RED-S in Female Sport: How Low Energy Availability Can Affect Performance, Hormones and Injury Risk

For female athletes, getting the balance between training, recovery and nutrition right is essential for long-term health and performance. However, when the body consistently uses more energy than it receives through food, it can begin to compromise important physiological processes. This is known as Relative Energy Deficiency in Sport (RED-S), a condition that can affect athletes at every level, from recreational runners to elite competitors.

RED-S is particularly important to recognise in female sport because its effects can extend beyond athletic performance, influencing menstrual health, bone strength, recovery and the risk of injury. Understanding the warning signs can help athletes, coaches and healthcare professionals identify problems early and support sustainable participation in sport.

What is RED-S?

RED-S develops when an athlete experiences low energy availability (LEA) over time. Energy availability describes the energy remaining for the body's normal physiological functions after the energy expended during exercise has been accounted for.

In simple terms, if an athlete consistently eats too little to support both their training and their body's basic needs, there may not be enough energy available to maintain normal health.

This can happen deliberately, such as when an athlete restricts food intake to achieve a particular body weight, but it can also occur unintentionally. Busy schedules, increased training loads, poor appetite after exercise, inadequate fuelling during long sessions and a lack of nutritional knowledge can all contribute.

Importantly, RED-S can affect athletes of any body size. Someone can appear fit, strong and healthy while still experiencing inadequate energy availability. It is not simply an eating disorder, nor is it limited to athletes who are underweight.

Why are female athletes particularly vulnerable?

Female athletes may face several challenges that increase the risk of low energy availability. Endurance sports, including running, cycling and triathlon, can involve substantial energy expenditure. Sports that emphasise leanness, aesthetics or a particular weight can also create pressure to restrict food intake.

Some athletes mistakenly believe that being lighter will automatically improve performance. Although body mass can influence performance in certain sporting contexts, reducing weight without appropriate nutritional support can undermine the very adaptations an athlete is trying to achieve.

Hormonal changes are another important consideration. When energy availability is insufficient, the body may reduce reproductive hormone signalling, affecting the menstrual cycle. However, the absence of menstrual symptoms does not rule out RED-S, particularly in athletes using hormonal contraception, which can mask changes in their natural cycle.

RED-S is also relevant during adolescence, when young athletes need sufficient energy not only for exercise but also for growth, bone development and the establishment of long-term reproductive health.

How does RED-S affect the body?

Low energy availability can influence multiple physiological systems rather than producing one isolated problem.

1. Menstrual and reproductive health

One of the recognised consequences of inadequate energy availability is disruption to the menstrual cycle. Reduced signalling between the brain and reproductive system can suppress the hormones involved in ovulation and menstruation.

Possible signs include:

• Irregular menstrual cycles.

• Periods becoming less frequent.

• Missing periods altogether.

• Reduced libido.

Menstrual disruption should not be dismissed as a normal consequence of hard training. Although other medical conditions can cause these symptoms, they warrant appropriate assessment.

2. Bone health and stress fractures

Bone health is a major concern in female athletes with RED-S. Bone is living tissue that continually remodels in response to mechanical loading, nutrition and hormonal influences.

When energy availability is inadequate, changes in reproductive hormones and bone metabolism can impair bone formation and maintenance. Over time, this may contribute to reduced bone mineral density and increase susceptibility to bone stress injuries.

For runners, this is particularly significant. A bone stress injury develops when repetitive loading exceeds the bone's capacity to adapt and repair. Early symptoms may include localised pain during running, tenderness over a specific area and discomfort that progressively worsens with activity.

RED-S is not the only cause of bone stress injuries, and not every stress fracture indicates RED-S. However, recurrent injuries, injuries occurring with relatively modest training loads or a history of menstrual disturbance should prompt consideration of underlying energy availability and bone health.

3. Reduced performance and slower recovery

An athlete may initially tolerate an increase in training despite inadequate fuelling. Eventually, however, the consequences can become apparent.

Potential signs include:

• Persistent fatigue and reduced motivation.

• Declining performance despite continued training.

• Difficulty recovering between sessions.

• Reduced strength and training adaptations.

• Increased susceptibility to illness.

• Problems with concentration and mood.

Athletes may respond to declining performance by training harder, creating a cycle in which energy demands increase while recovery becomes progressively more difficult.

4. Metabolic and hormonal changes

The body can adapt to inadequate energy intake by reducing energy expenditure and altering several hormonal pathways. Changes may involve thyroid hormones, reproductive hormones and the hormones involved in appetite and stress regulation.

These adaptations are part of the body's attempt to conserve energy. However, they can interfere with normal physiological function and make it increasingly difficult to sustain training, recover effectively and maintain overall health.

