Heavy legs, declining performance and slow recovery are easy to blame on a demanding training plan. But sometimes the problem is not how much you are training – it is how little energy your body has left to adapt to it.
Training is supposed to make you tired. A long run should feel like work, and a hard interval session may stay in your legs the following day.
But there is a difference between the temporary fatigue that follows a challenging session and the persistent exhaustion that appears when your body is not receiving enough energy.
For runners, under-fuelling can be surprisingly easy to miss. Your weekly mileage increases, but your meals remain the same. You squeeze a run in before breakfast, rush into work and do not eat properly until lunch. You focus on protein but forget that carbohydrate is the primary fuel for harder running. None of it may feel particularly extreme.
Over time, however, the gap between the energy you use and the energy you replace can begin to affect your performance, recovery and health.
The short version
One difficult run does not mean you are under-fuelled. Look for a pattern: easy runs feeling unusually hard, recovery taking longer, performance declining, repeated illness or injury, changes to your menstrual cycle or sex drive, low mood, poor concentration or persistent fatigue that follows you beyond training.
These symptoms can have several causes, so they should not be used to diagnose yourself. But if several are happening together, it is worth reviewing both your nutrition and your training load – and speaking to an appropriate healthcare professional.
What does under-fuelling actually mean?
Every run uses energy, but exercise is only one item on your body’s daily bill. Energy is also needed to repair tissue, maintain bone health, support the immune system, regulate hormones and keep the brain and other organs functioning normally.
Low energy availability, often shortened to LEA, occurs when there is not enough dietary energy left to support these functions after the energy used during exercise has been accounted for.
Short periods of lower energy availability are not automatically a medical condition. The concern is problematic LEA: a mismatch that is severe, prolonged or repeated often enough to affect health and performance. Sustained problematic LEA can contribute to Relative Energy Deficiency in Sport, known as REDs or RED-S.
According to the International Olympic Committee, REDs can affect both female and male athletes. It may influence areas including bone health, immunity, metabolism, hormonal function, psychological health, recovery and athletic performance.
Importantly, under-fuelling is not always intentional and is not always connected to an eating disorder. It can happen when training increases without food intake increasing alongside it, when runners have limited time to prepare meals, or when they simply underestimate how much energy their training requires.
It can also happen at any body size. A stable weight does not prove that a runner is eating enough; the body can conserve energy and maintain weight while reducing the energy available for other processes.
Normal training fatigue or something more?
| Normal response to hard training | Possible under-fuelling pattern |
|---|---|
| Fatigue follows a clearly demanding session or training block | Fatigue begins affecting easy runs and everyday life |
| You improve after suitable food, sleep and recovery | You remain unusually tired despite taking normal recovery measures |
| Performance returns as you recover | Pace, power or motivation continue to decline |
| A minor niggle settles with appropriate management | Injuries, illnesses or gut problems repeatedly return |
| Your mood and concentration are broadly normal | Irritability, low mood or poor concentration become more frequent |
This table is a guide, not a diagnostic tool. Persistent fatigue can also be related to iron deficiency, illness, poor sleep, stress, thyroid conditions and several other issues that may require medical assessment.
8 signs runners commonly miss
1. Easy runs no longer feel easy
Every runner has off days. But if familiar routes and comfortable paces repeatedly feel harder than expected, your body may be struggling to meet the demands of training.
You might notice that your heart rate is higher at your usual pace, your legs feel flat from the start or you need much longer to settle into a run. Weather, stress and sleep can all influence this, but consistently poor sessions deserve attention – especially when your training has increased and your nutrition has not.
2. You are training more but performing worse
More training only produces improvement when the body has enough time and energy to adapt. Without that support, adding mileage can result in declining rather than improving performance.
Possible signs include repeatedly missing target paces, losing your finishing speed, feeling weaker in the gym or making little progress despite completing the planned work. It is easy to respond by pushing harder, but that can deepen the problem if insufficient energy is the real cause.
