Why more athletes are turning to fat as a primary fuel source — and what the research actually says.
Endurance is Evolving
For decades, endurance nutrition has been synonymous with high-carb fueling: energy gels, pasta parties, and hourly glucose hits. But a growing number of ultra-athletes are flipping the script — turning instead to fat-adapted performance models, often rooted in ketogenic or low-carbohydrate diets.
The question is no longer “Can it work?”
It’s “When, how, and for whom does it work best?”
One of the most pivotal studies on this topic — the FASTER study — shifted the conversation. Let’s break down what it found, and why it matters.
The F.A.S.T.E.R. Study: Key Takeaways
Published in 2016, the FASTER (Fat Adapted Substrate utilization in Trained Elite Runners) study compared two groups of elite endurance athletes:
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High-Carb (HC): 59% carbohydrate intake
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Low-Carb / Keto (LC): <10% carbohydrate intake, 70%+ fat
Each group performed a 3-hour treadmill test at 64% VO₂ max (a moderate, sustainable intensity for endurance).
What Did They Find?
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Fat Oxidation Rates
LC athletes showed twice the rate of fat oxidation (1.54g/min vs 0.67g/min in HC group) — the highest ever recorded in humans. -
Glycogen Sparing
Despite common concerns, muscle glycogen use was similar between both groups during endurance efforts — showing that being fat-adapted does not mean you “run out” of glycogen. -
Stable Performance Output
Performance during steady-state exercise remained comparable — proving low-carb fueling doesn’t compromise endurance at moderate intensities. -
Metabolic Adaptation Takes Time
All LC athletes had been on a strict low-carb protocol for at least 6 months, showing the need for true adaptation before performance benefits appear.
Interpreting the Science: What This Actually Means
Fat-adapted athletes can:
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Burn fat more efficiently, reducing reliance on external carbs
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Extend performance duration by sparing glycogen
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Avoid sugar crashes, GI distress, and reactive hypoglycemia
However, they may face:
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Challenges at high intensities, where glucose becomes essential
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An adaptation curve of 4–6 weeks with potential performance dips
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Nutritional complexity — it’s not just “cutting carbs,” it’s rebuilding metabolic pathways
Related reading: Glycogen Sparing: The Key to Going Further Without Crashing
Why This Matters for Ultra-Endurance
In races that last 10, 20, or even 35+ hours, energy management becomes a survival skill. The body can store about 2,000 kcal of glycogen, but over 40,000 kcal in fat reserves — even in lean athletes.
Being fat-adapted means unlocking access to that massive fuel tank — and using it with control and confidence.
It also means:
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Stable energy levels over hours
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Lower inflammation
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Fewer gastrointestinal issues, especially in high-heat, long-distance efforts
This is why more ultra runners, cyclists, and ski mountaineers — including Team Holyfat athletes — are adopting a ketogenic or hybrid fueling approach.
Related reading: Metabolic Flexibility: Why It's the Ultimate Performance Advantage
How Holyfat Fits In
Fueling on a fat-adapted protocol isn’t just about macros — it’s about choosing the right fats.
Holyfat products are:
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Low-carb and high in MCTs, which are rapidly absorbed and used for ketone production
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Designed for long-burning, clean energy
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Free from added sugars or synthetic fillers that can cause GI distress
Whether you’re fully keto or just reducing carb intake to train metabolic flexibility, Holyfat delivers fuel you can trust in motion.
Conclusion: Fuel Smarter, Last Longer
Keto is not a magic bullet. But for ultra-endurance athletes who understand the science and commit to the process, it opens up a powerful new dimension of fueling — one built on stability, clarity, and longevity.
Don’t just burn calories. Burn clean. Burn smart. Burn long.