Signs You Are Underfueling as a Runner (Low Energy Availability Explained)
Endurance athletes — including runners, cyclists, and triathletes — have higher energy needs than most people. If you’re not eating enough to support your training, you may be experiencing low energy availability (LEA).
This is one of the most common — and overlooked — issues in running nutrition.
In this article, you’ll learn:
The key signs of underfueling as a runner
How low energy availability affects your body and performance
What to do to start fuelling properly
What is Low Energy Availability?
Low Energy Availability (LEA) occurs when your energy intake (calories and carbohydrates) is too low to support both training and normal body functions.
Importantly:You do not have to be underweight to be underfueling.
When energy is too low, the body reduces energy to systems like:
Hormones
Bone health
Recovery and adaptation
LEA is the underlying cause of Relative Energy Deficiency in Sport (REDs), a clinical condition requiring medical support.
Signs You May Be Underfueling as a Runner
Hormonal changes
Loss of period (females, >3 months)
Reduced testosterone, libido, or morning erections (males)
Bone health issues
Bone stress injuries or fractures
Low bone mineral density
Performance decline
Constant fatigue
Reduced endurance
Poor recovery
Decreased strength
Metabolic changes
Feeling cold often
Slowed metabolism
Hormonal imbalances (e.g. elevated cortisol)
Other symptoms
Digestive issues
Poor sleep
Irritability or anxiety
Frequent illness or slow healing
Disordered eating patterns
Restrictive eating
Fear around food
Focus on weight or body composition
How to Fix Underfueling
If you suspect low energy availability, start with these key strategies:
Eat regularly (including rest days)
Aim for:
3 main meals
2–3 snacks daily
2. Fuel before every run
Choose easy-to-digest carbohydrates:
Banana, toast, cereal, rice
Sports drinks, gels, or lollies
3. Fuel during long runs
For sessions over 90 minutes:Aim for 40–120g of carbs per hour
4. Prioritise recovery
Eat within 60 minutes post-run.A smoothie or flavoured milk is a simple option if you’re not ready for a full meal.
5. Adjust training if needed
Reduce double sessions and prioritise recovery if symptoms are present.
When to Seek Help
Speak to a GP or sports dietitian if you experience:
Loss of period for >3 months
Recurrent injuries
Hair loss, easy bruising, or frequent illness
Early support can prevent progression to REDs.
Final Thoughts
Underfueling is common in runners — but often missed.
The good news:It is completely reversible with the right nutrition approach.
Fueling your body properly will help you:
Run stronger
Recover faster
Support long-term health
Need Help With Your Running Nutrition?
If you want a personalised plan to fuel your training and avoid gut or energy issues:
Book a 1:1 consultation below
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