What Is RED-S? Relative Energy Deficiency in Sport

RED-S, short for Relative Energy Deficiency in Sport, is a syndrome that occurs when an athlete consistently takes in too few calories relative to the energy they burn through exercise. It was formally named by the International Olympic Committee in 2014 to replace the older concept of the “Female Athlete Triad,” which focused narrowly on disordered eating, menstrual dysfunction, and bone loss in women. The newer term reflects a wider reality: energy deficiency disrupts far more than three body systems, and it affects male athletes too.

Where the Old “Triad” Fell Short

For decades, the dominant framework was the Female Athlete Triad. It captured something real, but it also implied the problem was limited to women and to just three linked issues. In practice, clinicians noticed that under-fueled athletes showed cardiovascular changes, immune suppression, mood disturbances, and declining performance that didn’t fit neatly into three boxes. An IOC expert panel convened to update the science and concluded that the condition needed a broader name. RED-S, they wrote, “points to the complexity involved and the fact that male athletes are also affected.”¹ The shift wasn’t cosmetic. It expanded the clinical lens from a women’s health issue to a systemic metabolic problem in any athlete who chronically undereats for their training load.

What “Low Energy Availability” Actually Means

The core of RED-S is a concept called energy availability. It’s calculated by taking the calories you eat, subtracting the calories you burn during exercise, and dividing by your lean body mass.² What’s left over is what your body has to run everything else: digestion, immune function, hormone production, bone remodeling, brain chemistry. When that leftover number drops too low, your body starts triaging. It dials down the systems it considers non-essential for immediate survival, and the consequences ripple outward.

The threshold researchers most often cite is around 30 kilocalories per kilogram of fat-free mass per day. Below that level, measurable hormonal changes begin to appear in studies of young athletes.³ That number isn’t a cliff edge everyone falls off at the same point, but it serves as a rough boundary. Some athletes dip below it intentionally through restrictive dieting, others do it accidentally by ramping up training volume without eating more, and some land there through a combination of both.

The Hormonal Cascade

When energy availability drops, the body’s hormonal signaling changes in predictable ways. Leptin, the hormone that signals energy stores are adequate, falls. Ghrelin, the hunger hormone, rises. Cortisol, a stress hormone, climbs, partly because depleted glycogen stores put the body in a stress state. Thyroid hormone output slows, which lowers metabolic rate. The growth-hormone-and-IGF-1 axis gets disrupted, which matters for tissue repair and muscle maintenance.⁴

None of these shifts happen in isolation. The drop in leptin, for instance, has downstream effects on reproductive hormones. Elevated ghrelin can suppress the signaling hormones that drive the menstrual cycle in women. Elevated cortisol affects bone turnover, immune function, and sleep quality.⁵ The result is a web of interconnected problems, not a single symptom you can isolate and treat on its own.

Reproductive Health in Female Athletes

The most visible sign of RED-S in women is often the loss of their menstrual period, a condition called functional hypothalamic amenorrhea. The body essentially decides that reproduction is not a priority when fuel is scarce and shuts down the hormonal chain that drives the menstrual cycle.⁶ Some athletes and coaches still treat missed periods as normal or even as a sign of fitness. It is neither. It is a clinical red flag that the body is in energy deficit.

The loss of estrogen that accompanies amenorrhea isn’t just a reproductive issue. Estrogen plays a direct role in maintaining bone density, regulating cardiovascular function, and supporting mood. In adolescent athletes, the stakes are higher because the teenage years are when the skeleton is supposed to be building toward its lifetime peak bone mass. Disrupting that window can leave lasting deficits that are difficult to fully recover later.⁷

A common clinical mistake is prescribing oral contraceptives to “fix” the missing period. The pill produces a withdrawal bleed that mimics menstruation, but it doesn’t address the underlying energy deficit. The hormonal signaling from the brain to the ovaries remains suppressed, and the metabolic problems persist beneath a cosmetic solution.⁸

