Written by Rich Saxton
A recent BBC Sport article highlighted an important issue in sport: Relative Energy Deficiency in Sport (RED-S) can affect men just as much as women.
Understanding Relative Energy Deficiency in Sport (RED-S), the warning signs to look out for, and why eating enough is essential for performance, recovery and long-term health.
For many athletes, training harder, becoming lighter and pushing their limits can feel like the key to better performance. But what happens when the body simply doesn't have enough energy to keep up?
Relative Energy Deficiency in Sport (RED-S) is a condition that can affect athletes of all ages, abilities and sports. It occurs when the energy available from food is insufficient to meet the demands of exercise and the body's essential physiological functions.
At Run Serve Health Physiotherapy in Wilmslow, we recognise that getting the best out of your body isn't just about how much you train. It's also about how well you fuel, recover and look after your overall health.
What is RED-S?
RED-S develops when an athlete consistently consumes insufficient energy to support both exercise and normal bodily functions.
This is known as low energy availability. It can happen unintentionally, for example when training volume increases without a corresponding increase in food intake. It can also be associated with intentional food restriction, disordered eating or pressure to maintain a particular body weight.
Although RED-S is often associated with elite endurance athletes, it can affect recreational runners, cyclists, swimmers, gym-goers, young athletes and people participating in many other sports.
Importantly, you do not need to be underweight to experience RED-S. Someone can appear fit, strong and successful in their sport while their body is struggling to meet its energy requirements.
What are the signs and symptoms of RED-S?
RED-S can affect multiple systems throughout the body, meaning the symptoms are not always obvious. Some athletes initially attribute them to a demanding training programme, stress or simply doing too much.
Common warning signs include:
Repeated injuries: particularly bone stress injuries, stress reactions and stress fractures.
Persistent fatigue: feeling tired despite adequate sleep or struggling to maintain usual training intensity.
Poor recovery: soreness that lasts longer than expected and difficulty adapting to training.
Declining performance: workouts becoming harder, race results plateauing or performance worsening despite increased training.
Hormonal changes: irregular or absent periods in women, or reduced testosterone and libido in men.
Changes in mood and concentration: irritability, low mood, difficulty concentrating or disrupted sleep.
Reduced bone health: increased risk of stress fractures and, over time, reduced bone mineral density.
Not everyone will experience all these symptoms, and they can have other causes. However, a combination of recurring injuries, fatigue and declining performance should not be ignored.
RED-S affects both men and women
RED-S was historically associated primarily with female athletes because menstrual changes can provide an important warning sign of inadequate energy availability.
However, the condition affects men as well as women, and the signs in men can be less obvious.
In women, possible symptoms include missed or irregular periods, changes in reproductive hormones and reduced bone health.
In men, possible symptoms include reduced libido, lower testosterone levels, persistent fatigue, poor recovery and recurring bone injuries.
For both sexes, inadequate fuelling can affect metabolic function, immunity, cardiovascular health, reproductive health and psychological wellbeing.
The key message is that RED-S is not a female-only condition, and it is not limited to elite athletes.
Why is RED-S so common in sport?
One of the biggest contributing factors is the belief that being lighter automatically makes you faster.
In endurance sports such as running and cycling, athletes may feel pressure to reduce body weight to improve their power-to-weight ratio or running economy. In other sports, body composition may be linked to aesthetic expectations, selection or perceived performance advantages.
Although weight changes can sometimes influence performance, deliberately losing weight without considering the consequences can be counterproductive.
When the body does not receive enough energy, it attempts to conserve resources. Processes that are not immediately essential for survival, including some aspects of reproductive function, bone maintenance and recovery, can be downregulated.
Initially, an athlete may continue training and even perform well. Over time, however, the effects of inadequate fuelling can accumulate.
The result may be more injuries, reduced training quality, slower recovery and declining performance.
In other words, training more while fuelling less can leave you further away from your goals, not closer to them.
The link between RED-S and stress fractures
Bone stress injuries are an important concern for athletes experiencing RED-S.
Bones constantly adapt to the loads placed upon them. With appropriate nutrition and recovery, bone tissue can repair and remodel in response to training.
However, when energy availability is low, the hormonal and metabolic processes involved in maintaining bone health can be disrupted. This can reduce bone strength and impair the body's ability to repair the small amounts of damage that accumulate during exercise.
If training loads continue to exceed the body's capacity to recover, a bone stress reaction or stress fracture may develop.
For runners, these injuries can be particularly frustrating because they often interrupt training for weeks or months.
Repeated stress fractures should prompt consideration of the wider picture, including nutritional intake, training load, recovery, hormonal health and bone health, rather than focusing only on the injured area.
