The science behind carbohydrate fuelling, RED-S prevention and how one marathon runner fuelled her way to a 3:15 finish
Names have been changed to protect our runner’s privacy.
You’re training for your first marathon, cycling event, HYROX, triathlon, competition or endurance challenge.
You start looking into nutrition.
And everywhere you look, you see one message:
Protein. Protein. Protein.
Protein bars. Protein shakes. High-protein snacks. Protein everything.
And yes — protein matters.
Protein plays an essential role in muscle repair, recovery and adaptation. After training, your body uses protein to help rebuild and strengthen muscle tissue.
But there is another part of performance nutrition that often gets forgotten:
Fuel.
Because athletes do not just need to recover from training.
They need enough energy to complete the training that creates the adaptation in the first place.
And one of the most important fuel sources for endurance and high-intensity exercise?
Carbohydrates.
Your muscles store carbohydrate as glycogen, which is one of the key energy sources used when you run, cycle, lift, sprint or train at higher intensities.
Without enough carbohydrate available, training can feel harder, recovery can suffer and your ability to consistently complete quality sessions can decline.
Protein helps you repair.
Carbohydrates help you perform.
You need both.
The research: Female endurance athletes know carbohydrates matter… but still aren’t fuelling enough
A 2025 study published in Nutrients explored the nutrition habits, behaviours and experiences of female endurance athletes to understand why so many athletes struggle to match their fuelling with their training demands.
The researchers used a mixed-method approach:
- 72 female endurance athletes completed weighed food diaries
- 20 athletes took part in interviews exploring their experiences, beliefs and barriers around nutrition
The biggest finding?
Athletes understood that carbohydrates were important.
They knew carbs fuel performance.
They knew nutrition affected training.
But many athletes were still not eating enough carbohydrate to match the demands of their training.
The problem was not a lack of knowledge.
It was the gap between knowing what to do and consistently applying it.
The carbohydrate gap: Training increases, but fuel doesn’t always follow
The study found that carbohydrate intake became increasingly inadequate as training demands increased.
On rest days:
- Only around half of athletes met carbohydrate recommendations
- Average carbohydrate intake was approximately 3g/kg/day, which sits at the lower end of recommendations for endurance athletes
As training volume increased, the gap became larger:
- Moderate training: approximately 1.4g/kg/day below recommendations
- High training: approximately 3.5g/kg/day below recommendations
- Very high training: approximately 5.5g/kg/day below recommendations
This means the athletes doing the most training were often the athletes who needed the most fuel — but were not increasing carbohydrate intake enough to support the work they were asking their bodies to do.
Your training load changes.
Your fuelling needs should change too.
The protein problem: It wasn’t that athletes weren’t eating protein
One of the most interesting findings from the research was that protein was not the missing piece.
Athletes were generally consuming enough protein.
Average protein intake was approximately:
1.7g/kg/day
This sits within the commonly recommended range for endurance athletes:
1.2–2.0g/kg/day
Athletes were prioritising:
Protein snacks.
Protein recovery foods.
Protein bars.
High-protein meals.
But carbohydrates — the nutrient that directly supports endurance training intensity and glycogen replacement — were often being overlooked.
This highlights an important point:
A high-protein diet does not automatically equal a high-performance diet.
Performance nutrition is not about choosing one nutrient.
It is about matching your nutrition to what your body is being asked to do.
Why are athletes underfuelling?
The research showed that athletes were rarely intentionally trying not to eat enough.
Many genuinely believed they were making good choices.
Several themes appeared.
Knowing what to do, but struggling to apply it
Many athletes understood that carbohydrates were important before hard sessions.
However, some underestimated the needs of shorter sessions, easier sessions or everyday training.
A common mindset was:
“I only need fuel if the session is long.”
But every training session creates stress.
Your body needs energy not only to complete the session, but also to recover, adapt and prepare for what comes next.
Fear around carbohydrates
Some athletes described complicated relationships with food.
