1. RED-S in endurance sports: causes, definition and relevance

Relative Energy Deficiency in Sport (RED-S) is one of the most significant, yet often underestimated, risks in endurance sports – and it affects not only professionals, but also dedicated recreational runners, triathletes, and cyclists. The core of the problem: those who train too much and consume too little energy (calories) over an extended period not only jeopardize their performance, but also their health.

What does RED-S mean and why is it so relevant?

RED-S stands for relative energy deficit in sports. This means that your body uses more energy through training and everyday activity than you consume through food. The risk is particularly high in endurance sports such as running, triathlon, swimming, or cycling because the energy demand is very high and the deficit often occurs unconsciously [1][2].

The consequences are often underestimated:
- Decline in performance, constant fatigue
- Hormonal disorders (e.g. menstrual irregularities, low testosterone levels)
- Reduced bone density, increased risk of stress fractures
- Increased susceptibility to infection and impaired regeneration

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#### Who is affected? Differentiation from the “Female Athlete Triad”

Contrary to previous assumptions, RED-S does not only affect women. The "Female Athlete Triad" primarily refers to energy deficiency, menstrual irregularities, and reduced bone density, while RED-S as a clinical condition also affects men: In addition to decreased performance, they exhibit typical hormonal changes such as significantly reduced testosterone levels, loss of libido, and increased fatigue [1][2].

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#### Energy availability as a key factor

Energy availability (EA) describes the amount of energy your body has left after deducting the energy spent during exercise – based on your fat-free mass (FFM). Scientific recommendations suggest a minimum of 45 kcal per kilogram of FFM per day. If this number consistently falls below 30 kcal/kg FFM, important bodily functions are no longer optimally supported. For example, a 75 kg athlete with 15% body fat often falls below this threshold – a dangerous zone for bones, hormones, and metabolism [2].

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#### Causes and Risk Factors

Typical triggers for RED-S are:
- Excessive training volume without an adjusted diet
- Conscious calorie saving or insufficient knowledge of one's own energy requirements
- Psychological pressure regarding body weight or competitive performance

The risk is high in sports where weight or appearance is considered relevant to performance – especially in endurance sports [1][2].

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#### Recognize warning signs and take action

Symptoms such as persistent fatigue, decreased performance, training plateaus, frequent infections, menstrual irregularities (in women), loss of libido (in men), or even stress fractures should always be taken seriously. RED-S often develops gradually, and many coaches or athletes mistake it for overtraining [1][2].

**Important:** Only those who know and regularly monitor their energy requirements can minimize the risk. Apps, calorie tables, and knowledge of one's individual lean body mass can be helpful – consulting a nutritionist or sports physician may also be advisable.

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**Conclusion:**
RED-S applies to all endurance athletes, regardless of gender and body weight. Those who keep an eye on their energy requirements, regularly check their training and nutrition data, and also consider psychological factors can minimize risks and remain healthy and perform at their best in the long term [1][2].

Sources
[1] RED-S Syndrome: A Guide to Relative Energy Deficit ... https://blackroll.com/de/artikel/red-s-syndrom
[2] Risk of RED-S in endurance athletes https://junkmiles.de/red-s-energie-defizit-ausdauersport/