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Definition and Aetiology of the RED-S Syndrome
Relative Energy Deficiency in Sport (RED-S) is a comprehensive clinical syndrome caused by low energy availability (LEA). It represents a functional impairment of a wide range of physiological systems that are necessary not only for training but also for maintaining homeostasis (= balance) and the basic functions of the body.
Low Energy Availability (LEA)
Low energy availability (LEA) is the central pathophysiological cause of RED-S. It describes the state in which the energy available to the body, after subtracting the energy expenditure from physical activity (EEE = exercise energy expenditure), is insufficient to maintain vital functions in their entirety.
Energy availability (EA) is defined as follows:
EA = (energy intake – energy expenditure through training) / fat-free mass (FFM)
- Energy availability is expressed in kcal/kg FFM/day.
- LEA is present when energy availability remains permanently below 30 kcal/kg FFM/day. The body begins to downregulate vital processes in order to conserve energy.
- For gaining mass, an energy availability > 45 kcal/kg FFM/day should be targeted.
- Optimal energy availability is considered to be 45 kcal/kg FFM/day.
- For weight loss, a slight deviation below this value is possible. An energy availability of 30–45 kcal/kg FFM/day is considered tolerable for a limited time.
Low energy availability may arise unintentionally (e.g., due to insufficient knowledge about increased caloric requirements during high training volume) or intentionally (e.g., in the context of an eating disorder or a deliberate attempt to reduce body weight).
In my work with patients or athletes, I often observe attempts to reduce weight in the hope of improving performance. Frequently, there is implicit or explicit external pressure on athletes from coaches, parents, etc. Social media and pervasive “beauty ideals” further reinforce this.
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Systemic Effects and Clinical Manifestations
RED-S is not limited to a single bodily system; it can affect all physiological systems. The following clinical manifestations are characteristic and can vary depending on sex.
Hormonal and Reproductive Disorders
The hypothalamic–pituitary–gonadal axis is suppressed by low energy availability (hypogonadotropic hypogonadism), which restricts reproductive function.
- In women: absence of menstruation (amenorrhoea), menstrual irregularities, or oligomenorrhoea.
- In men: low testosterone levels, reduced libido, decreased ejaculate volume, or erectile dysfunction. Reduced morning erections may also be a warning sign.
- Low-T3 syndrome (reduction in the thyroid hormone fT3 without an underlying thyroid disorder)
Cardiovascular Disorders
- Sinus bradycardia
- QT prolongation
- Low blood pressure
- Increased cholesterol levels
Growth and Development
- Due to reduced secretion of growth hormones (IGF-1), children/adolescents may experience delayed or absent physical growth.
Skeletal and Bone Health
- Reduced secretion of sex hormones (oestrogen, testosterone) and changes in calcium and vitamin D metabolism lead to decreased bone density.
- Increased risk of stress fractures, particularly in the lower extremity.
- Development of osteopenia up to osteoporosis.
Gastrointestinal Symptoms
- Reduced intake of fibre and carbohydrates can lead to changes in the microbiome.
- Impaired barrier function due to reduced energy availability.
- Psychological factors such as stress and anxiety contribute to gastrointestinal complaints.
Immune System
- Reduced immune function, leading to increased susceptibility to upper respiratory tract infections and prolonged illness duration.
Psychological and Cognitive Symptoms
- Mood swings, irritability, memory impairment
- Sleep disturbances
- Motivation problems
Sport-Specific Symptoms
- Reduced adaptation to training stimuli
- Reduced training availability due to illness and injury
- Impaired recovery
- Decline in cognitive performance
- Reduced muscle strength
- Reduced endurance performance
It is important to note that all these symptoms may also occur independently of low energy availability. Other potential causes should always be excluded through medical diagnostics.
What to Do in Case of RED-S?
First, RED-S should be properly diagnosed. This often requires extensive assessment and substantial expertise regarding this still insufficiently known condition. Ideally, affected athletes should be evaluated in a specialised sports medicine centre.
In Berlin, the sports medicine outpatient clinic at Charité offers a consultation for RED-S patients, connected to a sports gynecology network. A similar network exists in Tübingen.
How Can I Prevent RED-S?
Ensure adequate energy intake through a balanced diet. Ideally, you know your fat-free mass (which can be measured via skinfold measurements with a caliper, a professional body composition analysis, or approximately using a home body fat scale). Once you know your fat-free mass, you can calculate the amount of energy you require for optimal energy availability using the formula above. Below are two calculation examples:
Runner
65 kg, 20% body fat, approx. 1500 kcal energy expenditure through activity.
Energy intake: 3800 kcal
Fat-free mass = 52 kg
Energy Availability = (3700 kcal – 1500 kcal) / 52 kg = 44.2 kcal/kg FFM/day
→ quasi-optimal energy availability
Bodybuilder
98 kg, 4% body fat, approx. 1300 kcal energy expenditure through activity.
Energy intake: 4000 kcal
Fat-free mass = 94 kg
Energy Availability = (4000 kcal – 1300 kcal) / 94 kg = 28.7 kcal/kg FFM/day
→ low energy availability
The runner is well supplied with energy and is likely not currently at risk for RED-S, whereas the bodybuilder in this example is in a state of low energy availability which, if sustained long enough, will likely result in RED-S.
How long low energy availability must persist to induce RED-S is unknown; however, it is likely that mildly reduced energy availability can persist longer before resulting in RED-S compared to severely reduced energy availability.
Conclusion
RED-S is a complex and potentially serious syndrome that can significantly impair both health and athletic performance. Early recognition of warning signs and an awareness that chronic energy deficiency is not merely a nutritional issue but a systemic issue of load and recovery is essential. Through targeted prevention, continuous education, and interdisciplinary support, athletes can significantly reduce their risk of RED-S. Adequate energy availability is not optional. It is a fundamental prerequisite for long-term performance development, health, and well-being. Early recognition of warning signs and adequate energy availability are crucial for healthy performance improvement. Therefore, at Enduure, in our individual coaching with athletes, feedback and regular communication with our athletes are particularly important, as this allows us to identify potential issues, such as RED-S, and provide appropriate guidance.
References
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;57:1073–1098.