Why RED-S Is Not Just a Women’s Problem

Male athletes with RED-S often fly under the radar because the most conspicuous marker in women, a missing period, has no direct equivalent in men. But energy deficiency suppresses testosterone production in men through the same basic mechanism: the brain dials down reproductive signaling when it senses insufficient fuel. A case report of a collegiate male swimmer documented strikingly low total, free, and bioavailable testosterone with otherwise normal lab values, pointing to a central shutdown rather than a problem with the testes themselves.⁹

Low testosterone in male athletes can show up as fatigue, declining performance, reduced libido, mood changes, and slower recovery from training. These symptoms overlap heavily with overtraining syndrome, which makes diagnosis tricky. Research suggests the condition is usually transient and resolves when energy intake is corrected.¹⁰ But because there’s no single obvious biomarker equivalent to amenorrhea, male RED-S tends to go unrecognized for longer, which means the downstream damage, especially to bone, accumulates quietly.

Bone Damage and Stress Fractures

The skeletal consequences of RED-S are among its most serious and least reversible effects. A retrospective analysis of elite athletes found that RED-S was diagnosed in about a quarter of the cohort, and stress fractures were dramatically more common in those athletes: 70% of athletes with RED-S had experienced stress fractures compared to 25% of those without.¹¹ High-resolution imaging of the affected athletes’ bones revealed reduced bone volume, fewer trabecular struts (the internal scaffolding that gives bones their strength), and thinner cortical shells, with the load-bearing tibia hit hardest.

The mechanism is a double hit. Markers of bone formation, like osteocalcin and a protein called P1NP, were lower in athletes with RED-S, while markers of bone breakdown were elevated.¹² So the body is simultaneously building less bone and dismantling more of it. Over months or years, the cumulative loss weakens the skeleton in ways that a standard bone density scan might underestimate, because the internal architecture can deteriorate even when overall density looks borderline acceptable.

Endurance athletes seem especially vulnerable. The combination of chronic energy deficit and repetitive impact loading creates a perfect storm for stress injuries. A narrative review noted that female endurance athletes with amenorrhea had significantly degraded trabecular microarchitecture compared to peers who menstruated normally, and that these structural deficits were associated with a history of stress fractures.¹³ Data on whether oral estrogen-progestin therapy helps are discouraging; studies of oral contraceptive pills in amenorrheic athletes have not shown meaningful bone density gains.¹⁴ Transdermal estradiol patches combined with cyclic progesterone have shown better results, with treated groups gaining roughly two and a half percent in spinal bone density over about a year and a half.¹⁵ But clinicians generally agree that addressing energy availability itself is the most important intervention.

How Common Is It, and Who Is Most at Risk

The numbers are sobering. A study of Finnish female athletes competing at the national or international level found that roughly 70% were at some risk of RED-S. Among endurance athletes specifically, the figure was 82%, with endurance athletes having about 2.4 times the odds of being at risk compared to non-endurance athletes.¹⁶ Nearly 18% of the endurance athletes fell into the most severe risk category, while none of the non-athlete controls did. Nearly a third of endurance athletes reported amenorrhea, and roughly half showed suppressed thyroid hormone or a low resting metabolic rate.

Bone stress injuries were reported by about half of both the endurance and speed-and-power athletes, compared to less than one in five in other sport groups.¹⁷ These are elite-level athletes with access to sports science support, which suggests the problem isn’t just about ignorance. The training demands in certain sports make energy deficiency structurally difficult to avoid unless nutrition is proactively planned and monitored.

Sports that emphasize leanness, aesthetics, or weight categories carry the highest risk. Distance running, cycling, rowing, gymnastics, figure skating, combat sports with weight classes, and dance are frequently cited. But the Finnish data show that even athletes in non-endurance sports had substantial risk levels, reinforcing the IOC’s point that RED-S is not confined to a narrow athletic profile.

The Coaching and Culture Problem

RED-S is partly a nutritional problem and partly a cultural one. The environments in which athletes train can actively drive energy deficiency. In qualitative research with athletes and staff at a professional netball club, athletes described coaches making weight-based selection decisions that triggered restrictive eating. One player recalled being told she was “too big” for a touring squad, prompting her to eat almost nothing for two weeks. The psychological fallout persisted for years, affecting her confidence on court.¹⁸

These dynamics aren’t rare outliers. Many athletes report that restrictive eating is normalized in their training environment, that coaches comment on body composition without understanding the metabolic consequences, and that thinness is treated as synonymous with fitness. Even well-meaning coaches can push athletes toward energy deficiency by prescribing high training volumes without any corresponding nutritional guidance. The gap between sports science knowledge and coaching practice remains wide in many settings.

