Under-Fuelling Explained: The Hidden Risks Of REDs For Athletes

Under-fuelling is more common in athletes than people realise, and it doesn't always look like extreme dieting.

You can have the perfect training program and all the motivation in the world, but if you're not eating enough, your body can't keep up. Food isn't just fuel for your next session: it repairs muscle, maintains bone, regulates hormones, and keeps your brain functioning. When intake consistently falls short of what training demands, low energy availability (LEA) can develop, and over time this can lead to Relative Energy Deficiency in Sport (REDs), affecting both health and performance.

Often, it's not about restriction, just not eating enough to match a heavy training load. So how do you know if you're under-fuelling, and what can you do about it?

Table Of Contents

• What Is Under-Fuelling Or REDs?

• What Under-Fuelling Actually Looks Like

• The Real Cost Of Under-Fuelling

• How To Know If You're Under-Fuelling

• Who's Most At Risk?

• How to Fuel For Your Training

• What If You're Already Under-Fuelling?

• Key Takeaways

What Is Under-Fuelling Or REDs?

Low energy availability happens when the energy you eat doesn't cover both what you burn training and what your body needs for everyday functions ¹.

Think of your daily energy like a budget. Training uses part of it, but your body still needs energy left over for things like hormone production, bone maintenance, immune function, digestion and recovery. If there's consistently not enough left over, the body starts to cut back on some of these processes.

This doesn't automatically mean REDs. Energy availability sits on a continuum, from short-term, "adaptable" low energy availability through to problematic LEA, where the health and performance effects become more serious ¹.

REDs is the broader condition that can develop when problematic LEA starts affecting the body and mind. It can impact reproductive function, bone health, metabolism, immunity, heart health, mental health and sports performance ¹.

Importantly, you don't need to be underweight to have REDs. An athlete can look fit and maintain a stable weight while still not eating enough to meet their body's needs.

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Energy availability is commonly expressed as the amount of dietary energy remaining after exercise expenditure, relative to fat-free mass. However, the IOC now emphasises that there isn't one universal energy-availability number that predicts REDs for every athlete. Individual responses can vary depending on factors such as the duration and severity of under-fuelling, training demands and individual characteristics ¹.

This is one reason why REDs should not be diagnosed simply by plugging numbers into a calculator.

What Under-Fuelling Actually Looks Like

Under-fuelling can be intentional or completely accidental.

You might think you're eating enough, but suddenly your training increases from five sessions a week to eight. Your appetite doesn't change, your meals stay the same and suddenly there is a significant gap between what your body is using and what you're eating.

The Australian Institute of Sport identifies several situations that can contribute to low energy availability, including increased training loads, limited time or skills for meal preparation, and not understanding individual energy requirements ².

Some common examples include:

• Skipping breakfast before an early training session

• Going several hours without eating between sessions

• Avoiding snacks because you're worried about gaining weight

• Not increasing food intake during a heavy training block

• Training frequently without adequate carbohydrate intake

• Finishing training and waiting hours before eating

• Trying to lose body fat while maintaining a high training load

• Accidentally eating less because you're busy with work, studying or travelling

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Under-fuelling can also occur when athletes focus heavily on protein or "clean eating" while unintentionally cutting back on total energy and carbohydrate intake ¹,² .

 

The Real Cost Of Under-Fuelling

Under-fuelling isn't just about feeling tired during training. When energy availability remains too low, the body can begin making changes to conserve energy. These changes can affect multiple systems throughout the body ¹,⁵ .

• Hormones and Reproductive Health →Low energy availability can interfere with hormonal function and reproductive health. For female athletes, this can include changes to menstrual function. For male athletes, problematic LEA can also affect reproductive hormones, including testosterone-related pathways ¹,⁵ . A missing or irregular period shouldn't simply be accepted as "normal for an athlete." It can be a warning sign that the body isn't receiving enough energy.
  
  

• Bone Health → Your bones are living tissue and require energy and nutrients to continually remodel and repair. Problematic LEA can disrupt bone metabolism and is associated with poorer bone health and increased risk of bone stress injuries ¹,⁵ . For an athlete, a bone stress injury can mean weeks or months away from normal training,  a significant price to pay for consistently under-fuelling.
  
  

• Immune Function → Your immune system also requires energy to function properly. The IOC identifies immune disturbances as one of the potential consequences associated with problematic LEA. This may contribute to an athlete experiencing more frequent illness or struggling to maintain consistent training ¹.
  
  

• Performance and Recovery → Here's the ironic part, under-fuelling can make all that training less effective. When your body doesn't have enough energy available, recovery and adaptation can be compromised ¹. So while eating less might seem like a way to become leaner or lighter, consistently under-fuelling can ultimately make it harder to train, recover and perform.

 

How To Know If You're Under-Fuelling

So, how do you actually know if you're not eating enough? There isn't one symptom that proves an athlete is under-fuelling. Instead, it's about recognising patterns.

Potential warning signs include:

• Persistent fatigue or low energy
• Poor recovery between sessions
• Declining training or competition performance
• Recurrent injuries or bone stress injuries
• Frequent illness
• Changes in menstrual function
• Changes in mood or behaviour
• Difficulty concentrating
• Unexplained weight loss
• Increased preoccupation with food or body weight
• Feeling cold more often
• Struggling to complete normal training sessions ². 

