for 2 hours without the hunger and energy fluctuations I’d experienced before. My recovery was noticeably better.”
No gels, no sports drinks. Electrolyte tablets dissolved in water every 5 miles. At mile 20 — where many runners hit the wall due to glycogen depletion — Claire felt strong. “I picked up pace in the last 6 miles. I’ve never experienced that before.”
4 hours 12 minutes — a personal best by 23 minutes. “Keto didn’t ruin my running. It transformed it.”
High protein and fat: salmon and beef-heavy dinners, morning keto breakfasts, and strategic use of MCT oil pre-long-run.
Fat-adapted endurance performance is not a myth. For those willing to commit through the adaptation period, the metabolic flexibility keto provides is a genuine competitive advantage.