A 19-year-old middle-distance runner comes to the outpatient clinic with pain on the inner side of the tibia that worsens with every training session. When asked, she says her periods stopped five months ago, that she has been feeling tired and that she cut carbohydrates to "get lighter" before races. Her weight, she says, is stable. In the same week, a 34-year-old cyclist arrives with a drop in performance, repeated colds and low sex drive; he trains 15 hours a week and eats little at dinner to stay in shape on the climbs. Both cases are fictional and show the same problem seen from different sides.

The name of this problem has changed. For decades people spoke of the female athlete triad, with three points: low available energy, menstrual change and low bone mass (De Souza, 2014). In 2014 the International Olympic Committee proposed a broader concept, relative energy deficiency in sport, which includes men and many other systems beyond bone and ovary. The 2023 consensus updated the model, adopted the acronym REDs and brought a staged assessment tool (Mountjoy, 2023).

This text follows the path from the triad to REDs, explains what energy availability is, shows the effects on the body, looks at men and at Paralympic athletes, describes screening and diagnosis, presents the debate the concept still provokes and ends with treatment and return to sport.

Key points

• REDs is a syndrome of physical and psychological impairment caused by low energy availability that is problematic, prolonged or intense, in women and men (Mountjoy, 2023).
• Indicators of REDs appear in 23 to 79.5 out of every 100 female athletes and in 15 to 70 out of every 100 male athletes, a wide range because the syndrome is a diagnosis of exclusion (Mountjoy, 2023).
• The classic threshold of 30 kcal per kilogram of fat-free mass per day comes from short studies with sedentary women and serves poorly as an individual cutoff (Areta, 2021).
• Stable weight does not rule out the problem, because the body adapts and spends less (De Souza, 2014).
• In a study of 50 male cyclists, 28 out of every 100 had low energy availability and 44 out of every 100 had low bone density in the lumbar spine (Keay, 2018).
• The central treatment is to restore available energy, above all by increasing intake, with a team of a physician, a sports nutritionist and a mental health professional (De Souza, 2014).

Summary

• From the female athlete triad to REDs
• Energy availability: the calculation the body makes
• What lack of energy does to the body
• Men and Paralympic athletes
• Screening and staged assessment
• Differential diagnosis and the debate about the syndrome
• Treatment and return to sport
• How NEXO helps train this reasoning
• Frequently asked questions
• Take-home messages

From the female athlete triad to REDs

The triad was born from the observation of female athletes with amenorrhea and stress fractures. The triad coalition described three components, each on a continuum from healthy to ill: energy availability, with or without disordered eating; menstrual function, from normal to functional hypothalamic amenorrhea; and bone health, from ideal bone mass to osteoporosis (De Souza, 2014). The coalition's rule of thumb remains useful: anyone with one component should be investigated for the other two.

The model left out two things. The first was men, who also eat less than they spend and also lose bone. The second was the systems that low energy affects beyond the ovary and the skeleton, such as metabolism, immunity, gut, heart, blood and mood. That is what the IOC tried to correct in 2014, with an update in 2018, and again in 2023 (Mountjoy, 2023).

What the 2023 consensus brought

The 2023 group gathered 17 specialists, 10 women and 7 men, and used a formal agreement method, with a threshold of 80 votes out of every 100 to accept each statement. Of the 144 approved statements, 27 had minority disagreement, a sign that the topic is still being built (Mountjoy, 2023).

The consensus also shows where the evidence is weak. Since 2018, about 178 studies have been published, with about 23,822 participants, and 80 out of every 100 participants were women. Only 20 out of every 100 studies included men. The acronym changed from RED-S to REDs, and the definition began to speak of "problematic" low energy availability, to separate the brief and adaptable drop from the drop that causes harm.

Aspect	Female athlete triad	REDs (IOC, 2023)
Who is included	Women athletes	Women and men athletes, including Paralympic athletes
Central cause	Low energy availability, with or without eating disorder	Problematic low energy availability, prolonged or intense
Outcomes	Menstrual function and bone health	More than ten systems, plus performance and mental health
Tool	Cumulative risk score for clearance	CAT2, in three steps, with a four-color traffic light
Sources	De Souza, 2014	Mountjoy, 2023

Energy availability: the calculation the body makes

Energy availability is the energy left over for the body to function after exercise. The calculation is simple on paper: energy intake minus exercise expenditure, divided by fat-free mass, per day (Areta, 2021). When little is left over, the body saves energy: it lowers resting metabolism, reduces thyroid hormones, suppresses the reproductive axis and decreases bone formation.

