Are You at Risk of RED-S?

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Ruby Wyles

By: Ruby Wyles

Runner, triathlete, and passionate coach, Ruby is most fulfilled by helping athletes overcome limiting beliefs with joy. She is also a proud science nerd, and advocate for athletes' mental and physical health.

It’s no coincidence that proper fueling, particularly eating enough, is a hot topic across all Freetrail outlets. Recently, both the SubHub and Trail Society podcasts have discussed Relative Energy Deficiency in Sports (RED-S) and the consequences of underfueling, so synthesizing it here seemed timely. Sports dietitian to some of the highest profile trail and ultra runners, such as Canyons by UTMB 100 kilometer winner Rod Farvard, Vic Johnson, spoke with Emkay Sullivan and Dani Moreno on this recent episode of the SubHub podcast: Candid Coaching | Understanding RED-s with MPH, RDN Vic Johnson. While physician and leading female athlete researcher Dr. Emily Kraus came on the Trail Society podcast to chat about all things female athlete health, as well as the consequences of under fueling on bone health, performance, and more. If you haven’t already, add both episodes to your next long-run playlist!

What is RED-S?

Relative Energy Deficiency in Sports (RED-S) is a complex syndrome that affects athletes when there is an imbalance between their energy intake and the energy expended during exercise and daily activities.This mismatch in energy intake and expenditure is known as low energy availability (LEA), and RED-S is the consequence of prolonged LEA. RED-S was initially understood within the scope of the Female Athlete Triad, which includes disordered eating, menstrual dysfunction, and decreased bone mineral density. However, the concept of RED-S broadens this understanding, recognizing that it affects both male and female athletes, and can occur independent of disordered eating, and encompasses a wider range of health and performance consequences. 

Causes and Mechanisms

The primary cause of RED-S is low energy availability (LEA), an insufficient caloric intake relative to the amount of energy expended. Both Dr. Emily Kraus and Vic Johnson, RDN, highlighted recent research suggesting that low-carbohydrate availability, the result of not consuming enough carbohydrates, in particular, has a direct effect on RED-S and hormonal suppression, as illustrated in the diagram below.

Proposed theoretical model; low carbohydrate availability (LCA) has independent health and performance consequences.
Proposed theoretical model; low carbohydrate availability (LCA) has independent health and performance consequences.

In endurance sports like trail running, when athletes are putting in many hours of training each week, often fitting it in around an incredibly busy life – a full-time job, family, travel, and other life commitments. Needless to say, athletes’ energy expenditure can be extremely high, potentially even higher for non-elite athletes juggling so many additional life demands alongside training. As a result, LEA can occur unintentionally due to a lack of awareness about athletes’ nutritional requirements. However, energy deficits can be intentional, such as through restrictive dieting or overtraining without adequate nutritional support, often with athletes falsely believing being lighter will make them faster, or that attaining a “runner’s body” will aid performance. To further understand the risk of low energy availability, disordered eating, exercise dependence, and fueling habits in trail runners, check out this article from Keely Henninger, More Fuel For The Fire.

When the body does not receive enough energy, it prioritizes essential functions such as maintaining core body temperature and vital organ function over other processes. This adaptation can lead to a cascade of physiological disruptions. For instance, reproductive functions may be suppressed, leading to menstrual irregularities and amenorrhea (the loss of a menstrual cycle) in women and decreased testosterone levels in men. Similarly, metabolic rate may decrease, bone health can become compromised, and the immune system weakened, making athletes more susceptible to illnesses and injuries, as well as, ironically, worsening body composition.