3. Recovery takes longer than it used to
Heavy legs after a long run are expected. Feeling as though every session remains in your body for several days is different.
Under-fuelling can reduce the resources available for replenishing glycogen and repairing muscle and connective tissue. You may notice persistent soreness, a lack of freshness between sessions or a growing need to shorten workouts because you have not recovered from the previous one.
4. Niggles, injuries or illnesses keep returning
Recurrent injuries and illnesses are recognised warning signs of low energy availability. Problematic LEA can affect bone health, immunity and the body’s response to training, potentially increasing the risk of bone stress injuries and prolonging recovery.
One cold or one sore calf does not establish a pattern. Repeated infections, recurring pain or an injury that is taking unusually long to resolve should be investigated rather than accepted as an inevitable part of running.
5. Your hunger – or your relationship with food – changes
Some under-fuelled runners feel constantly hungry. Others experience a reduced appetite, particularly after intense training, and find it difficult to replace the energy they have used.
You may also find yourself thinking about food more frequently, feeling anxious about eating on rest days or trying to “earn” meals through exercise. These experiences deserve support. Under-fuelling can be accidental, but it can also overlap with disordered eating, and the solution is not always as simple as adding another snack.
6. Your mood, motivation or concentration drops
Fuel supports the brain as well as the working muscles. Low mood, irritability, poor concentration and a loss of enjoyment in training are all associated with problematic low energy availability.
This can create a difficult cycle: training feels worse, confidence falls and the runner assumes they need to become more disciplined. In reality, the more productive response may be additional food, more recovery or professional support.
7. Your menstrual cycle or sex drive changes
Irregular or absent periods can be an important warning sign for runners who naturally menstruate. Although cycle changes can have several causes, they should not be dismissed as a normal consequence of serious training. Hormonal contraception can also make menstrual changes more difficult to interpret.
In runners of any sex, a noticeable reduction in libido can be another possible sign that the body is conserving energy. These changes are worth discussing with a GP or sports-medicine professional, particularly when they occur alongside fatigue, injury or declining performance.
8. Your weight is stable, so you assume everything is fine
Under-fuelling does not always cause obvious weight loss. The body may adapt by reducing energy use elsewhere, which means weight can remain relatively stable while health, recovery or performance begins to suffer.
This is why the scale is a poor screening tool on its own. How you feel, recover, perform and function matters more than whether your weight has changed.
Why runners are particularly vulnerable
Distance running creates a combination of high energy expenditure and a culture that can sometimes reward doing more while eating less. Under-fuelling may begin when:
- weekly mileage or intensity increases without a corresponding change in food intake;
- early-morning runs regularly replace breakfast;
- busy schedules leave long gaps between meals;
- runners remove foods or entire food groups without suitable replacements;
- protein receives all the attention while carbohydrate intake falls too low;
- appetite is suppressed after hard or hot-weather training;
- weight, body composition or “running light” becomes a major focus; or
- social-media advice encourages fasting, restriction or copying another athlete’s diet.
Not every fasted run is harmful, and not every runner needs to eat during every session. The wider pattern matters: the length and intensity of the run, the rest of the day’s intake, the overall training load and the runner’s individual health.
What to do if you think you may be under-fuelling
Match your intake to your training
Your food intake should not look identical on every day if your training does not. Long runs, hard intervals and high-volume weeks generally require more energy – particularly more carbohydrate – than rest days or short, easy sessions.
You do not need to count every calorie to respond. Begin by noticing whether food increases when training increases, or whether you are asking more from your body while giving it exactly the same amount.
Build fuel around your runs
A practical starting point is to support the sessions that place the greatest demand on you:
- Before: Eat a familiar carbohydrate-rich meal or snack before long or hard sessions. Toast, a banana, cereal, a bagel or an oat bar can all be useful, depending on timing and tolerance.
