There is a condition that affects a significant number of female athletes across every sport, every competitive level, and every age group. It disrupts hormones, degrades bone density, impairs performance, affects mood, suppresses immune function, and in some cases causes damage that does not fully reverse. Most of the women who have it do not know. Many of the coaches, trainers, and doctors they work with do not know how to recognize it either.
The condition is RED-S – Relative Energy Deficiency in Sport. And understanding it is one of the most important things a female athlete can do for her long-term health and career.
What RED-S Is
RED-S stands for Relative Energy Deficiency in Sport. It was formally defined by the International Olympic Committee in 2014 as a syndrome caused by insufficient energy availability relative to the demands of exercise and daily physiological function. The term replaced and significantly expanded an older framework called the Female Athlete Triad, which focused narrowly on three interconnected issues in female athletes: low energy availability, menstrual dysfunction, and low bone density.
RED-S kept those three concerns at its core but recognized that the downstream effects of low energy availability extend far beyond bone and hormones. The IOC’s updated framework identifies impaired function across at least ten physiological systems — endocrine, cardiovascular, gastrointestinal, immunological, haematological, growth and development, metabolic, psychological, and reproductive — all driven by the same upstream cause: not enough energy coming in to support both the demands of sport and the demands of keeping a body alive and healthy.
That upstream cause has a specific name in the literature: low energy availability, or LEA. Energy availability is calculated as dietary energy intake minus the energy expended in exercise, divided by fat-free mass. When that number drops below roughly 30 kilocalories per kilogram of fat-free mass per day — a level significantly below what most athletes are actually consuming — the body begins to downregulate systems in a sequence that prioritizes survival and deprioritizes everything else.
The reproductive system goes first. Then bone. Then the immune system, the cardiovascular system, and metabolic function. The brain’s cognitive function and mood regulation are affected. Growth and development are affected in younger athletes. All of it, from one cause.
How It Happens — and Why It Is So Often Invisible
RED-S does not require intentional restriction to develop. This is one of the most important and most misunderstood facts about the condition. The majority of female athletes with RED-S are not on diets. They are not skipping meals on purpose. They are not trying to lose weight.
What they are doing is training at a volume and intensity that has increased their energy expenditure significantly, without a corresponding increase in what they eat. Sometimes this happens because appetite is genuinely suppressed after high-intensity training. Sometimes it happens because the athlete does not realize how much more fuel a significant training load requires. Sometimes it is the result of general healthy eating patterns — lots of vegetables, lean proteins, limited processed food — that are nutritionally sound but calorically insufficient for someone running fifty miles a week.
In athletic cultures, the undereating that drives RED-S is often rewarded rather than identified. Leanness is associated with performance in many sports. Athletes who are disciplined about food are praised. The appearance of control is treated as a virtue. And the early signs of RED-S — fatigue, mood changes, declining performance — are almost always attributed to something other than food.
An athlete might be told she is overtraining. Or not sleeping enough. Or going through a stressful period. She might be prescribed iron supplements for low ferritin without anyone asking what is driving the depletion. She might visit three different specialists for three different symptoms without any of them connecting those symptoms into a single picture.
This diagnostic gap is not a failure of individual providers. RED-S is genuinely underrecognized in primary care, and its presentation varies significantly between individuals. But it is a gap that costs female athletes years — of health, of performance, of bone density that may not fully return.
The Signs of RED-S in Female Athletes
Because RED-S affects multiple systems simultaneously, its presentation can look very different from one athlete to another. The following signs, particularly in combination, warrant a serious conversation with a sports dietitian and a sports medicine physician.
Menstrual cycle changes. This is the most important marker and the one that gets normalized most often. A lost period, an irregular cycle, or a cycle that has become unpredictable is not a normal adaptation to training. It is a physiological signal that reproductive function has been downregulated due to insufficient energy. The clinical term for this is functional hypothalamic amenorrhea, and it is caused by low energy availability — not by training load itself, not by low body fat per se, and not by being a competitive athlete. A regular menstrual cycle is a vital sign. Its disruption is a warning sign, not a badge of fitness.