5. Psychological wellbeing

RED-S can also affect psychological health. Some athletes experience irritability, low mood, anxiety, poor concentration or an increased preoccupation with food, body weight and exercise.

The relationship can work in both directions: psychological stress or disordered eating may contribute to low energy availability, while the physiological consequences of inadequate fuelling may further affect mood and wellbeing.

Recognising these signs without judgement is important, particularly in sporting environments where restrictive eating or exercising through exhaustion may be normalised.

d body composition.

These changes can overlap with some of the symptoms associated with RED-S, including fatigue, changes in menstrual patterns and an increased concern about bone health.

However, menopause and RED-S are not the same condition. Menstrual changes during perimenopause should not automatically be attributed to inadequate fuelling, and fatigue should not automatically be attributed to either hormonal changes or training.

For active women in midlife, adequate energy intake, sufficient protein, appropriate carbohydrate availability and a balanced resistance-training programme can all contribute to maintaining performance and musculoskeletal health.

Women who are increasing their training volume while reducing their food intake to manage body composition may be particularly vulnerable to inadequate fuelling. Paying attention to recovery, dietary intake and changes in performance is therefore important throughout the menopausal transition.

How is RED-S diagnosed?

There is no single blood test that definitively diagnoses RED-S. Assessment generally involves a detailed clinical history, consideration of training and nutritional habits, physical examination where appropriate and investigations guided by the individual's symptoms.

The International Olympic Committee's RED-S Clinical Assessment Tool (REDs CAT2) provides a structured framework to help appropriately trained healthcare professionals assess risk and guide decisions about sport participation.

Depending on the presentation, assessment may include:

• Menstrual and reproductive history.

• Dietary intake and training-load assessment.

• Review of previous injuries, particularly bone stress injuries.

• Blood tests to investigate relevant hormonal, nutritional or other medical abnormalities.

• Bone mineral density assessment using DXA when clinically indicated.

• Assessment for eating disorders or disordered eating where appropriate.

It is important to investigate other possible explanations for symptoms, including iron deficiency, thyroid disorders and other medical conditions. Athletes with suspected RED-S should seek assessment from a suitably qualified healthcare professional, ideally with experience in sports medicine and nutrition.

How can RED-S be managed?

The main objective is to restore adequate energy availability and support the body's normal physiological functions. Management should be individualised according to the severity of the problem, the athlete's symptoms and any associated injuries.

1. Improve nutritional intake

Increasing energy intake is often central to recovery. This may involve eating more regularly, increasing portion sizes, adding snacks and ensuring adequate carbohydrate intake around training.

Carbohydrates are particularly important for supporting higher-intensity exercise and replenishing muscle glycogen. Protein supports muscle repair and adaptation, while dietary fats provide energy and essential fatty acids and contribute to normal physiological function.

Calcium and vitamin D are also important for bone health, although supplementation alone will not correct RED-S if overall energy availability remains inadequate.

A sports dietitian can help athletes develop a practical fuelling strategy that meets their training requirements without unnecessary dietary restriction.

2. Adjust training when necessary

Continuing to train through significant under-fuelling can prolong recovery and increase health risks. Depending on the clinical assessment, reducing training volume or intensity, incorporating additional recovery days or temporarily restricting participation may be necessary.

Athletes with suspected bone stress injuries require particular care. High-impact activity may need to be modified or stopped until the injury has been appropriately assessed and a safe return-to-running plan established.

Rest is not a failure of training. It is part of the process through which the body adapts and recovers.

3. Monitor recovery and health

Recovery should be assessed using more than sporting performance alone. Relevant measures may include energy levels, menstrual function where applicable, injury symptoms, recovery between sessions and the athlete's relationship with food and exercise.

Where menstrual disruption or other significant symptoms are present, medical assessment is important. Hormonal contraception should not be used simply to mask menstrual dysfunction without investigating its underlying cause.

4. Take a multidisciplinary approach

Effective management may involve a sports physician, GP, sports dietitian, physiotherapist or osteopath, and a mental health professional where appropriate.

Each professional has a different role. Musculoskeletal clinicians can help identify concerning injury patterns, assess loading tolerance and support rehabilitation, but they should not attempt to diagnose RED-S from an injury alone.

Communication between the athlete, coach and healthcare team can help ensure that training expectations support rather than undermine recovery.

What can athletes and coaches do to reduce the risk?

Prevention begins with recognising that training and nutrition must work together.

Athletes should aim to fuel before, during and after exercise according to the demands of their sport, avoid unnecessary dietary restriction and increase nutritional intake when training demands rise. Coaches should be alert to persistent fatigue, declining performance and recurrent injuries rather than assuming that greater training effort is always the answer.

Sporting environments should also avoid placing undue emphasis on body weight or appearance. Education about energy availability, menstrual health and bone stress injuries can help athletes recognise problems before they become more serious.