The figure of 30 kcal per kilogram of fat-free mass per day became famous as a threshold. It comes from experiments of a few days in young sedentary women, in which energy was controlled in the laboratory, and Areta's review shows that these thresholds are debated in women and unknown in men (Areta, 2021). The 2023 consensus adds that, in men, changes seem to appear at lower values, between 9 and 25 (Mountjoy, 2023).

Why the calculation is difficult in real life

Measuring intake and expenditure outside the laboratory is imprecise. Food records underestimate what is eaten, training expenditure varies and fat-free mass requires a body composition exam. That is why the consensus prefers to speak of a continuum, from adaptable low availability, brief and without harm, to problematic low availability, which is prolonged or intense and leads to clinical signs (Mountjoy, 2023).

The timing of intake also matters. Logue's review gathers studies that measure energy balance hour by hour and suggests that spending long periods of the day in deficit, even with a similar daily total, is associated with hormonal changes (Logue, 2020). Training after a long fast and eating a large dinner can hide a deficit that the daily average does not show.

Carbohydrate, beyond calories

The consensus highlights a theme that has gained strength: low carbohydrate availability seems to amplify the effects of low energy, with worsening of iron and bone markers in studies of a few days (Mountjoy, 2023). Carbohydrate-restricted diets, common in endurance sport, thus enter the clinical conversation.

What lack of energy does in the body

The IOC model organizes the effects into health and performance. Health includes reproductive function, bone, metabolism, blood, growth, immunity, gut, heart, mental health and other systems (Mountjoy, 2023). Performance includes endurance, strength, recovery, coordination, concentration and response to training.

Menstrual cycle and bone

In women, suppression of the hypothalamic axis leads to functional hypothalamic amenorrhea, which is a diagnosis of exclusion: first, pregnancy, thyroid and prolactin changes and other causes of amenorrhea must be ruled out (De Souza, 2014). Lack of estrogen and low energy, together, reduce bone formation and increase bone resorption.

Bone is the most serious point for those who are young. About 90 out of every 100 parts of peak bone mass are reached by age 18, and years of amenorrhea in adolescence can leave a deficit that is difficult to recover (De Souza, 2014). In bone densitometry of young athletes, the coalition advises using the Z-score, which compares with people of the same age, and not the T-score.

Metabolism, blood and immunity

Energy deficiency lowers resting metabolism and active thyroid hormones, raises cortisol and alters iron (Areta, 2021). Repeated respiratory infections, slow-healing injuries and persistent fatigue are common complaints that the athlete attributes to training.

Overtraining or low energy?

The consensus points to a large overlap with overtraining syndrome. In a review cited by the IOC, 18 of 21 studies on overtraining showed signs of low energy availability, which raises the question of how many cases attributed to excessive training were in fact a lack of food (Mountjoy, 2023).

Men and Paralympic athletes

The cyclist from the opening has a profile that the literature has come to recognize. In men, the reproductive signs are subtler: lower libido, fewer morning erections and low testosterone (Mountjoy, 2023). An attempt to validate a questionnaire for men, the LEAM-Q, showed that, among the symptoms tested, only low libido separated cases of low energy availability from controls well (Lundy, 2022).

Studies with men are still small, but they point in the same direction. In a study of 50 competitive road cyclists, 28 in 100 had low energy availability by clinical interview, 44 in 100 had a lumbar Z-score below minus 1, and energy availability was the main determinant of lumbar bone density (Keay, 2018). Among cyclists with low energy, those who had never done impact sports had the more fragile bone.

In a survey of 108 men in endurance sports, recreational but competitive, 47 in 100 fell in the risk range (Lane, 2019). The number needs a caveat: the authors used cutoffs derived from women, which may not apply to men. In a study of 53 male endurance athletes, exercise dependence was associated with eating disorder symptoms, a more negative energy balance and higher cortisol (Torstveit, 2019).