Persistent fatigue. The kind that does not respond to rest, that makes easy training days feel hard, and that shows up as a generalized depletion that is qualitatively different from normal tiredness. When the body is in a state of chronic low energy availability, it reduces metabolic rate to conserve resources. Recovery slows. The fatigue compounds.
Declining performance despite consistent training. Performance adaptation requires energy. When energy availability is insufficient, the body cannot complete the adaptive processes that make training productive — rebuilding muscle, strengthening connective tissue, consolidating cardiovascular improvements. An athlete can train and train and see little or no progress because the fuel is not there to convert effort into adaptation.
Recurrent stress fractures or slow-healing injuries. Bone is living tissue that requires ongoing energy and hormonal support to maintain its density. When estrogen drops due to menstrual disruption and energy availability is chronically low, bone remodeling is impaired — less new bone is formed and the structural integrity of existing bone declines. Stress fractures in the feet, shins, or hips in a female athlete, particularly recurrent ones or ones that occur at low training loads, are a significant red flag.
Frequent illness. The immune system is among the systems the body sacrifices under chronic energy restriction. Athletes with RED-S get sick more often, recover from illness more slowly, and are more susceptible to upper respiratory infections during and after training.
Mood changes, anxiety, and depression. Low energy availability affects the hormonal and neurological environment in ways that directly impair mood regulation. The psychological effects of RED-S are significant and frequently attributed to life stress, training pressure, or personality rather than to the physiological state the athlete is in.
Food preoccupation and difficult food relationships. Chronic energy restriction increases the brain’s focus on food. This can manifest as constant thinking about food, rigidity around eating, and the kind of obsessive food planning or restriction that looks like discipline but is actually a physiological response to scarcity. In some athletes, the relationship between restriction and food preoccupation becomes entrenched in ways that have both nutritional and psychological dimensions.
Cold intolerance. Feeling consistently colder than others in the same environment, particularly cold hands and feet, is a sign of metabolic downregulation and reduced peripheral circulation — the body conserving heat in response to low energy availability.
Hair loss or thinning. Hormonal disruption from low energy availability shows up in hair, skin, and nails as well as in the menstrual cycle.
RED-S Versus the Female Athlete Triad: Understanding the Difference
The Female Athlete Triad, first described in the early 1990s, identified three interconnected conditions in female athletes: disordered eating, amenorrhea, and osteoporosis. It was a meaningful clinical advance at the time and remains relevant as a framework.
RED-S expanded this in several important ways. First, it recognized that the effects of low energy availability extend far beyond the three conditions in the Triad to encompass the full range of physiological systems described above. Second, it acknowledged that the spectrum of each component matters — you do not need clinical amenorrhea to have hormonal disruption, and you do not need osteoporosis to have compromised bone health. Subclinical presentations across any of these systems are meaningful and worth addressing. Third, it recognized that male athletes can also be affected, though the hormonal and reproductive consequences differ.
For female athletes, the practical takeaway from both frameworks is the same: the menstrual cycle and bone health are early warning systems for a broader energy availability problem. When they signal, the signal deserves to be taken seriously.
Who Is at Risk
RED-S has been documented across virtually every sport and competitive level. Research consistently identifies higher prevalence in endurance sports — distance running, cycling, swimming, triathlon — but it is also well-documented in aesthetic sports, weight-class sports, and team sports. Recreational athletes are affected, not only elite competitors. The common denominator is not the sport or the level. It is the gap between energy in and energy out, however that gap came to be.
Female athletes who have a history of dieting or weight-focused eating are at higher risk, as are those in sports where leanness is associated with performance or appearance. Athletes who have significantly increased training volume without adjusting intake are at risk. Athletes who eat in ways that are generally considered healthy but are calorically inadequate for their training demands are at risk.
Adolescent athletes warrant particular attention. The middle and high school years represent a window of critical bone development. Compromised bone accrual during this period has consequences that extend into adulthood and do not fully reverse with later nutritional correction.
