RED-S in runners occurs when the body does not have enough available energy to support training and normal physiological functions. This can affect runners of any sex, ability or body size. It may impair performance, hormones, bone health, immunity, mood and cardiovascular health, but early recognition and properly supported changes to fuelling and training can help recovery.
Key takeaways
- Low energy availability (LEA) means too little dietary energy remains for the body after exercise demands are accounted for.
- Relative Energy Deficiency in Sport (RED-S) is the wider syndrome of health and performance problems that can develop with problematic LEA.
- Female and male runners can be affected, whether under-fuelling is intentional or accidental.
- Possible signs include persistent fatigue, declining performance, recurrent illness or injury, menstrual changes, reduced libido, mood changes and bone stress injuries.
- There is no single diagnostic test. Assessment should be led by a doctor and may involve a sports dietitian and other qualified professionals.
- Supplements cannot compensate for inadequate energy intake.
What are low energy availability and RED-S?
Energy availability is the energy from food that is left for the body’s essential functions after the energy cost of exercise has been considered. When too little remains, the body may adapt by reducing energy spent on functions such as reproduction, bone turnover, metabolism, immunity and recovery.
A short, mild mismatch on an isolated day is not automatically RED-S. The 2023 International Olympic Committee (IOC) consensus distinguishes between exposure that may be adaptable and problematic LEA. The effect depends on factors including its severity, duration and frequency, as well as the individual runner and their environment.
RED-S—relative energy deficiency in sport—is the syndrome of harmful health and performance outcomes associated with problematic LEA. It expands on the older “female athlete triad” concept. Menstrual function and bone health remain important, but RED-S recognises that many body systems can be affected and that male athletes are also at risk.
Why runners may become under-fuelled
Under-fuelling runners are not always restricting food deliberately. Running volume can rise quickly while appetite, meal planning or food access fails to keep pace. Common routes into LEA include:
- adding mileage, speed sessions, gym work or double-training days without increasing food;
- long gaps between meals, skipped breakfasts or frequent fasted runs;
- poor fuelling before, during and after longer sessions;
- busy work, study, family or travel schedules;
- reduced appetite after intense exercise, heat, illness or gastrointestinal symptoms;
- limited food budgets, dietary exclusions or insufficient knowledge about sports nutrition;
- trying to lose weight or change body composition too aggressively;
- pressure from sport culture, coaches, social media or comparison with other runners;
- disordered eating or an eating disorder.
Low carbohydrate availability may be part of the picture even when total food intake appears reasonable. A runner can also have LEA at a stable weight, at a higher or lower body weight, or without looking visibly unwell. Appearance cannot rule it in or out.
Signs of RED-S in female and male runners
No single symptom proves RED-S, and signs vary. Patterns and changes from your normal baseline matter more than one bad run. Possible warning signs include:
- persistent tiredness, poor recovery or unusually heavy legs;
- performance plateau or decline despite continued training;
- recurrent colds, illnesses or slow wound healing;
- repeated niggles, bone stress reactions or stress fractures;
- irritability, low mood, anxiety, difficulty concentrating or changed sleep;
- feeling cold, gastrointestinal disturbance or reduced appetite;
- loss of strength, power, coordination or training response;
- menstrual cycles becoming irregular, less frequent or stopping;
- reduced libido and, in men, fewer morning erections;
- unexpected weight change—or no weight change despite other symptoms.
Hormonal contraception can alter or mask menstrual bleeding, so a withdrawal bleed does not necessarily confirm normal natural menstrual function. In adolescents, delayed growth, puberty or menstrual development also warrants clinical attention. Men should not dismiss sexual or hormonal changes as simply the result of hard training.
Health and running-performance risks
Problematic LEA can affect multiple systems. The IOC’s RED-S model includes reproductive function, bone health, immunity, metabolism, blood health, gastrointestinal function, cardiovascular health, sleep and mental health. Mental health difficulties can be a consequence of RED-S, a contributor to under-fuelling, or both.
For runners, one of the most serious concerns is impaired bone health. Reduced bone formation and hormonal disruption can increase the risk of bone stress injury, including the injuries discussed in our guide to stress fractures in runners. Some effects, particularly on bone, may take much longer to reverse than day-to-day fatigue.
Performance may suffer through reduced endurance, strength, power, concentration, coordination and ability to adapt to training. More training is therefore not always the solution to a disappointing race or stalled progress. If the body lacks enough energy to recover, adding load may deepen the problem.
RED-S, overtraining or iron deficiency?
RED-S overlaps with other common causes of running fatigue, so self-diagnosis is unreliable.
RED-S versus overtraining syndrome
Both can involve persistent fatigue, declining performance, mood disturbance, poor sleep and recurrent illness. Overtraining syndrome is associated with prolonged maladaptation to excessive training load and inadequate recovery. RED-S is specifically linked to problematic low energy availability. A runner may have one, both, or another condition entirely. Our article on overtraining syndrome in runners explains the overlap in more detail.
RED-S versus iron deficiency
Iron deficiency can cause fatigue, breathlessness, poor concentration and reduced endurance. It can coexist with RED-S, particularly when overall intake is low, but it is not the same condition. Iron status is assessed with appropriately interpreted blood tests; LEA cannot be confirmed or excluded by ferritin alone. Read our guide to blood tests for runners, and do not start high-dose iron unless a qualified clinician recommends it.