Mental Health and Disordered Eating

The relationship between RED-S and mental health runs in both directions. Energy deficiency itself changes brain chemistry: cortisol rises, thyroid output drops, and mood-regulating neurotransmitters are affected. Athletes with RED-S frequently report irritability, difficulty concentrating, loss of motivation, and depressive symptoms. At the same time, conditions like orthorexia nervosa (an obsessive fixation on “clean” or “healthy” eating) and muscle dysmorphia can drive the restrictive behaviors that produce RED-S in the first place.¹⁹

This bidirectional loop makes treatment complicated. An athlete whose energy deficiency is driven by a clinical eating disorder needs psychological intervention alongside nutritional rehab. An athlete whose deficiency is driven by ignorance or poor coaching might respond to education and meal planning alone. Distinguishing between the two requires careful clinical assessment, and many sports medicine settings still lack the integrated mental health support needed to do that well.

Recovery and What Gets Better (and What Might Not)

The good news is that many consequences of RED-S are reversible once energy availability is restored. Menstrual function typically returns, testosterone levels in men normalize, thyroid output recovers, and mood improves. The bad news is that bone is the slowest tissue to rebuild. Endocrinology research has found that low bone density is highly prevalent in athletes with amenorrhea and carries an elevated risk of stress and fragility fractures. Young athletes who fail to achieve their peak bone mass during the critical developmental window may never fully close the gap.²⁰

Recovery timelines vary. Some athletes see hormone levels normalize within weeks of eating adequately. Bone density improvements are measured in years, not months, and gains are modest even with pharmacological support. The longer the energy deficit has persisted, the more entrenched the metabolic adaptations become, and the harder the road back. Athletes who catch the problem early, ideally before stress fractures or prolonged amenorrhea develop, have a much better prognosis than those who push through symptoms for seasons.

Screening and the Problem of Measurement

One persistent challenge is that energy availability is conceptually straightforward but practically difficult to measure. Calculating it requires accurate data on food intake, exercise energy expenditure, and body composition, and each of those measurements has its own error margin. Athletes often underreport food intake, exercise energy expenditure is hard to capture outside a lab, and body composition methods vary in precision. Some researchers have argued that calculating energy availability using the principle of energy balance rather than relying on self-reported dietary intake offers a more reliable picture.²¹

In practice, clinicians often rely on a cluster of indirect markers rather than a single energy availability number: menstrual status, resting metabolic rate, thyroid function, bone density scans, blood lipid profiles, and clinical questionnaires. The IOC and other governing bodies have developed screening tools that flag athletes at risk based on a combination of these indicators. No single test definitively diagnoses RED-S, which is part of why it goes unrecognized so often. An athlete with “normal” bloodwork on one axis can still have serious deficits elsewhere.

What Changes When You Know About RED-S

For athletes, the most actionable insight is that persistent fatigue, frequent injuries, and declining performance are not always signs to train harder. They can be signs to eat more. This runs against deep instincts in competitive sport, where discipline is equated with restriction and toughness means pushing through discomfort. Reframing adequate fueling as a performance strategy rather than a lack of discipline is one of the most effective shifts in sports nutrition education.

For coaches and support staff, awareness of RED-S means understanding that comments about an athlete’s weight can trigger clinically dangerous behavior, that a missing period is not a benign side effect of training, and that lean does not equal fast. Periodizing nutrition alongside training, ensuring athletes have access to sports dietitians, and creating environments where athletes feel safe reporting symptoms are all structural interventions that reduce risk without requiring anyone to diagnose a medical condition.

For parents of young athletes, the key takeaway is that adolescence is the worst possible time for chronic energy deficit. The skeleton is actively growing, hormonal systems are still maturing, and the consequences of disrupting those processes echo far beyond the competitive career. A teenager who loses their period or develops a stress fracture needs clinical evaluation, not reassurance that “serious athletes just push through it.”