Paralympic athletes

The 2023 consensus devotes attention to athletes with disabilities, a group almost absent from research. In one of the cited studies, 62 in 100 Paralympic athletes had tried to change their weight, and 44 in 100 women reported menstrual changes (Mountjoy, 2023). Spinal cord injury, amputation and wheelchair use alter energy expenditure and body composition, which makes the usual formulas even less reliable.

Why they go unnoticed

The pressure for a lighter body comes from coaches, from peers and, increasingly, from social media, and it reaches men too (Wasserfurth, 2020). Thinking of REDs only when facing a thin female athlete is a base rate error: the disease exists in those who do not fit the stereotype.

Screening and stepwise assessment

The triad coalition recommends screening at the pre-participation evaluation, with questions about the menstrual cycle, fractures, eating and weight concerns, and investigating the other components whenever one appears (De Souza, 2014). It warns of a point that confuses: weight can stay stable even with low energy availability, because the body reduces expenditure.

The IOC CAT2 tool

The 2023 consensus presented the REDs CAT2 clinical assessment tool, in three steps (Mountjoy, 2023). The first is screening, with questionnaires and warning signs. The second is assessment of severity and risk, with primary, secondary and potential indicators, which include menstrual data, hormones, bone density, a history of stress fractures, blood tests and mental health. The third is diagnosis, made by a physician, after ruling out other causes.

The result translates into a four-color traffic light, from green to red, with participation guidance ranging from full training to withdrawal. The consensus makes clear that the tool has its own license, that there is no single test that gives the diagnosis and that screening questionnaires alone are not enough. Thinking about sensitivity, specificity and predictive values helps to understand why a positive questionnaire is the start, and not the end, of the investigation.

Body composition with care

Measuring body fat can help, and it can also harm. The consensus recommends that body composition be assessed below age 18 only for medical reasons, with methods such as skinfold measurements, DXA and ultrasound, by a trained professional, and with confidential results (Mountjoy, 2023). Public weigh-ins in the locker room have no place in this care.

The conversation that opens the investigation

Much of the information that matters only appears when the athlete feels heard. The clues the patient offers often come sideways: "I skipped dinner", "my period disappears in season", "I train more when I eat more". Asking about thoughts, concerns and expectations shows what the athlete believes about weight and performance.

Differential diagnosis and the debate about the syndrome

REDs is a diagnosis of exclusion. Fatigue, drop in performance, amenorrhea and fractures have many causes, and the consensus asks the physician to rule out the alternatives before concluding (Mountjoy, 2023). For amenorrhea, the list includes pregnancy, thyroid disease, hyperprolactinemia, polycystic ovary syndrome and ovarian insufficiency (De Souza, 2014). For fatigue, it includes anemia from other causes, celiac disease, inflammatory bowel disease, type 1 diabetes, infections and depression.

Putting two similar pictures side by side is the central exercise. A runner with amenorrhea and weight loss may have hyperthyroidism; a cyclist who is tired and has low libido may have hypogonadism from another cause. The reverse care also applies: ordering tests without a clear question feeds cascades of care that help the athlete little.

Eating disorder and mental health

Altered eating behaviors and eating disorders are risk factors for REDs, and low energy itself can worsen mood, sleep and anxiety (Mountjoy, 2023). Not every athlete with REDs has an eating disorder: many simply do not eat enough for the training load, because of lack of time, knowledge or access to food.

When there are signs of an eating disorder, the mental health professional joins the team from the start. The conversation that captures what the scale does not capture also matters here, because severe eating disorders can come with life-threatening risk and with suffering the athlete hides. Talking about food without judgment and without commenting on the body is part of the treatment.

Does REDs exist as a syndrome?

The concept has critics. Jeukendrup and colleagues argue that the model attributes to low energy availability symptoms that may have many other causes, such as stress, poor sleep, poorly planned training and illness, and that studies showing causality in athletes outside the laboratory are lacking (Jeukendrup, 2024). They propose a broader approach, with a checklist of eight categories of possible causes for health and performance problems.

Point	IOC consensus view	Critical view
Cause of symptoms	Problematic low energy availability	Many possible causes, of which energy is one
Evidence	Experimental, cross-sectional and intervention studies	Studies of causality in athletes are lacking
Diagnosis	CAT2 in three steps, with exclusion of differentials	Broad investigation of eight categories of causes
Risk highlighted	Underdiagnosis, especially in men	Overdiagnosis and excessive focus on calories
Sources	Mountjoy, 2023	Jeukendrup, 2024

The two views agree on more points than it seems. Both call for careful differential diagnosis and a multidisciplinary team. For the clinician, the debate is a reminder that the REDs hypothesis must be tested, not presumed.