Other explanations can include infection, thyroid disease, coeliac disease, pregnancy, diabetes, sleep problems, medication effects and other nutritional deficiencies. Our overview of running fatigue causes is useful background, but ongoing or worsening symptoms need individual medical assessment.
Why there is no single RED-S test
The IOC RED-S Clinical Assessment Tool version 2 (CAT2) states that there is no single validated diagnostic test. Risk factors, signs and symptoms are numerous, and the runner’s environment matters. Body weight, a calorie estimate, a hormone result or one blood panel cannot provide a definitive answer in isolation.
CAT2 is designed for qualified clinical professionals, not as an online self-score. It uses three broad steps: initial screening; severity and risk stratification using primary and secondary indicators; and a physician-led final diagnosis and treatment plan. Its four-level traffic-light system—green, yellow, orange and red—also informs training and competition recommendations.
What clinician-led assessment may involve
A GP or sports and exercise medicine doctor may ask about training load, food intake, weight history, injury, menstrual or sexual function, sleep, mood, medication and social pressures. Depending on the history and examination, they may arrange blood tests, heart assessment, bone imaging or bone-density testing and consider alternative diagnoses.
Care may also involve a registered sports dietitian, physiotherapist and, where body image, anxiety, disordered eating or an eating disorder is present, an appropriately qualified mental health professional. The aim is to protect health while building a safe, sustainable return to training.
How runners can reduce the risk
- Match food to training demand. Increase intake as mileage, intensity or strength work rises rather than waiting for severe hunger.
- Eat regularly. Use meals and snacks across the day, especially when schedules are busy.
- Fuel key sessions. Plan carbohydrate before and, where appropriate, during longer or harder runs, then eat afterwards.
- Include all major macronutrients. Carbohydrate supports training; protein supports repair; dietary fat supports health and helps provide energy.
- Avoid aggressive body-composition targets. Any intentional weight change in a serious training block should be cautious and professionally supported.
- Track useful health signals. Notice menstrual changes, libido, recurrent illness, injuries, mood, sleep and performance—not just pace or weight.
- Build rest into the plan. Recovery days and sensible progression help energy intake keep pace with exercise expenditure.
For a practical food-first overview, see our running nutrition guide. Daily nutritional foundations can sit alongside adequate food and professional care, but supplements cannot compensate for inadequate energy intake and should never be presented as treatment for RED-S.
What to do if you are concerned
- Do not push through. Pause attempts to lose weight and avoid adding more training.
- Book a medical appointment. Explain the changes in performance, health, cycles, libido, injuries, eating and training. Ask whether RED-S and other causes should be considered.
- Seek sports nutrition support. A registered sports dietitian can help increase energy availability without rigid rules or guesswork.
- Adjust intake and training with your care team. Recovery commonly requires more energy intake, lower exercise expenditure, or both. Some runners may need temporary restrictions from training or competition.
- Address psychological factors early. Seek specialist support if food restriction, fear of weight gain, compulsive exercise or body-image distress is involved.
Recovery time varies. Energy, mood and training response may improve before hormones or bone health do. Return-to-sport decisions should follow clinical risk, not a target date or pressure to race.
When to seek urgent help
Call 999 or go to A&E for collapse, chest pain, severe breathlessness, confusion, or another immediately life-threatening symptom. Seek urgent medical advice for fainting, a racing or irregular heartbeat, rapidly worsening weakness, inability to keep food or fluids down, or severe mental health distress. Stop running and seek prompt assessment for focal bone pain, pain at rest or pain that changes your gait, as this may indicate a bone stress injury.
Frequently asked questions
Can recreational runners develop RED-S?
Yes. RED-S is not limited to elite athletes. Any runner can become under-fuelled when food intake is insufficient relative to exercise demand, including beginners increasing mileage and busy recreational runners.
Can you have RED-S without losing weight?
Yes. Weight may stay stable while the body adapts by reducing energy allocated to other functions. A normal or higher body weight does not rule out low energy availability.
Do regular periods rule out RED-S?
No. Menstrual disturbance is an important sign, but it is not present in every case. Hormonal contraception may also mask changes. Clinicians assess the whole pattern of risks, signs and symptoms.
Do men get RED-S?
Yes. Male runners may experience impaired bone health, reduced libido, hormonal changes, fatigue, illness, injury and poorer performance. RED-S should not be treated as a female-only condition.
Can a supplement fix RED-S?
No. Supplements cannot compensate for inadequate energy intake. Treatment must address the underlying mismatch between dietary intake and exercise demand, with medical and dietetic support where appropriate.
Should I stop running completely?
Not everyone needs complete rest, but some runners require reduced or temporarily stopped training because of their clinical risk. A doctor-led assessment should guide participation, especially where there is a bone stress injury, cardiovascular concern, significant hormonal disturbance or an eating disorder.
Sources
- International Olympic Committee: 2023 consensus statement on Relative Energy Deficiency in Sport (RED-S). British Journal of Sports Medicine. PMID: 37752011. DOI: 10.1136/bjsports-2023-106994.
- IOC Relative Energy Deficiency in Sport Clinical Assessment Tool, version 2 (IOC RED-S CAT2). British Journal of Sports Medicine. PMID: 37752002. DOI: 10.1136/bjsports-2023-106914.
Medical disclaimer: This article is for general information and is not a substitute for individual medical advice, diagnosis or treatment. If you are concerned about low energy availability, RED-S, an eating disorder or a bone stress injury, speak to a qualified healthcare professional. Do not delay urgent care because of information in this article.