Symptoms and Consequences

Figure 1 The REDs 2023 conceptual models

The symptoms and consequences of RED-S can be divided into health and performance categories:

Health Consequences:

  • Menstrual Dysfunction: Irregular or absent menstrual cycles in women.
  • Hormonal Imbalances: Decreased levels of estrogen in women and testosterone in men.
    • In women, hormonal imbalances are easy to track, with a regular period being the best sign that their bodies are primed to adapt to training. 
    • Men should pay attention to the frequency of morning erections, with decreased frequency and intensity being a sign of decreased testosterone levels.
  • Bone Health: Increased risk of bone stress injuries, such as stress fractures and reactions, as well as osteopenia and osteoporosis due to reduced bone density.
    • As Dr. Emily Kraus explained, estrogen is essential for optimal bone health, so hormonal suppression reduces bone remodeling, leaving athletes more susceptible to bone injuries.
  • Metabolic Issues: Slowed metabolic rate and impaired nutrient absorption.
    • Athletes may intentionally under-eat to try to lose fat and improve body composition, but this can have the opposite effect and actually lead to worsening body composition. When a body is underfueled, it will break down lean muscles for energy, while holding on to fat for basic survival functions like keeping warm.
  • Psychological Effects: Increased risk of depression, anxiety, and irritability.
    • Not to mention eating disorders and obsessive compulsive disorder: an undernourished brain is more susceptible to fixation on food, among other things.
  • Cardiovascular Health: Increased risk of cardiovascular disease due to altered lipid profiles.
    • Underfueling often increases cholesterol and triglyceride levels in the blood, which may further motivate athletes’ to restrict their intake under the false assumption that they are overconsuming fats, when in fact the opposite is likely true.

Performance Consequences:

  • Decreased Endurance: Reduced glycogen stores and muscle mass impair endurance and strength.
  • Increased Injury Risk: Poor recovery and weakened bone health increase susceptibility to injuries.
  • Impaired Adaptation: Reduced ability to build muscle and adapt to training stimuli.
    • A common sign of RED-S, as highlighted by Vic Johnson, RDN, is a slower than normal (or expected) recovery from training, as well as a lack of improvement, or even decreasing performance.
  • Concentration and Coordination Issues: Cognitive function may be impaired, affecting focus and coordination, not to mention academic achievement and work productivity.

Diagnosis and Management

RED-S is what’s called a diagnosis of exclusion, meaning that there are no definitive tests for RED-S, instead, other similarly-presenting health conditions must be ruled out before RED-S is diagnosed. This requires a multidisciplinary approach involving healthcare providers, dietitians, and sports scientists. Key indicators include a thorough assessment of dietary intake, training loads, menstrual history in women, and clinical signs such as weight changes, bone health status, and injury and illness history. 

Blood tests

While there is no definitive “RED-S blood test”, certain blood biomarkers may be red flags.That said, blood tests in isolation cannot be used to diagnose RED-S.

  • Low ferritin
  • Low folate
  • Low vitamin B12
    • Both vegan athletes and those with RED-S often present with low B12 levels
  • Low vitamin D
  • Low testosterone in men
  • Low estrogen and follicle-stimulating hormone (FSH) in women
  • High sex-hormone-binding globulin (SHBG)
  • High cholesterol
  • High cortisol 

During the diagnostic process, doctors should also check athletes’ complete blood count (CBC), iron panel, thyroid function, and metabolic profile. Iron deficiency, with or without anemia, thyroid and metabolic diseases, depression, eating disorders, and other mental illnesses often present with similar symptoms to RED-S, so healthcare providers must screen for and treat these conditions first.

RED-S Recovery

Management of RED-S focuses on restoring energy balance through nutritional interventions and adjusting training loads. Strategies include:

