RED-S – Relative Energy Deficiency in Sport – is a syndrome that occurs when energy intake is insufficient to meet both the demands of training and basic bodily functions. It is a serious health problem that goes far beyond what was formerly known as the 'Female Athlete Triad'.

The original triad described the link between energy deficiency, menstrual irregularities, and reduced bone density in female athletes. The updated RED-S concept recognizes that the effects are much broader and can also affect men—although women are more frequently and severely affected.

The central mechanism is low energy availability (LEA). This is the energy remaining for bodily functions after deducting the energy expended during training. If this is too low, the body switches to a conservation mode and reduces or stops non-essential functions – including reproduction.

The effects impact virtually every bodily system: hormonal imbalances, bone weakness, slowed metabolism, cardiovascular impairment, mental health, immune function, protein synthesis, and more. Paradoxically, athletic performance also suffers – even though the energy deficit often arises from the pursuit of improved performance.

RED-S can affect anyone – elite athletes and recreational athletes, in endurance and strength sports, whether consciously restricting calories or experiencing unintentional energy depletion due to high training volume. Detection and prevention are essential for long-term health and, ironically, also for long-term athletic performance.

Low energy availability can have various causes – from intentional restriction to unintentional mismatch between demand and supply. Understanding these causes is important for prevention and intervention.

Intentional calorie restriction is a common cause. The desire to lose weight or achieve a specific body image leads to a conscious reduction in food intake. This pressure is particularly high in sports with weight classes, aesthetic requirements, or a perceived advantage through low weight.

Eating disorders – anorexia nervosa, bulimia nervosa, binge eating disorder, and atypical forms – may be the underlying cause. The line between disordered eating and a clinical eating disorder is fluid, and RED-S can occur at various stages. Exercise can both mask and exacerbate eating disorders.

Unintentional energy depletion occurs when training volume increases without a corresponding adjustment to nutrition. High training hours, intense phases, double training sessions – energy requirements can easily be underestimated, especially when appetite is suppressed by training.

A lack of nutritional knowledge can lead to not eating enough or the wrong things, despite good intentions. Low-fat diets, an excessive focus on 'clean' eating, and the avoidance of entire food groups can all contribute to energy deficiencies.

Time constraints – busy schedules, training alongside work or studies, travel – can lead to skipped or insufficient meals. The practical aspect of nutrition is often underestimated.

The symptoms of RED-S can be subtle and are often not immediately associated with low energy. In women, menstruation is an important – but not the only – indicator.

Menstrual irregularities are a classic warning sign. Amenorrhea – the absence of menstruation for three or more months – is a late sign. Earlier warning signs include oligomenorrhea – irregular cycles with intervals of more than 35 days – a shortened luteal phase, anovulatory cycles, or lighter bleeding than before.

But beware: Taking hormonal contraceptives masks these warning signs. A pill break is not a real period and provides no information about the underlying hormonal balance. Women on the pill can experience red spotting without noticing any obvious menstrual irregularities.

A performance plateau or decline despite hard training is an important sign. The body doesn't have enough energy to adapt and improve. Stagnation or decline can be frustrating and lead to the mistaken conclusion of training even more or eating less – a vicious cycle.

Fatigue beyond normal post-workout fatigue, concentration problems, irritability, and mood swings can indicate LEA (Leakage-Induced Anti-Emotional Deficiency). The body prioritizes energy for basic functions, and 'luxury functions' like a good mood suffer.

Frequent injuries and illnesses indicate that the immune system and tissue repair mechanisms are under-supplied. Stress fractures are a particularly serious warning sign of compromised bone health.

Sensitivity to cold – constantly feeling cold, cold hands and feet – is a sign of a slowed metabolism. The body conserves energy by reducing heat production.

One of the most serious consequences of RED-S is the impairment of bone health. The effects can be irreversible and impact health far beyond an athlete's active career.

Estrogen deficiency due to hormonal suppression is a major contributing factor. Estrogen protects bones and promotes their formation. With low estrogen levels – as in amenorrhea – bone resorption predominates, and bone density decreases.

But it's not just estrogen that's affected. The energy deficiency itself directly impacts bone metabolism. Insufficient nutrients – calcium, vitamin D, protein – limit bone formation. Elevated cortisol levels due to chronic stress promote bone loss.

Adolescence and early adulthood are critical periods for bone formation. During this time, maximum bone mass is reached – the 'bone bank' that will be drawn upon later. RED-S during this phase can prevent this maximum mass from ever being reached – with lifelong consequences.

Stress fractures are an acute manifestation of bone problems. These fatigue fractures result from repetitive stress on weakened bones. They can interrupt training and competitions and are a clear warning sign.

The long-term outlook is worrying. Women with a history of RED-S have an increased risk of osteoporosis and fractures later in life. Some bone loss is not fully reversible, even after normalizing energy intake.

RED-S does not only affect bones and hormones. Cardiovascular and metabolic health are also at risk, with potentially serious consequences.

The resting metabolic rate decreases in response to the energy deficit. The body attempts to conserve energy by reducing its basal metabolic rate. This makes further weight loss more difficult and facilitates weight gain when intake normalizes – often frustrating for those affected.

Cardiac function may be impaired. Bradycardia – a slowed heartbeat – is a common sign of severe energy deficiency. Heart rate variability may be altered. In cases of extreme deficiency, structural changes in the heart are possible.

Paradoxically, the lipid profile can be worsened. Even with reduced food intake, LDL cholesterol can rise – a sign of impaired metabolism. This increases long-term cardiovascular risks.

Blood sugar regulation may be impaired. Low blood sugar – hypoglycemia – can occur during exercise, with symptoms such as dizziness, weakness, and confusion. Long-term insulin sensitivity may be disrupted.

