What is the cause of RED-S?

December 11, 2025 •

4 min read

According to the International Olympic Committee, the primary cause of Relative Energy Deficiency in Sport (RED-S) is low energy availability, a state where the energy consumed from food is insufficient to support the body's energy expended during exercise and for normal physiological functions. This deficit triggers a series of adverse health consequences impacting multiple body systems.

Quick Summary

Low energy availability is the fundamental cause of RED-S, a condition that occurs when dietary intake fails to meet exercise energy expenditure. This energy deficit compromises multiple bodily functions, including hormonal regulation, bone health, and metabolic processes, affecting both male and female athletes.

Key Points

Low Energy Availability (LEA): The foundational cause of RED-S is a caloric deficit where energy intake is less than energy expenditure.
Intentional and Unintentional Causes: LEA can be caused by deliberate dietary restriction for performance or appearance, or by simply underestimating high training energy needs.
Hormonal Disruption: Chronic energy deficit suppresses reproductive and metabolic hormones in both male and female athletes.
Impaired Bone Health: A significant drop in bone mineral density is a hallmark of RED-S, increasing the risk of stress fractures and osteoporosis.
Multi-System Impairment: RED-S negatively impacts nearly every physiological system, including metabolic, cardiovascular, gastrointestinal, and immune function.
Performance Decline: Despite the goal of losing weight for performance, RED-S ultimately leads to fatigue, weakness, and decreased athletic capability.
Psychological Component: Mental health issues like anxiety and disordered eating are both causes and consequences of RED-S.

The Core Imbalance: What Drives RED-S

At its heart, the cause of RED-S is a state of persistent low energy availability (LEA). This is not simply a matter of under-eating, but a critical energy imbalance where the body does not have enough fuel left over after training to perform its essential functions. When an athlete's energy expenditure from daily activities and exercise outweighs their energy intake from food, the body enters a state of preservation, shutting down or impairing non-essential systems to conserve energy. This cascade effect is the origin of the widespread symptoms associated with RED-S.

Factors Contributing to Low Energy Availability

Several factors, both intentional and unintentional, can lead to the energy deficit that causes RED-S. Often, a combination of these elements pushes an athlete into a state of LEA.

Intentional under-fueling: Athletes may deliberately restrict calorie intake in an effort to lose weight or alter body composition, often due to aesthetic pressures from their sport (e.g., gymnastics, ballet) or personal body image concerns. They may mistakenly believe that a lower body weight will lead to better performance.
Unintentional under-fueling: In some cases, athletes simply underestimate the massive energy requirements of their high-volume or high-intensity training. They may eat what feels like a normal amount of food, but it is insufficient to cover both their basal metabolic rate and their extensive exercise expenditure. This is common in endurance sports like running and cycling.
Increased training load: A sudden increase in training intensity, duration, or volume without a corresponding increase in food intake can rapidly create an energy deficit. This is a frequent trigger, especially during competitive seasons.
Poor nutritional timing and quality: Even with adequate daily calorie totals, poor timing or an inadequate balance of macronutrients (especially carbohydrates) around training sessions can lead to functional LEA. Proper fueling before, during, and after exercise is critical for energy availability.
Disordered eating behaviors: A spectrum of disordered eating habits, which may not meet the criteria for a clinical eating disorder, can lead to LEA. Athletes may develop restrictive tendencies or unhealthy relationships with food due to the competitive environment or personal psychological factors.

The Physiological Consequences

Once LEA is established, the body's defensive response triggers a broad range of physiological changes that define the RED-S syndrome. These changes affect nearly every body system.