  • Nutritional Rehabilitation: Athletes’ should work with a registered dietitian to guide them on an adequate caloric and nutrient intake to meet their energy demands, paying particular attention to optimal carbohydrate consumption. As Vic Johnson, RDN, suggested in the recent SubHub podcast, this is often a gradual process of adding in extra snacks and increasing portion sizes. 
  • Training Interventions: Research suggests that athletes’ recovering from RED-S, in particular the hormonal suppression, benefit from reducing training volume, especially sustained endurance activities. Strength training should be encouraged to promote bone health. That said, if athletes’ aren’t meeting their caloric needs, any training can be detrimental to athletes’ health and keep them in a state of RED-S.
  • Education: Athletes, coaches, and support staff need to be educated about the importance of energy balance, the risks of RED-S to health and performance, trustworthy nutrition advice, and proper athlete development, especially through puberty. This graph below illustrates the significant lack of knowledge of RED-S among female cross-country athletes and support staff, thus the need for education here is paramount.
  • Monitoring: Regular monitoring of health and performance indicators to ensure recovery and prevent recurrence. This may include: blood testing, DEXA scans to assess bone health, body weight and composition, training metrics, diet, and sleep.
  • Psychological Support: Increasing caloric intake, reducing training load, and, if necessary, increasing body weight can be very psychologically challenging for athletes’ dealing with underlying eating disorders, disordered eating behaviors and thought patterns, exercise compulsions, and/ or other mental health issues. Seeking support from a therapist is essential in order for RED-S recovery to be successful long-term.
This graph illustrates the significant lack of knowledge of RED-S among female cross-country athletes and support staff.
This graph illustrates the significant lack of knowledge of RED-S among female cross-country athletes and support staff.


Preventing RED-S involves fostering a culture that values health and long-term performance over short-term gains. This includes promoting healthy eating habits, ensuring adequate rest and recovery, and providing education about the risks of energy deficiency. Sports dietitians and psychologists have a pivotal role here, educating athletes on the importance of adequate nutrition, maintaining a regular menstrual cycle, and debunking misleading beliefs that weighing less will improve performance. Coaches and sports organizations also play a crucial role in creating an environment where athletes feel supported in maintaining their health while striving for excellence.


RED-S is a significant issue in sports, impacting athletes’ health and performance, and will continue to be a topic of discussion across Freetrail’s various media outlets. While in the short-term, weight loss may yield slightly improved performance, any advantage is short-lived, and sustained performance and progression is hindered by underfueling and RED-S, not to mention athletes’ health. By understanding the causes, recognizing the symptoms, and implementing comprehensive management and prevention strategies, the sports community can help athletes achieve their full potential in a healthy and sustainable manner. 


Burke, L. M., Ackerman, K. E., Heikura, I. A., Hackney, A. C., & Stellingwerff, T. (2023). Mapping the complexities of Relative Energy Deficiency in Sport (REDs): development of a physiological model by a subgroup of the International Olympic Committee (IOC) Consensus on REDs. British journal of sports medicine, 57(17), 1098–1108. https://doi.org/10.1136/bjsports-2023-107335

Cabre, H. E., Moore, S. R., Smith-Ryan, A. E., & Hackney, A. C. (2022). Relative Energy Deficiency in Sport (RED-S): Scientific, Clinical, and Practical Implications for the Female Athlete. Deutsche Zeitschrift fur Sportmedizin, 73(7), 225–234. https://doi.org/10.5960/dzsm.2022.546 

Correction: 2023 International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). (2024). British journal of sports medicine, 58(3), e4. https://doi.org/10.1136/bjsports-2023-106994corr1 

Lodge, M. T., Ward-Ritacco, C. L., & Melanson, K. J. (2023). Considerations of Low Carbohydrate Availability (LCA) to Relative Energy Deficiency in Sport (RED-S) in Female Endurance Athletes: A Narrative Review. Nutrients, 15(20), 4457. https://doi.org/10.3390/nu15204457 

Lodge, M. T., Ackerman, K. E., & Garay, J. (2022). Knowledge of the Female Athlete Triad and Relative Energy Deficiency in Sport Among Female Cross-Country Athletes and Support Staff. Journal of athletic training, 57(4), 385–392. https://doi.org/10.4085/1062-6050-0175.21 

Mountjoy, M., Ackerman, K. E., Bailey, D. M., Burke, L. M., Constantini, N., Hackney, A. C., Heikura, I. A., Melin, A., Pensgaard, A. M., Stellingwerff, T., Sundgot-Borgen, J. K., Torstveit, M. K., Jacobsen, A. U., Verhagen, E., Budgett, R., Engebretsen, L., & Erdener, U. (2023). 2023 International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British journal of sports medicine, 57(17), 1073–1097. https://doi.org/10.1136/bjsports-2023-106994 

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