Thyroid function is downregulated. The body reduces the production of the active thyroid hormone T3 to conserve energy. This contributes to a slower metabolism and can cause symptoms such as cold intolerance, dry skin, and hair loss.

The irony of RED-S is that the energy deficit – often in the pursuit of better performance – sabotages the performance itself. The short-term 'advantages' of a lower weight are negated by a variety of impairments.

Training adaptation is reduced. Without sufficient energy, the body cannot carry out the repair and building processes that make training effective. The result: hard training without progress. Frustration often leads to even more training or even less food – a vicious cycle.

Recovery is impaired. Recovery between training sessions takes longer. Muscle soreness persists, fatigue accumulates. Performance in consecutive training sessions decreases.

Strength is reduced because muscle growth is suppressed and muscle mass may even be lost. The body uses muscle protein as an energy source when other sources are depleted.

Endurance performance suffers due to reduced glycogen stores, compromised oxygen supply, and a limited ability to reach and maintain high intensities.

The risk of injury increases – not only stress fractures, but also muscle injuries, tendon problems, and general susceptibility. Immune function is reduced, leading to more frequent infections and missed training sessions.

Regularly monitoring your health parameters can uncover early warning signs before your performance drops significantly.

The link between RED-S and mental health is bidirectional. Psychological factors can contribute to RED-S, and RED-S can worsen mental health. This interaction makes treatment complex.

Eating disorders and disordered eating behavior are both a cause and a complication. The perfectionism, body image issues, and need for control that often occur in competitive athletes can lead to restrictive eating. The physiological effects of energy deficiency then exacerbate psychological symptoms.

Depression and anxiety disorders are more common in RED-S. The energy deficiency affects neurotransmitters and hormonal axes that regulate mood. Serotonin, the 'happiness hormone', requires sufficient nutrients for its production.

Cognitive function suffers. Concentration problems, memory impairment, slow reactions – the brain is an energy-hungry organ and suffers when it is undersupplied. For female students, athletes, or working professionals, this can have serious consequences.

The relationship with sport can become dysfunctional. Training becomes an obligation, rest days trigger anxiety, and one's identity becomes completely tied to athletic performance. These patterns can hinder recovery.

Recognizing psychological components is important for treatment. Often, psychological support is an essential part of the intervention, not just nutritional counseling. Treating RED-S requires a multidisciplinary team.

The diagnosis of RED-S is based on a combination of symptoms, physical examination, laboratory results, and medical history. There is no single test that confirms or rules out RED-S.

The medical history is crucial: training volume and type, dietary habits, menstrual history, injury history, weight development, mood, and energy levels. Questionnaires such as the LEAF-Q – Low Energy Availability in Females Questionnaire – can be helpful in screening.

The physical examination may reveal signs of malnutrition: low body weight, bradycardia, hypotension, dry skin, hair loss, dental problems during vomiting.

Laboratory tests typically include: blood count, hormone levels – LH, FSH, estrogen, progesterone, thyroid hormones, cortisol –, metabolic markers, electrolytes, iron levels, and vitamin D. Low LH and estrogen levels, reduced T3, and elevated cortisol may indicate LEA.

A bone density measurement – ​​DEXA – is indicated in cases of long-standing menstrual disorders or stress fractures. Low bone density confirms the severity of the situation and indicates the urgency of treatment.

Calculating energy availability is theoretically possible, but practically difficult. It requires precise measurement of energy intake and training effort, which is prone to error. Clinical signs are often more informative.

Prevention is the best approach with RED-S. Developing healthy attitudes towards nutrition and the body, as well as being aware of warning signs, protects against this harmful spiral.

Energy intake must match training volume. Higher training volume requires more food. This sounds obvious, but is often overlooked. Appetite is not a reliable indicator – it can be suppressed by training, stress, and other factors.

A practical guideline: At least 45 kcal per kilogram of fat-free mass per day should be available after deducting training expenditure. Problems typically arise below 30 kcal/kg FFM. This calculation is complex, but it illustrates the order of magnitude.

Nutritional quality is important, but not as a substitute for quantity. 'Clean' eating with too few calories leads to LEA just as much as an unhealthy diet. All macronutrients are necessary: ​​sufficient carbohydrates, protein, and fat.

Rest days and recovery are part of training, not laziness. The body needs time and energy to adapt. More training isn't always better – sometimes it's worse.

Use menstruation as a health indicator. Regular cycles show that the body has enough energy. Absence or irregularities are warning signals, not normal 'athlete' symptoms.

Critically examine your weight. Is your target weight realistic and healthy? Does weight loss truly improve performance? The assumption that 'lighter is faster' is often false and dangerous.

Treating RED-S requires a multidisciplinary approach and patience. It is not a quick fix, but a process that can take months to years, depending on the severity and duration of the condition.

Increasing energy intake is the key. This often means eating more than feels comfortable, especially if your metabolism is downregulated. Working with a sports nutritionist is very helpful.

Training reduction may be necessary. In cases of severe RED-S, a temporary break from sports or a significant reduction in training intensity is required to give the body the energy needed for healing. This is emotionally very difficult for performance-oriented athletes, but often essential.

Psychological support is necessary in many cases. Overcoming restrictive thought patterns, developing a healthier body image, and treating potential eating disorders require professional help.

Menstruation is a marker of recovery. The resumption of regular cycles indicates that the body has enough energy. For some women, this takes months after normalizing their diet.

Bone health requires special attention. This includes sufficient calcium and vitamin D, bone-strengthening exercise – if the general condition allows – and possibly medication in cases of severe osteoporosis.

The return to full training is gradual and only occurs with stable recovery. The desire to quickly return to the previous level must give way to reality. In the long run, a complete recovery is more important than short-term performance.