Hormonal dysfunction: The energy crisis leads to a suppression of the hypothalamic-pituitary-gonadal (HPG) axis. In females, this reduces the production of estrogen, leading to irregular or absent periods (amenorrhea). In males, it lowers testosterone levels, impacting libido and strength.
Metabolic rate suppression: The body attempts to conserve energy by slowing down the basal metabolic rate, making it harder to maintain body temperature and complicating the energy balance further.
Compromised bone health: Low levels of sex hormones, particularly estrogen, impair bone formation and maintenance. This leads to decreased bone mineral density (osteopenia or osteoporosis) and significantly increases the risk of stress fractures and other bone injuries, especially in weight-bearing sports.
Impaired immunity and illness: The body diverts energy away from the immune system, leaving the athlete more susceptible to illnesses, colds, and infections. Recovery from sickness also takes longer.
Cardiovascular and gastrointestinal issues: Cardiovascular health can be negatively impacted, and gastrointestinal function, including gut motility and nutrient absorption, is impaired.

RED-S vs. Overtraining Syndrome

While closely linked, RED-S and Overtraining Syndrome (OTS) are distinct conditions. The primary difference lies in the root cause, though they often co-exist due to the high stress levels and poor recovery associated with RED-S.


Aspect	Relative Energy Deficiency in Sport (RED-S)	Overtraining Syndrome (OTS)
Primary Cause	Chronic low energy availability (LEA).	Excessive training load combined with inadequate rest/recovery.
Key Driver	The energy deficit is the central issue, triggering a multi-system impairment.	The stress of training itself is the main factor, leading to a state of chronic fatigue and decreased performance.
Impact on Body	Widespread systemic effects affecting hormonal, metabolic, and bone health.	Impairs the nervous, hormonal, and immune systems, leading to exhaustion and a decline in performance.
Overlap	LEA is a risk factor for OTS, as the under-fueled body cannot adapt to high training loads.	OTS can contribute to LEA through high energy expenditure, but is not the sole cause.

The Psychological Underpinnings of RED-S

It is crucial to recognize that psychological factors are intertwined with the physiological causes of RED-S. For many athletes, mental health issues contribute to and are worsened by the condition. The relentless drive for perfection, often combined with pressure from coaches or teammates, can lead to disordered eating behaviors. A decline in performance or mood swings resulting from RED-S can further increase anxiety and depression. Recognizing this psychological component is vital for a comprehensive treatment plan.

Conclusion

In summary, the cause of RED-S is the physiological stress response to a persistent mismatch between energy intake and energy expenditure, known as low energy availability (LEA). This imbalance, which can be intentional or unintentional, forces the body to prioritize immediate survival over long-term health and optimal performance. By suppressing crucial physiological functions such as hormonal regulation, bone building, and immune response, RED-S creates a cycle of health and performance detriments. Recognizing and addressing the root cause through a multi-disciplinary approach involving nutrition, training adjustments, and psychological support is essential for an athlete's full recovery.

Optional Outbound Link: For more detailed information on preventing and treating RED-S, a resource for athletes and practitioners can be found on the REDinSport website.

Frequently Asked Questions

The primary cause of RED-S is low energy availability (LEA), which is a persistent energy imbalance where an athlete's dietary intake is insufficient to cover their energy expenditure from daily activities and exercise.

When the body detects an energy deficit from low energy availability, it conserves energy by suppressing non-essential physiological functions, leading to hormonal imbalances, slowed metabolism, and impaired immune function that cause the wide range of RED-S symptoms.

No, RED-S affects athletes of all genders. While originally known as the Female Athlete Triad, the name was changed to RED-S to reflect that the syndrome and its consequences also affect male athletes.

Yes, RED-S can be caused by unintentional under-fueling. This often happens when an athlete simply underestimates their high energy requirements, especially during periods of intense training, despite having healthy eating habits.

RED-S is not an eating disorder, though it can overlap with or be a result of disordered eating behaviors or a clinical eating disorder. The primary driver is the energy deficit, which can be caused by reasons other than body image concerns.

Psychological factors like perfectionism, high performance pressure, and anxiety can lead to behaviors that cause RED-S, such as intentional calorie restriction and overtraining. The resulting health issues can, in turn, worsen mental health.

Low energy availability disrupts hormone production throughout the body. In females, it can cause a drop in estrogen, leading to menstrual dysfunction. In males, it can lower testosterone. These imbalances affect numerous systems, including bone health and metabolism.