This included:
- Fear around weight gain
- Guilt around certain foods
- Labelling foods as “good” or “bad”
- Pressure to become lighter for performance
However, endurance athletes often require more energy because they train more.
Reducing carbohydrates in an attempt to change body composition can unintentionally reduce:
- Training quality
- Recovery
- Adaptation
- Long-term consistency
The goal is not simply to become lighter.
The goal is to become stronger, healthier and able to train consistently.
Busy lifestyles
Many athletes were balancing:
Work.
Family.
Training.
Social commitments.
Food preparation became another challenge.
This is why practical fuelling options matter.
Good sports nutrition does not need to be complicated.
It needs to be realistic.
The RED-S connection: Why enough fuel matters
Underfuelling is not always obvious.
It does not always look like skipping meals.
Sometimes it looks like:
“I eat healthy.”
“I prioritise protein.”
“I train consistently.”
But the missing piece is enough energy to support the work being done.
Low energy availability (LEA) occurs when the body does not have enough energy available after exercise demands are considered.
Over time, this can contribute to Relative Energy Deficiency in Sport (RED-S).
RED-S can affect:
- Recovery
- Training adaptation
- Bone health
- Hormonal health
- Immune function
- Performance consistency
Fitness is not built from training alone.
Training creates the stimulus.
Fuel gives your body the resources to adapt.
Case study: How a first-time marathon runner fuelled her way to a 3:15 finish
Sally was preparing for her first marathon.
She was not an elite athlete with a private chef or a perfectly controlled training environment.
She was a normal runner balancing training with everyday life.
Her goal was simple:
Arrive at race day healthy, fuelled and confident.
She did not follow a restrictive approach.
She did not remove carbohydrates.
She did not obsessively track every calorie.
Instead, she focused on a simple strategy:
Fuel before training.
Fuel during longer sessions.
Recover afterwards.
The result?
Her first marathon finish:
3:15.
Breakfast fuelling: Food first, not coffee first
Many athletes start their morning with coffee and delay eating.
Coffee can absolutely be part of an athlete’s routine.
But it should not replace fuel.
Before longer runs, Sally prioritised carbohydrates first.
Her go-to breakfast was:
RED-SCUE Fuelling Jar
Made using:
- Cassava
- Plantain
- Whole-food carbohydrate sources
- Fibre
- Naturally occurring micronutrients
She enjoyed it with:
- Yoghurt
- Fruit
- Fluids
- Coffee afterwards if wanted
The aim was simple:
Start the session with available energy.
Why cassava and plantain?
RED-SCUE was built around the idea of carbohydrate diversity.
Carbohydrates do not have to come from the same sources every day.
Cassava and plantain are traditional carbohydrate staples used around the world, providing athletes with:
- Carbohydrate for training fuel
- Fibre
- Potassium
- Naturally occurring micronutrients
- Greater variety within the diet
Plantain is not simply a banana replacement.
Unlike relying only on refined sugars, plantain provides carbohydrate alongside naturally occurring nutrients and fibre.
This creates a food-first approach to fuelling.
Not just quick sugar.
Not just gels.
Real-food carbohydrates designed around athlete needs.
RED-SCUE Fuelling Jar: A practical carbohydrate-first breakfast
The Fuelling Jar was designed as an easy alternative to traditional breakfast options such as:
- Chia seed puddings
- Overnight oats
- Complicated meal preparation
It is designed to be:
- Easy to transport
- Convenient for early mornings
- Practical for commuting
- Useful for race travel and training camps
A simple way to get carbohydrate in before training.
Before training: Build your fuel stores
For longer or harder sessions, eating carbohydrates beforehand can help:
- Top up glycogen stores
- Improve training quality
- Reduce early fatigue
- Support performance
A simple example:
RED-SCUE Fuelling Jar + yoghurt + fruit
Starting fuelled means you are not asking your body to perform on empty.
During training: Practise fuelling before you need it
Fuelling is a skill.
Just like you train your muscles to run further, you train your gut to tolerate carbohydrate during exercise.
Many athletes wait until they feel tired before taking fuel.