For female runners, a particularly important message is that missed periods, recurring stress injuries and persistent fatigue should not be regarded as inevitable consequences of training hard.d body composition.

These changes can overlap with some of the symptoms associated with RED-S, including fatigue, changes in menstrual patterns and an increased concern about bone health.

However, menopause and RED-S are not the same condition. Menstrual changes during perimenopause should not automatically be attributed to inadequate fuelling, and fatigue should not automatically be attributed to either hormonal changes or training.

For active women in midlife, adequate energy intake, sufficient protein, appropriate carbohydrate availability and a balanced resistance-training programme can all contribute to maintaining performance and musculoskeletal health.

Women who are increasing their training volume while reducing their food intake to manage body composition may be particularly vulnerable to inadequate fuelling. Paying attention to recovery, dietary intake and changes in performance is therefore important throughout the menopausal transition.

How is RED-S diagnosed?

There is no single blood test that definitively diagnoses RED-S. Assessment generally involves a detailed clinical history, consideration of training and nutritional habits, physical examination where appropriate and investigations guided by the individual's symptoms.

The International Olympic Committee's RED-S Clinical Assessment Tool (REDs CAT2) provides a structured framework to help appropriately trained healthcare professionals assess risk and guide decisions about sport participation.

Depending on the presentation, assessment may include:

• Menstrual and reproductive history.

• Dietary intake and training-load assessment.

• Review of previous injuries, particularly bone stress injuries.

• Blood tests to investigate relevant hormonal, nutritional or other medical abnormalities.

• Bone mineral density assessment using DXA when clinically indicated.

• Assessment for eating disorders or disordered eating where appropriate.

It is important to investigate other possible explanations for symptoms, including iron deficiency, thyroid disorders and other medical conditions. Athletes with suspected RED-S should seek assessment from a suitably qualified healthcare professional, ideally with experience in sports medicine and nutrition.

How can RED-S be managed?

The main objective is to restore adequate energy availability and support the body's normal physiological functions. Management should be individualised according to the severity of the problem, the athlete's symptoms and any associated injuries.

1. Improve nutritional intake

Increasing energy intake is often central to recovery. This may involve eating more regularly, increasing portion sizes, adding snacks and ensuring adequate carbohydrate intake around training.

Carbohydrates are particularly important for supporting higher-intensity exercise and replenishing muscle glycogen. Protein supports muscle repair and adaptation, while dietary fats provide energy and essential fatty acids and contribute to normal physiological function.

Calcium and vitamin D are also important for bone health, although supplementation alone will not correct RED-S if overall energy availability remains inadequate.

A sports dietitian can help athletes develop a practical fuelling strategy that meets their training requirements without unnecessary dietary restriction.

2. Adjust training when necessary

Continuing to train through significant under-fuelling can prolong recovery and increase health risks. Depending on the clinical assessment, reducing training volume or intensity, incorporating additional recovery days or temporarily restricting participation may be necessary.

Athletes with suspected bone stress injuries require particular care. High-impact activity may need to be modified or stopped until the injury has been appropriately assessed and a safe return-to-running plan established.

Rest is not a failure of training. It is part of the process through which the body adapts and recovers.

3. Monitor recovery and health

Recovery should be assessed using more than sporting performance alone. Relevant measures may include energy levels, menstrual function where applicable, injury symptoms, recovery between sessions and the athlete's relationship with food and exercise.

Where menstrual disruption or other significant symptoms are present, medical assessment is important. Hormonal contraception should not be used simply to mask menstrual dysfunction without investigating its underlying cause.

4. Take a multidisciplinary approach

Effective management may involve a sports physician, GP, sports dietitian, physiotherapist or osteopath, and a mental health professional where appropriate.

Each professional has a different role. Musculoskeletal clinicians can help identify concerning injury patterns, assess loading tolerance and support rehabilitation, but they should not attempt to diagnose RED-S from an injury alone.

Communication between the athlete, coach and healthcare team can help ensure that training expectations support rather than undermine recovery.

What can athletes and coaches do to reduce the risk?

Prevention begins with recognising that training and nutrition must work together.

Athletes should aim to fuel before, during and after exercise according to the demands of their sport, avoid unnecessary dietary restriction and increase nutritional intake when training demands rise. Coaches should be alert to persistent fatigue, declining performance and recurrent injuries rather than assuming that greater training effort is always the answer.

Sporting environments should also avoid placing undue emphasis on body weight or appearance. Education about energy availability, menstrual health and bone stress injuries can help athletes recognise problems before they become more serious.

For female runners, a particularly important message is that missed periods, recurring stress injuries and persistent fatigue should not be regarded as inevitable consequences of training hard.