Treatment and return to sport

The central treatment is to restore energy availability, without medication, above all by increasing intake and, when necessary, reducing the training load (Mountjoy, 2023). The triad coalition describes the minimum team: physician, sports nutritionist and mental health professional, with the coach and family informed as the case requires (De Souza, 2014).

Each system has its own timeline

Recovery does not happen all at once. Available energy improves in days to weeks, menstruation returns in months and bone mass takes years (De Souza, 2014). Explaining these timelines to the athlete prevents frustration and dropout, and asking the athlete to repeat the plan in their own words, as in teach-back, shows whether the message got through.

Component	Expected recovery time	Observation
Energy availability	Days to weeks	Depends on increasing intake and adjusting training
Menstrual function	Months	Return of menstruation is a sign of axis recovery
Bone mineral density	Years	Slow gain, and part of the adolescent deficit may remain
Source	De Souza, 2014

The role of hormones

The oral contraceptive, widely used to "regulate" menstruation, creates bleeding that masks amenorrhea and does not restore bone mass (De Souza, 2014). The coalition reserves hormone therapy for those who have not responded to at least one year of nutritional treatment, or for those with significant fractures and bone loss. The 2023 consensus cites a trial in which transdermal estradiol with cyclic progesterone increased bone density, but restoring energy remains the foundation (Mountjoy, 2023).

The trials show how hard treatment is. In a 12-month trial with increased intake, 57 out of every 100 participants dropped out of the study (Mountjoy, 2023). Adherence depends on trust and on a feasible plan, and techniques such as the OARS skills of motivational interviewing help when the athlete resists eating more.

Clearance to train and compete

The triad coalition proposed a cumulative risk score that leads to full, provisional or limited clearance, restriction or temporary disqualification, with a written contract between athlete and team (De Souza, 2014). The IOC CAT2 follows similar logic to the four-color traffic light. In severe eating disorders, the recommendation is to restrict training until clinical stabilization.

Deciding together with the athlete matters as much as the color of the traffic light. Shared decision-making explains risks, agrees on short-term goals and makes clear what changes the plan. Removing an athlete without conversation usually pushes the athlete away from the health service.

Prevention

The 2023 consensus notes that fewer than half of the coaches and physicians surveyed could name the three components of the triad (Mountjoy, 2023). Education for coaches, athletes and families, an end to comments about the body, access to food at the training center and an environment that values health above the scale are part of prevention (Wasserfurth, 2020).

Back to the two athletes

The runner has the three components of the triad in play, with tibial pain that calls for investigation of a stress fracture. The cyclist has signs that recall REDs and that also fit other diseases. In both, the reasoning starts with the question about energy and continues through the differential diagnosis before any label.

How NEXO helps train this reasoning

In NEXO, the case of the day is fictional, with a real clinical presentation and up to six progressive clues, and you propose the diagnosis by searching for the ICD-10 code. Separating REDs from a thyroid disease, from anemia of another cause or from an eating disorder is the kind of reasoning that is trained clue by clue. The post about affective bias helps you notice when the image of the disciplined athlete gets in the way of the right question.

The limits on using the game are described on the NEXO economy page. If you research the teaching of clinical reasoning, the page for researchers explains how to get in touch. Professionals who write or review cases find information under curation and under creators, and institutions can learn about the proposal on the partnerships page.

Frequently asked questions

Is REDs the same as the female athlete triad?

The triad is a subset of REDs. REDs includes men and other systems beyond menstruation and bone, while the triad describes three components in women (Mountjoy, 2023).

Can men have REDs?

They can. The most cited signs are decreased libido, fewer morning erections, low testosterone and bone loss, and studies with cyclists and runners show that the problem exists (Keay, 2018).

If weight is stable, can I rule out low energy availability?

Stable weight does not rule it out. The body reduces expenditure to adapt to low energy, and weight can stay the same while hormones and bone suffer (De Souza, 2014).

Is there a test that confirms REDs?