But by that point, fatigue has already started.
A better approach?
Fuel before you need it.
Practising during training helps you:
- Find what works for you
- Train your gut
- Reduce the chance of energy crashes
- Build confidence before race day
For sessions lasting longer than 60–90 minutes, carbohydrate during exercise becomes increasingly important.
Current sports nutrition guidance commonly recommends:
30–60g carbohydrate per hour
for many endurance sessions.
Longer events may require higher intakes depending on the athlete, intensity and duration.
Sally’s long run fuelling strategy
During longer runs, Sally used:
RED-SCUE Energy Bars
Each bar provides approximately:
38g carbohydrates
Made with:
- Cassava
- Plantain
- Whole-food ingredients
- Fibre
- Micronutrients
Rather than waiting until she felt depleted, she practised taking fuel consistently during longer sessions.
Her goal was not to rescue a bad run.
Her goal was to prevent arriving at that point.
Why choose RED-SCUE instead of only gels?
Gels can be a useful option for many athletes.
But they are not the only way to fuel.
Some athletes prefer food-first fuelling because it provides:
- More familiar ingredients
- Greater carbohydrate diversity
- A different texture and experience
- A less processed approach
RED-SCUE is not designed around a restrictive, low-carb or “less is more” approach.
It is designed around giving athletes practical carbohydrate fuel.
RED-SCUE Energy Bars
Available flavours:
Non-cacao options
Vanilla Cinnamon
Lemon Drizzle
Orange Zest
Cacao options
Chocolate Chip
Salted Caramel
Cacao-based flavours
Each bar provides approximately:
38g carbohydrates
Built around:
Cassava.
Plantain.
Whole-food carbohydrate sources.
After training: Recovery is carbohydrates + protein
The common message after training is:
“Get your protein in.”
And protein matters.
But recovery requires more.
Your body needs:
Protein
To support:
- Muscle repair
- Adaptation
Carbohydrates
To:
- Restore glycogen
- Prepare for your next session
- Support recovery
A simple recovery option:
RED-SCUE Fuelling Jar + yoghurt/protein source + fruit
Followed by a balanced meal containing:
- Carbohydrates
- Protein
- Healthy fats
- Micronutrients
Why fuelling supports consistency and performance
Underfuelling does not only affect race day.
It affects the training that gets you there.
When your body does not have enough energy available, it can impact:
- Recovery between sessions
- Training quality
- Adaptation
- Ability to stay consistent
The athletes who improve are not always the ones who train the hardest.
They are often the ones who can train consistently.
And consistency requires fuel.
Moving away from restrictive sports nutrition
The current nutrition world can feel confusing.
High protein.
Low carb.
Low calorie.
Cutting foods.
Trying to be smaller.
But endurance training is not built on restriction.
Your body needs enough energy to:
Train.
Recover.
Adapt.
Perform.
Stay healthy.
Eating enough is not a lack of discipline.
It is part of being an athlete.
The biggest lessons from the research and Sally’s marathon
The research teaches us:
1. Knowing how to fuel and fuelling enough are different things.
Athletes can understand nutrition and still unintentionally underfuel.
2. More training requires more fuel.
Your carbohydrate needs should increase with your training demands.
3. Fuelling is a skill.
Practise before race day.
4. Food-first fuelling can be simple.
You do not need complicated tracking.
You need reliable habits.
The simple athlete fuelling formula
Before training
Carbohydrates → fuel the session
Example:
RED-SCUE Fuelling Jar
During longer sessions
Carbohydrates → maintain energy
Example:
RED-SCUE Energy Bar
After training
Carbohydrates + protein → recover and adapt
Example:
RED-SCUE Fuelling Jar + yoghurt/protein source
Stop asking:
“Am I eating enough protein?”
Start asking:
“Am I fuelling enough for the training I’m doing?”
Because endurance athletes do not run on protein alone.
They run on fuel.
Fuel first. Perform better. Recover stronger.
RED-SCUE Energy Bars & Fuelling Jars
Carbohydrate-first fuel built for athletes.
