Adaptation vs Performance Fueling: Finding the Balance for Optimal Endurance Performance

In the world of endurance sports, we tend to focus heavily on race-day nutrition - what gel to take, how often to sip your drink, which electrolyte mix gives you the best edge. And while those things matter (a lot), they’re only one half of the equation.

The other half? How you fuel your body in training to build the engine in the first place.

This is where the concept of adaptation vs performance fueling comes in and why it’s one of my favourite (and most misunderstood) topics in sports nutrition.

What is Adaptation Fueling?

Adaptation fueling is about training the body to become more efficient, metabolically flexible, resilient, and able to perform across a wider range of conditions. This means sometimes training with lower carbohydrate availability to:

Encourage fat adaptation
Enhance mitochondrial biogenesis (building more energy-making powerhouses)
Improve glycogen storage efficiency
Build overall metabolic flexibility

Essentially, we’re teaching the body to become a better engine, not just a faster one.

What is Performance Fueling?

Performance fueling, on the other hand, is about maximising output in the moment.

That means giving your body the fuel it needs to go harder, faster, and longer usually in the form of higher carbohydrate availability.

This is the strategy you want on race day.

The goal here is to reduce fatigue, preserve glycogen, and improve your pace, power, and recovery.

Why the Distinction Matters

Endurance athletes often fall into one of two camps:

“Train fasted, low-carb, or keto all the time.”
“Carbs all day, every day, for every session.”

Both extremes have their downsides.

Training low all the time can reduce your ability to hit high intensities, blunt your immune system, and cause under-recovery.

Training high all the time can limit your body’s ability to burn fat efficiently and make you overly reliant on carbs (and bonk risk).

The real magic happens when you periodise your nutrition, just like you do your training.

Adaptation Fuelling vs Performance Fueling for Endurance

Strategic Low-Carb Training: When and Why

Training with low carbohydrate availability can be done in several ways:

Fast overnight and easy morning session
Back-to-back sessions without full refueling
‘Sleep low’ method (no carbs post-training, then a fasted AM session)

These sessions are typically lower in intensity and shorter in duration designed to trigger adaptation, not crush you.

I don’t recommend doing these strategies willy-nilly. They need to be planned and paired with the right sessions to be effective and safe.

Strategic High-Carb Training: When and Why

When your sessions are:

Long (>90 min)
High intensity or intervals
Race simulations
Back-to-back high-demand days

That’s when you want to go all-in on your carbs. This supports performance, reduces muscle breakdown, and improves training outcomes - especially if your session goal is speed, power, or recovery.

This is also your chance to train the gut so your stomach learns to tolerate race-day fuel under stress.

The Metabolic Flexibility Sweet Spot

Metabolic flexibility is the ability to switch between fuel sources:

burning carbs when you need to go hard, and
burning fat when you need to go long.

This is what we want.

Strategic fueling helps you build this flexibility in training, and harness it in competition.

Putting It Into Practice

Here’s how you might approach a week with both adaptation and performance fueling in mind:

Session Type	Fueling Strategy
Easy recovery ride	Low-carb / fasted
Strength or HIIT	Moderate-carb pre & post
Long run or brick	High-carb before and during
Tempo bike session	High-carb before, during & after
Easy swim	Low-carb, optional fasted

The key is intentionality. Fuel based on the purpose of the session, not a one-size-fits-all plan.

A Word of Caution

Low-carb training is a tool, not a lifestyle.
Used poorly, it can backfire.
Used well, it can unlock a new level of endurance, efficiency, and performance.

If you’ve got a history of disordered eating, hormonal imbalance, RED-S, or thyroid dysfunction, low-fuel training might not be appropriate. Always work with a qualified practitioner (hi, that’s me!) to tailor your approach.

UH endurance sport background