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But here's where things get complicated. Feeling tired doesn't automatically mean you have REDs. Fatigue can come from poor sleep, too much training, illness, stress, not enough carbs, or plenty of other causes. Many REDs symptoms overlap with these other explanations ⁴, so it's not something you can diagnose from a checklist. If several signs show up together, especially alongside low food intake or a big jump in training, it's worth talking to a sports dietitian or doctor.

 

Who's Most At Risk?

REDs can affect athletes of any sex, sport or body size, but some are more at risk than others:

• Sports where leanness is seen as a performance edge, like long-distance running, cycling, gymnastics, dance and weight-category sports, can create extra pressure to restrict food ². 
• It's not limited to these sports, though. Any athlete with a heavy training load can struggle to eat enough, especially when juggling multiple sessions, competitions, travel, school, uni or work.
• Some athletes cut back on food because they believe being lighter will automatically boost performance.
• Under-fuelling isn't always intentional. It can also happen simply from not eating more when training demands go up.
• A history of disordered eating raises the risk of problematic LEA, but under-fuelling can happen without one ¹,² .

 

How To Fuel For Your Training

The goal isn't to simply "eat more." It's to match your nutrition to your training demands - a principle known as "fuelling for the work required"  ⁶. On a rest day, your energy requirements may still be elevated as your body recovers and rebuilds from training, while on a day with multiple sessions, you'll need substantially more energy and carbohydrate to support performance and recovery.

Regular meals and snacks are one of the easiest ways to help prevent large gaps in energy intake. The exact amount and timing will depend on the athlete, sport and training schedule, but the principle remains the same: don't leave your body running on empty for hours at a time.

• Carbohydrates Matter

Carbohydrates are particularly important for athletes because they provide a major source of fuel during high-intensity exercise and help replenish muscle glycogen. Low carbohydrate availability can potentially magnify some REDs-related outcomes ¹. So if you're training hard but cutting carbohydrates at the same time, you may be creating an even bigger problem.

• Don't Forget Protein and Fat

Protein supports muscle repair and adaptation, while dietary fats provide energy and contribute to normal physiological functions. A balanced diet containing adequate carbohydrate, protein and fat is therefore important for supporting both performance and health.

• Fuel Around Training

Your food intake shouldn't stop once training begins. Eating before training can help provide the energy required for the session, while consuming food after training supports recovery and replenishment.

For athletes completing multiple sessions in a day, this becomes particularly important. Waiting until dinner to replace everything you've used during the day may make it much harder to meet your total energy requirements.

 

What If You're Already Under-Fuelling?

If an athlete is experiencing problematic LEA or REDs, simply being told to "eat more" isn't always enough. The AIS recommends increasing energy intake and, where necessary, reducing exercise demands as part of treatment. As a general guide, the IOC's practical approach suggests increasing energy intake by approximately 300-600kcal per day as a starting point clinicians may use - but this figure is not intended for self-directed use, and the right amount for you depends on individual assessment by a sports dietitian ². 

Energy requirements vary significantly between athletes, and accurately measuring energy availability can be difficult. Treatment may also need to address menstrual dysfunction, bone health, psychological factors or other health concerns.

For this reason, REDs is best managed with support from an appropriate health professional, such as a sports dietitian and sports medicine practitioner. A multidisciplinary approach may be required depending on the athlete's individual needs ².

 

Key Takeaways

Under-fuelling isn't simply about being hungry. When an athlete consistently doesn't consume enough energy to support both training and normal physiological functions, problematic low energy availability can develop and may contribute to REDs.

• You don't have to be underweight to be under-fuelled.
• Under-fuelling can happen intentionally or unintentionally.
• REDs can affect male and female athletes across many different sports.
• Persistent fatigue, poor recovery, recurrent injuries, illness or hormonal changes may be warning signs.
• Your nutritional needs increase when your training load increases.
• Carbohydrates are particularly important for supporting high-intensity training and recovery.
• Not every symptom means you have REDs, proper assessment is important.
• Early recognition and appropriate support can help protect both health and performance.

Most importantly, food isn't something you need to earn through training. Your body is doing far more than getting you through your next session. It's recovering, adapting, maintaining bone, regulating hormones, supporting your immune system and keeping you healthy.

If you want to perform at your best, you need to fuel the body that makes that performance possible.

 

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

 

References

1. Mountjoy M, Ackerman KE, Bailey DM, Burke LM, Constantini N, Hackney AC, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073-98.

2. Australian Sports Commission. Energy availability [Internet]. Canberra: Australian Institute of Sport; [cited 2026 Aug 19]. Available from: https://www.ausport.gov.au/ais/nutrition/energy-availability

3. Gallant TL, Ong LF, Wong L, Sparks M, Wilson E, Puglisi JL, Gerriets VA. Low energy availability and relative energy deficiency in sport: a systematic review and meta-analysis. Sports Med. 2025;55(2):325-39.

4. Jeukendrup AE, Areta JL, Van Genechten L, Langan-Evans C, Pedlar CR, Rodas G, et al. Does relative energy deficiency in sport (REDs) syndrome exist? Sports Med. 2024;54(11):2793-816.

5. Angelidi AM, Stefanakis K, Chou SH, Valenzuela-Vallejo L, Dipla K, Boutari C, et al. Relative energy deficiency in sport (REDs): endocrine manifestations, pathophysiology and treatments. Endocr Rev. 2024;45(5):676-708.

6. Impey SG, Hearris MA, Hammond KM, Bartlett JD, Louis J, Close GL, Morton JP. Fuel for the work required: a theoretical framework for carbohydrate periodization and the glycogen threshold hypothesis. Sports Med. 2018;48(5):1031-48.

Disclaimer:
The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.