What Recovery Looks Like
RED-S is treatable. The primary intervention is restoring adequate energy availability — eating enough to support both training and the physiological functions that have been compromised. In practice, this is harder than it sounds, for several reasons.
For many athletes, increasing intake after a period of restriction feels psychologically uncomfortable even when the intention is health rather than weight change. The relationship between food and performance has often become complicated in ways that go beyond simple nutrition advice. Working with a registered dietitian who understands both sports nutrition and the psychology of eating is the most effective way to navigate the nutritional piece.
For athletes with significant menstrual disruption, recovery of the cycle can take months after energy availability is restored, and in some cases longer depending on the duration and severity of the deficit. Bone density recovery is slower still and may not be complete, which is one of the strongest arguments for identifying and addressing RED-S early rather than waiting.
For athletes where the food relationship has become disordered — where restriction feels compulsive, where food preoccupation is significant, or where the pattern involves cycling between restriction and loss of control — working alongside a therapist who understands eating and athletes is strongly recommended alongside nutritional support. The nutritional and psychological components of RED-S interact closely, and addressing both simultaneously tends to produce better outcomes than addressing them in sequence.
What to Do If You Recognize These Signs
If you recognize yourself in this post — particularly if several of these signs are familiar, or if you have had a menstrual disruption you have not fully investigated — the most important step is getting a proper assessment rather than trying to self-diagnose or self-correct in isolation.
That assessment should include a sports dietitian who can evaluate your energy availability relative to your training load, a conversation about your menstrual history, and depending on how long the pattern has been present, a DEXA scan to assess bone density. A physician who understands RED-S, ideally a sports medicine doctor, is a valuable part of the care team.
You do not have to be at a crisis point to seek this out. The earlier low energy availability is identified and addressed, the more complete the recovery tends to be and the less permanent the consequences.
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Frequently Asked Questions
What is the difference between RED-S and the female athlete triad? The Female Athlete Triad is an older framework focusing on three interconnected conditions in female athletes: low energy availability, menstrual dysfunction, and low bone density. RED-S, defined by the IOC in 2014, expanded this to recognize that low energy availability affects at least ten physiological systems — not just those three — and that the spectrum of each matters, not just clinical presentations at the severe end. RED-S also recognizes that male athletes can be affected. For female athletes, the practical implications overlap significantly, but RED-S provides a more complete picture of what is actually at stake.
Can you have RED-S without losing your period? Yes. Menstrual disruption is the most visible and well-known marker of RED-S, but it is not required for the condition to be present or for other systems to be affected. Subclinical hormonal changes, impaired bone remodeling, immune suppression, and mood effects can all occur at levels of energy deficiency that do not yet produce amenorrhea. Waiting for a lost period to investigate energy availability misses a meaningful window for earlier intervention.
How do I know if I am eating enough as a female athlete? There is no universal number. Energy needs vary significantly with body size, training volume, training intensity, and individual metabolism. What tends to indicate adequate energy availability is a regular menstrual cycle, consistent energy throughout the day, the ability to complete training without feeling chronically depleted, good recovery between sessions, stable mood, and the absence of persistent food preoccupation. Working with a sports dietitian to assess your intake relative to your actual training demands is the most reliable way to understand whether your energy availability is adequate.
Does RED-S affect recreational athletes or only competitive athletes? RED-S affects athletes at all competitive levels, including recreational athletes who train consistently. The condition is not defined by how competitive you are but by the relationship between your energy intake and your total energy expenditure including training. Recreational runners, cyclists, and fitness enthusiasts are all represented in the literature on RED-S, and the health consequences are the same regardless of competitive level.
Can bone density lost due to RED-S be recovered? Partially, in many cases. Bone density can improve when energy availability is restored and the hormonal environment normalizes. However, recovery is often incomplete — particularly for bone density lost during adolescence or over extended periods — which is why early identification and treatment matters. Some studies show continued bone density improvements for years after restoration of adequate energy availability, but the rate and ceiling of recovery depend heavily on how long and how severe the deficit was.