Energy Availability Explained: Why Stress, Training Load and Fuel All Matter
When athletes hear the word fuelling, many people immediately think:
“It’s just about calories, right?”
Calories matter — but energy availability is a much bigger picture.
It is not just about how much food you eat.
It is about how much energy your body has left after training to run everything else.
What is energy availability?
Energy availability (EA) is the amount of energy your body has available after exercise has been accounted for to support normal functions.
The simple way to think about it:
Food coming in → minus exercise energy used → what is left for your body to function
That remaining energy supports:
- Hormones
- Immune system
- Bone health
- Metabolism
- Recovery
- Brain function
- Muscle repair
- Adaptation to training
So two athletes can eat the same number of calories but have completely different energy availability.
Why?
Because their training load, body size, stress levels and recovery demands may be different.
It’s not just about calories eaten
Imagine two people both eat 2,500 calories.
Athlete A:
- Desk job
- 3 easy gym sessions per week
- Good sleep
- Low stress
They may have plenty of energy available.
Athlete B:
- Full-time job
- High life stress
- 10 hours of training per week
- Poor sleep
- Hard sessions
The same 2,500 calories may not be enough.
Their body has more demands competing for that energy.
Training is only one type of stress
Many athletes forget that training is not happening in isolation.
Your body does not have separate “stress buckets”.
A hard training session, a stressful job, poor sleep and emotional stress all contribute to the total load your body experiences.
Think of your body like a bank account.
You have energy coming in.
You have energy going out.
The withdrawals are not just workouts.
They include:
Training stress
- Running
- Cycling
- Swimming
- Strength training
- Racing
Life stress
- Busy work periods
- Exams
- Travel
- Relationships
- Financial pressure
Recovery stress
- Poor sleep
- Illness
- Not enough rest days
When withdrawals keep exceeding deposits, your body starts making adjustments.
How does outside stress affect training?
You might notice:
Same workout feels harder
A session that normally feels easy suddenly feels exhausting.
Recovery slows
You feel sore for longer.
Your legs feel heavy.
Your motivation drops.
Performance plateaus
You train more but stop improving.
Your body becomes protective
When energy availability is low, your body prioritises survival functions.
It may reduce investment in areas that are not immediately essential for survival — including some aspects of reproduction, recovery and adaptation.
Why do some people need more fuel than others?
Because every athlete has a different demand.
Your fuelling needs depend on:
1. Training volume
Someone running 20 minutes three times per week has a different requirement from someone preparing for an ultra marathon.
More training = more energy used.
2. Training intensity
Not all exercise costs the same.
A hard interval session, race simulation or long ride creates a bigger demand than an easy recovery session.
3. Body size and muscle mass
Larger bodies generally require more energy.
More muscle tissue also increases energy needs.
4. Everyday movement
Two athletes can complete the same workout but have different daily demands.
One might:
- Sit most of the day
Another might:
- Walk 15,000 steps
- Have a physical job
- Stand all day
Their total energy needs are different.
5. Stress and recovery
This is the one many people miss.
Your body needs energy not only to move.
It needs energy to repair.
Poor sleep and high stress can increase the challenge of recovering from training.
Energy availability and RED-S
When energy availability stays too low for too long, it can contribute to Relative Energy Deficiency in Sport (RED-S).
RED-S is not simply:
“I didn’t eat enough today.”
It is a mismatch between:
- The energy your body needs
- The energy available after exercise
It can affect:
Performance
- Reduced endurance
- Lower strength
- Poor adaptation
- Increased fatigue
Health
- Bone health
- Hormonal function
- Immune health
- Recovery
This can happen to athletes of any level.
You do not have to be elite.
You do not have to be visibly underweight.
You simply need a period where the demands placed on your body exceed the fuel available.
Why carbohydrates matter in energy availability
Carbohydrates are often the first thing athletes cut.
But carbohydrate availability affects your ability to train.
Low carbohydrate availability can mean:
- Lower glycogen stores
- Harder sessions
- Higher perceived effort
- Slower recovery
Your body can use fat as fuel, but high-intensity exercise relies heavily on carbohydrate.
That is why endurance athletes, runners, cyclists and team sport athletes need carbohydrate in their fuelling strategy.
A better question than “How many calories should I eat?”
Instead ask:
“Is my body getting enough fuel to support the life and training I am asking it to do?”
Look at:
✅ Energy levels
✅ Training quality
✅ Recovery
✅ Sleep
✅ Mood
✅ Performance
✅ Injury patterns
✅ Hunger signals
Your body gives you feedback.
The athlete takeaway
Fuelling is not just about avoiding weight gain.
It is about creating enough energy availability so your body can:
Train → Recover → Adapt → Improve
The goal is not to eat the least amount possible while still training.
The goal is to provide your body with enough fuel to handle the demands you place on it.
Because performance is not built during the workout.
It is built when your body has enough energy to recover from the workout.
