Episode 7 - Fuel Utilisation Why Pace Changes What You Burn (and Why It Matters for Ultras)

This episode explains fuel utilisation in simple terms: your body is always using a mix of fat and carbohydrate, but the harder you run, the more you rely on carbs.

The key message is that many runners don’t “blow up” because they forgot to eat, they blow up because their pace created a higher carbohydrate demand than their fueling plan could support.

The episode uses a simple mental model of two fuel tanks:

• Fat tank = large, slower energy, supports easier efforts

• Carb tank = smaller, faster energy, increasingly important as intensity rises

It then shows how this appears in training and racing:

• Easy long runs often feel manageable

• Harder sessions, climbs, and surges can quickly increase carb demand and lead to fatigue if under-fueled

Common mistakes covered:

• Fueling by habit (same grams/hour for every run)

• Under-fueling key sessions to “train fat burning”

• Confusing training adaptations with race-day strategy

Practical advice:

• Match fueling to the session goal

• Practice race fueling in training

• Use pacing as part of your fueling strategy (surging early makes fueling harder)

Pace and fueling must work together: the harder the effort, the more carbohydrate you need to support it.

Main takeaway

Pace and fueling must work together: the harder the effort, the more carbohydrate you need to support it.

Key references:

• Jeukendrup (2014), Sports Medicine – carbohydrate intake during exercise, dose-response, multiple transportable carbohydrates, oxidation limits and practical recommendations.

• Wallis & Podlogar (2022), GSSI Sports Science Exchange – contemporary carbohydrate guidance for endurance athletes (before, during, after exercise; periodized carbohydrate intake).

• ISSN Position Stand on Ketogenic Diets (2024) – increased fat oxidation does not necessarily translate to improved endurance performance.