Fat Adaptation for Endurance Athletes: How Adam Szewc Fuels Ultra Performance on Keto/Carnivore

Originally featured in an interview with The Carnivore Bar; expanded for Fuel Fantastic. Credit to The Carnivore Bar and author Kathryn Kos for the original article.

Quick Take

Fat adaptation teaches the body to rely primarily on fat for fuel instead of constantly topping up carbs. For many endurance athletes, that can mean steadier energy, fewer GI issues, and more predictable performance on long outings. Research on long-term keto-adapted athletes shows very high rates of fat oxidation with muscle glycogen use and repletion similar to high-carb athletes during long efforts.

Adam’s Story: Ultra Goals, T1D, and Finding a Better Fuel

I’m Adam Szewc, an endurance athlete living with Type 1 diabetes. Transitioning from a standard high-carb approach to a keto/carnivore pattern simplified my fueling and helped stabilize blood sugar on long runs and stacked training weeks. Instead of micromanaging gels and insulin, I can focus on pacing, terrain, and the work—especially on 6–8 hour efforts—without the same fear of sudden crashes.

Fuel Fantastic grew out of this experience: practical, science-aware nutrition that supports real performance, recovery, and metabolic control.

“Fat adaptation gave me steadier energy and simpler fueling on long days.†— Adam Szewc

What Is Fat Adaptation?

Fat adaptation is the metabolic shift where your body becomes highly efficient at oxidizing fat (including body fat) for energy during endurance exercise. In long-term keto-adapted athletes, measured fat-burning rates during submax efforts are exceptionally high, while glycogen usage and restoration after multi hour training can look similar to high-carb athletes during long efforts.

Key upsides many athletes report:

• Steadier energy over long durations (less “roller-coaster†fueling).
• Fewer GI issues from constant gels/drinks.
• Simpler logistics on big training days and ultras.

A balanced perspective:

Short, abrupt low-carb high-fat (LCHF) phases can spike fat oxidation quickly but may impair economy at higher intensities. Translation: don’t expect a one week switch to feel magical if you’re targeting top‟end speed soon. Build the base first.

The Science in Plain English

• Long-term adaptation matters. Ultra runners chronically adapted to low-carb show very high fat burn rates without compromising glycogen status in long submax efforts.
• Mechanisms include greater mitochondrial efficiency and a transcriptomic shift toward fat oxidation pathways with habitual LCHF intake.
• Nuance at high intensities. Brief LCHF stints (days) can increase fat use but may raise oxygen cost and blunt all out performance — so time your nutrition with your race demands.

How I Made the Switch (and What I Tell Athletes)

1. Gradual carb taper over weeks, not days. Start with easy aerobic sessions while carbs come down.
2. Prioritize animal based foods (meat, eggs) and quality fats; keep protein sufficient for recovery.
3. Electrolytes & hydration — critical in the first month to avoid “keto flu†feelings.
4. Build your aerobic base: lots of Zone 2, some fasted easy runs/rides to reinforce fat use.
5. Rehearse race fueling: even fat adapted athletes sometimes take strategic carbs during racing; practice it.

Sample Keto/Carnivore Day for Endurance Training

• Morning (easy aerobic): Coffee + electrolytes; optional fasted session.
• Mid day: Ribeye or burger patties, eggs, salt.
• Pre PM session: Small portion of dairy or berries if tolerated (individual).
• Dinner: Ruminant meat (fatty cuts), bone broth, salt.
• All day: Sodium, potassium, magnesium dialed to sweat rate.

(Medical note: athletes with diabetes must coordinate changes with their clinical team and monitor closely.)

When to Add Carbs Anyway

If your event has repeated high intensity surges, a targeted small carb strategy can be layered on top of a fat adapted base. Practice it in training to avoid GI surprises.

Frequently Asked Questions

Does fat adaptation mean I’ll never use carbs?
No. It means your default is fat, and you selectively layer carbs based on the demands of the day.

Will my high intensity suffer?
In the very short term or if you only partially adapt — possibly. With a proper base and smart timing, many athletes perform well across intensities.

Is there research behind this?
Yes — both mechanistic and performance‟context studies, including the FASTER work on long‟term keto adapted athletes.

Work With Me

If you want help transitioning, troubleshooting electrolytes/GI, or building a hybrid fueling plan for your race calendar, reach out via Fuel Fantastic (coaching & consults).

Credit

This blog expands on an interview originally published on The Carnivore Bar blog, written by Kathryn Kos. Thanks to Kathryn and The Carnivore Bar team for featuring my story. Check out the original article

here.