At Run Serve Health, we believe that understanding why an injury has occurred is an important part of helping athletes return to sport safely and reduce the risk of future problems.
How can RED-S affect your performance?
It is easy to assume that better performance comes from completing more sessions, increasing mileage or pushing harder during every workout.
But training is only one part of the equation.
Performance improves when the body can respond to training stress, repair tissue, replenish energy stores and adapt to the demands placed upon it. Without sufficient energy, these processes become compromised.
Athletes experiencing RED-S may notice that:
Their usual training pace feels harder than normal.
They struggle to complete sessions they previously managed comfortably.
They experience recurring illnesses or injuries.
Their recovery between sessions deteriorates.
Their race performance plateaus despite increasing training volume.
Adequate fuelling is not simply about having enough energy for the session itself. It is about supporting the body's ability to recover and adapt afterwards.
How can athletes reduce their risk of RED-S?
There is no single nutritional target that suits everyone. Energy requirements vary depending on body size, age, training volume, intensity, growth and individual physiology.
However, several principles can help support a healthier approach to sport.
1. Match your fuelling to your training
As training duration or intensity increases, your energy requirements usually increase too. Make sure your food intake reflects the demands of your programme, including on demanding training days.
2. Don't neglect carbohydrates
Carbohydrates are an important fuel source for moderate- to high-intensity exercise. Consistently restricting carbohydrate intake while maintaining a demanding training schedule can make it harder to meet your energy needs.
3. Prioritise recovery
Rest days, adequate sleep and sufficient nutrition are essential parts of an effective training programme. Recovery is not time wasted; it is when your body adapts to training.
4. Avoid chasing a particular body weight
There is no universally ideal weight for a runner or athlete. Performance, strength, health and the ability to train consistently are more meaningful considerations than weight alone.
5. Recognise warning signs early
Persistent fatigue, repeated injuries, hormonal changes and declining performance deserve attention. Do not assume these are simply the price of training hard.
6. Seek appropriate professional support
If you are concerned about RED-S, speak to a healthcare professional. Assessment may involve your injury history, training programme, nutritional intake, symptoms and relevant investigations. A sports dietitian and a clinician experienced in RED-S can help guide an appropriate recovery plan.
Can RED-S be reversed?
In many cases, RED-S can improve with early recognition and appropriate treatment.
Management generally focuses on restoring adequate energy availability, addressing any nutritional deficiencies, modifying training when necessary and allowing the body time to recover.
Depending on the individual, treatment may also involve investigations into hormonal function and bone health, alongside psychological support where disordered eating or anxiety around food and body weight is present.
Recovery does not always happen overnight. Athletes with significant bone stress injuries or hormonal disruption may need a prolonged period of modified training and monitoring.
It is important to remember that returning to full training too quickly can increase the risk of further injury. A gradual, individualised return to sport should take account of symptoms, healing, overall health and the demands of the activity.
How can physiotherapy help?
Physiotherapy can play an important role in assessing and managing injuries associated with RED-S, particularly bone stress injuries and recurring running-related problems.
At Run Serve Health Physiotherapy, our approach looks beyond the immediate symptoms to consider the factors that may be contributing to an injury.
Depending on your needs, this may include:
Assessing your injury and current functional capacity.
Reviewing training load, recent changes in activity and recovery.
Developing an appropriate rehabilitation and return-to-running plan.
Progressively rebuilding strength and tolerance to exercise.
Identifying when further medical assessment or specialist referral is needed.
Physiotherapy is one part of the wider picture. Suspected RED-S requires appropriate medical assessment and, where indicated, support from a sports dietitian and other relevant healthcare professionals. Simply strengthening an injured area will not resolve an underlying energy deficiency.
Our aim is to help you return to activity safely while supporting the bigger picture of your long-term health and performance.
The bottom line: health comes first
It can be tempting to view fatigue, missed periods, reduced libido or repeated injuries as unavoidable consequences of being a committed athlete.
They are not something you should automatically accept.
RED-S can affect athletes of all abilities, and it may go unnoticed for months or years when the focus remains solely on training and performance.
Fuelling your body adequately, recovering well and responding to warning signs early are essential parts of becoming a healthier, more resilient athlete.
You do not need to sacrifice your long-term health to become a better runner, cyclist or sportsperson.
In fact, a healthy body is the foundation on which sustainable performance is built.
Concerned about recurring injuries or struggling to recover?
At Run Serve Health Physiotherapy in Wilmslow, we help runners and athletes understand their injuries, rebuild strength and return to sport with a plan tailored to their needs.
If you are experiencing recurring injuries, persistent fatigue or a decline in performance, we can help assess your musculoskeletal concerns and guide you towards the appropriate next steps.