In Part 3 of our Clarissa’s Ride or Die – How to… series, you’ll discover how your metabolism actually works – and how you can influence whether your body burns fat or carbohydrates.
With us again: Clarissa and her epic 1000 km ride without any external support – a real-world energy experiment.
How does your body decide whether to burn fat or carbohydrates? At first glance, this sounds like a purely scientific question – but in reality, it can make or break your performance in ultra-events.
Our athlete Clarissa found out the answer firsthand.
She trained her fat metabolism with intention.
When does your body prefer to burn fat or carbohydrates – and how can you influence this choice?
In Part 1 of our Ride or Die – How to… series, we introduced her unbelievable achievement:
1000 kilometers – from Basel to Barcelona in 3.5 days.
No help. No money. No support.
No hotels. No cafés. No food stops.
Clarissa wasn’t allowed to accept anything – no water, no food, no shelter. Everything she needed had to be packed in advance or found along the way.
She slept outdoors – in forests, on benches, or beside her bike.
Her goal: push through, learn, and experience everything fully – with total self-reliance.
This wasn’t just an adventure – it was a full-body, full-mind metabolic stress test.
👉 Read Part 1 here
Whenever you move – whether casually or at high intensity – your body relies on two key fuel sources:
🧠 Carbs = Turbo
🧠 Fat = Diesel for the long haul
Being able to switch between fat or carbohydrates – known as metabolic flexibility – is a key factor in endurance performance.
There are four main physiological factors:
Why?
While fat yields more energy per gram, it’s slower. In high-intensity situations, your body needs instant energy – and that means carbs.
The longer your effort, the more glycogen you burn. Eventually, your body relies increasingly on fat reserves.
Goal: Conserve glycogen, increase fat usage.
Well-trained athletes burn more fat efficiently because they:
👉 You can train this through consistent low to moderate-intensity sessions.
Research shows that strategies like “Train Low” (Burke et al., 2022; Jeukendrup, 2023) significantly improve the body’s ability to burn fat.
📎 More on fat metabolism in women here
→ 60–70 % of your max heart rate
→ Easy pace = optimal fat burning
→ 60–90 min in the morning before breakfast
→ Consider a protein shake instead of carbs – preserves muscle and lowers cortisol
→ Important: No intervals – keep it low and steady!
→ Ideally through spirometry testing
→ You’ll discover when you burn fat vs. carbs most efficiently
💡 On a budget?
Try the Engine Check from Swiss Cycling – simple, effective, affordable.
👉 Here’s how it works
→ Can you still talk easily? → You’re burning fat
→ Breathing heavily? → You’re burning carbs
Whether you’re racing ultra distances, bikepacking, or just riding longer:
Your ability to burn fat instead of carbohydrates is key to long-term performance and energy stability.
It’s not about cutting carbs – it’s about making your metabolism flexible and resilient.
Fat for the journey. Carbs for the boost.
Just like Clarissa.
And just like you – with the right training and metabolic awareness.
Part 1: Ride or Die
👉 Why female athletes should (and must) train fat oxidation
Read now
Part 2: Hydration in Sport
👉 Balancing thirst, performance, and risk
Go to article
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