Relative energy deficiency in sport (RED-S): when training too much and eating too little makes you ill
Chronically consuming less energy than training expends does not improve performance. It breaks down hormones, bone, immunity, and the mind, in both women and men.
What RED-S is and why it matters
The root is low energy availability: insufficient energy for the body's functions after the cost of exercise.
The acronym RED-S stands for relative energy deficiency in sport and names a syndrome of harm to health and performance caused by low energy availability, that is, insufficient energy relative to the cost of exercise. It was formalized by the International Olympic Committee and updated in 2018 (Mountjoy et al., 2018).
The logic is simple and counterintuitive: chronically eating less than training consumes does not make the athlete leaner and faster in a sustainable way. The body cuts functions it considers nonessential in order to save energy, and health and performance decline together.
Since the 2018 update, more than 170 new studies have broadened the field, including the role of low carbohydrate availability, the relationship with mental health, and the impact on male athletes (Mountjoy et al., 2023).
From the female athlete triad to a syndrome affecting everyone
The concept was born from the female athlete triad and grew to include men, with a drop in testosterone among the signs.
RED-S is the evolution of the female athlete triad concept, which linked low energy availability, menstrual dysfunction, and low bone density. The triad remains valid but describes only part of the picture (Mountjoy et al., 2018).
The expanded model recognizes that men also become ill with insufficient energy, with a drop in testosterone among the markers, and that the harms go well beyond bone and menstruation (Mountjoy et al., 2018).
The 2023 consolidation reinforced this broadening and brought mental health to the center, both as a cause and as a consequence of low energy availability (Mountjoy et al., 2023).
The systems that break down
The bill for energy saving is paid by several systems at once: hormones, bone, immunity, mood, and performance.
Low energy availability affects multiple systems at once. It is not an isolated symptom but a pattern that the attentive clinician recognizes by the combination (Mountjoy et al., 2018).
In the hormonal axis, menstrual dysfunction and amenorrhea appear in women and a drop in testosterone in men. In bone, mineral density is reduced, with more stress fractures. Resting metabolism slows to conserve energy.
The bill continues through lowered immunity, the gastrointestinal tract, the cardiovascular system, and the psychological sphere. And, contrary to what the rush for results promises, performance worsens, with less strength, endurance, and training capacity (Mountjoy et al., 2018).
| System | Typical sign |
|---|---|
| Hormonal | Amenorrhea in women, drop in testosterone in men |
| Bone | Low mineral density and stress fractures |
| Metabolic | Reduced resting metabolism |
| Immune | More infections and slow recovery |
| Psychological | Changes in mood and in the relationship with food |
| Performance | Less strength, endurance, and training capacity |
Health and performance: the 2023 models
The 2023 consolidation separated problematic exposure from adaptable exposure and introduced a physiological model.
The 2023 consolidation updated the conceptual models of health and performance and introduced a new physiological model, designed to represent the complexity of the problem (Mountjoy et al., 2023).
The central distinction is between problematic exposure and adaptable exposure to low energy availability. Not every momentary reduction becomes disease; what causes illness is prolonged exposure, combined with individual modulating factors (Mountjoy et al., 2023).
These individual factors explain why two athletes with the same diet and the same training can have different outcomes. Assessment, therefore, is individual, not a single yardstick.
How it is detected: the REDs CAT2 tool
The 2023 assessment uses a clinical tool with risk stratification and recommendations for training and competition.
The 2023 consolidation presented the second version of the REDs clinical assessment tool, which organizes diagnosis by cumulative severity and by risk stratification, with associated recommendations for training and competition (Mountjoy et al., 2023).
The document also outlined guidelines for a safe and effective body composition assessment, precisely to help prevent RED-S, rather than encouraging measurements that reinforce risky behaviors (Mountjoy et al., 2023).
In practice, screening begins with attention to the signs: menstrual changes, stress fractures, unexplained performance decline, persistent fatigue, and a dysfunctional relationship with food. Confirmation is clinical and multidisciplinary.
Prevention and treatment: eating enough
Treatment is restoring energy availability. There is no performance shortcut in undereating.
The basis of treatment is to increase energy availability, with more energy in the diet, less expenditure, or both, always under guidance. The approach is multidisciplinary, with a physician, a nutritionist, and, when necessary, psychological support (Mountjoy et al., 2023).
The message for the athlete and for anyone who trains hard is direct: chronically undereating is not a performance strategy, it is a disease factor. Sustainable results come from having enough energy to train, recover, and live.
In the face of signs of RED-S, the course of action is not to tighten the diet further, but to seek specialized evaluation. This text is educational and does not replace consultation with a health professional.
Why this matters for your care
RED-S describes what happens when the energy available to the body, after the cost of training is subtracted, becomes too low for a prolonged period. It is a health and performance problem, recognized by the International Olympic Committee, that affects athletes of both sexes. This text is educational and does not replace medical evaluation.
References
- Mountjoy M, Sundgot-Borgen JK, Burke LM, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine. 2018. doi:10.1136/bjsports-2018-099193
- Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine. 2023. doi:10.1136/bjsports-2023-106994
Educational and scientific content. It does not constitute diagnosis, prescription or individual clinical guidance, and does not replace a medical consultation. Management decisions must be individualized by a physician.