- During: For harder running lasting longer than around an hour, carbohydrate during the session can support performance. Established sports-nutrition guidance commonly recommends around 30–60g per hour for exercise lasting roughly one to two-and-a-half hours, adjusted for the session and individual tolerance. Longer endurance events may require a more personalised strategy.
- After: Follow demanding runs with carbohydrate to replenish energy stores and protein to support repair. If a full meal is not available soon, use a practical snack first – such as yoghurt and fruit, cereal with milk, a sandwich or a smoothie – then eat a balanced meal.
These are broad guidelines rather than an individual prescription. Your needs will vary with body size, training volume, session intensity, goals and medical history.
Avoid long, accidental gaps without food
Regular meals and snacks can make it easier to meet a runner’s energy requirements than trying to replace everything at dinner. This is particularly important for people training before work, running at lunchtime or completing more than one session in a day.
Prepare for the moments when convenience usually wins: keep an easy breakfast ready, pack a post-run snack and carry fuel on long runs rather than relying on motivation or appetite once you are already depleted.
Do not treat supplements as a substitute for energy
Iron, vitamin D and other nutrients matter, but a supplement cannot correct an overall shortage of energy or carbohydrate. Unexplained fatigue should be properly assessed; taking high-dose supplements without confirming a deficiency can be unnecessary and, in some cases, harmful.
Consider whether your training also needs to change
If symptoms are significant, simply adding food while maintaining an unsustainable load may not be enough. Recovery may require reducing training volume or intensity alongside increasing energy intake. This decision is best made with qualified support, particularly if injury, menstrual disruption or suspected REDs is involved.
A quick fuel check
Ask yourself:
- Has my mileage or intensity increased recently?
- Did my meals and snacks increase with it?
- Am I regularly running before breakfast and then delaying food afterwards?
- Are easy runs repeatedly feeling harder than they should?
- Am I recovering more slowly or becoming ill more often?
- Have I experienced recurring injuries, menstrual changes, reduced libido or persistent low mood?
- Am I cutting out foods because social media suggests they are “bad”?
- Do thoughts about food, weight or earning meals occupy a lot of mental space?
One “yes” does not diagnose under-fuelling. Several answers forming a persistent pattern are a reason to look more closely.
When to seek professional support
Speak to a GP, registered sports dietitian or appropriately qualified sports-medicine professional if you experience persistent fatigue, recurrent illness, an unexplained performance decline, irregular or absent periods, reduced libido, unintentional weight loss or repeated injuries.
Localised bone pain, pain that worsens with impact, or a suspected stress fracture should be assessed promptly. Seek support early if food, body image or exercise is causing anxiety or affecting daily life.
REDs cannot be diagnosed from a checklist or an online article. A clinician may need to consider other causes and use medical history, blood tests or scans as part of an assessment.
The bottom line
The goal of training is not to complete the greatest possible workload on the smallest possible amount of food. It is to give your body a reason to adapt—and enough energy to do it.
Feeling tired after a difficult run is part of the process. Feeling persistently depleted, injured or unable to perform is not something to wear as a badge of honour.
Before adding another session, look at the whole picture. Sometimes the smartest way to move forward is not more training. It is more fuel.
References
- Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee’s consensus statement on Relative Energy Deficiency in Sport. British Journal of Sports Medicine. 2023;57:1073–1097.
- Australian Institute of Sport. Energy Availability and Relative Energy Deficiency in Sport.
- High Performance Sport New Zealand. Low Energy Availability and Relative Energy Deficiency in Sport.
- British Dietetic Association. Sport and Exercise Nutrition. Updated May 2026.
- Thomas DT, Erdman KA, Burke LM. Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise. 2016;48(3):543–568.
- Athletics Australia. Relative Energy Deficiency in Sport: Athlete Fact Sheet. 2025.
Disclaimer
This article is for general educational purposes and is not a substitute for personalised medical or nutritional advice. Persistent fatigue and changes in health or performance can have many causes. Consult a GP, registered dietitian or suitably qualified healthcare professional if you are concerned about your symptoms, nutrition or relationship with food and exercise.