There is still no single test. The diagnosis combines history, clinical signs, tests and the exclusion of other causes, made by a physician (Mountjoy, 2023).

Does the contraceptive solve the athlete's lack of menstruation?

Oral contraceptives produce bleeding, but they do not treat the cause or restore bone mass, and they can hide the problem (De Souza, 2014).

Does every athlete with REDs have an eating disorder?

Many do not. Some cases come from lack of time, knowledge or access to food in the face of a high training load, but an eating disorder is a risk factor and must be investigated carefully (Mountjoy, 2023).

How long does recovery take?

It depends on the system. Energy improves in days to weeks, menstruation in months and bone in years (De Souza, 2014).

Take-home messages

• REDs arises from problematic low energy availability and affects women, men and Paralympic athletes.
• The threshold of 30 kcal per kilogram of fat-free mass per day is a laboratory reference, not an individual cutoff.
• Stable weight does not rule out the problem, and one component of the triad calls for looking for the others.
• In men, low libido and bone loss are signs to ask about.
• The diagnosis requires excluding other causes, and the concept itself is still debated.
• Treatment is restoring energy, with a team of a physician, a sports nutritionist and a mental health professional, and each system has its own timeline.
• Body composition, when necessary, is measured with care and confidentiality.

Play today's case on NEXO.

References

1. Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073-1097. PMID 37752011. DOI 10.1136/bjsports-2023-106994.
2. De Souza MJ, Nattiv A, Joy E, et al. 2014 Female Athlete Triad Coalition Consensus Statement on Treatment and Return to Play of the Female Athlete Triad: 1st International Conference held in San Francisco, California, May 2012 and 2nd International Conference held in Indianapolis, Indiana, May 2013. Br J Sports Med. 2014;48(4):289. PMID 24463911. DOI 10.1136/bjsports-2013-093218.
3. Areta JL, Taylor HL, Koehler K. Low energy availability: history, definition and evidence of its endocrine, metabolic and physiological effects in prospective studies in females and males. Eur J Appl Physiol. 2021;121(1):1-21. PMID 33095376. DOI 10.1007/s00421-020-04516-0.
4. Logue DM, Madigan SM, Melin A, et al. Low Energy Availability in Athletes 2020: An Updated Narrative Review of Prevalence, Risk, Within-Day Energy Balance, Knowledge, and Impact on Sports Performance. Nutrients. 2020;12(3):835. PMID 32245088. DOI 10.3390/nu12030835.
5. Jeukendrup AE, Areta JL, Van Genechten L, et al. Does Relative Energy Deficiency in Sport (REDs) Syndrome Exist? Sports Med. 2024;54(11):2793-2816. PMID 39287777. DOI 10.1007/s40279-024-02108-y.
6. Wasserfurth P, Palmowski J, Hahn A, et al. Reasons for and Consequences of Low Energy Availability in Female and Male Athletes: Social Environment, Adaptations, and Prevention. Sports Med Open. 2020;6(1):44. PMID 32910256. DOI 10.1186/s40798-020-00275-6.
7. Lundy B, Torstveit MK, Stenqvist TB, et al. Screening for Low Energy Availability in Male Athletes: Attempted Validation of LEAM-Q. Nutrients. 2022;14(9):1873. PMID 35565840. DOI 10.3390/nu14091873.
8. Lane AR, Hackney AC, Smith-Ryan A, et al. Prevalence of Low Energy Availability in Competitively Trained Male Endurance Athletes. Medicina (Kaunas). 2019;55(10):665. PMID 31581498. DOI 10.3390/medicina55100665.
9. Keay N, Francis G, Hind K. Low energy availability assessed by a sport-specific questionnaire and clinical interview indicative of bone health, endocrine profile and cycling performance in competitive male cyclists. BMJ Open Sport Exerc Med. 2018;4(1):e000424. PMID 30364549. DOI 10.1136/bmjsem-2018-000424.
10. Torstveit MK, Fahrenholtz IL, Lichtenstein MB, et al. Exercise dependence, eating disorder symptoms and biomarkers of Relative Energy Deficiency in Sports (RED-S) among male endurance athletes. BMJ Open Sport Exerc Med. 2019;5(1):e000439. PMID 30792881. DOI 10.1136/bmjsem-2018-000439.