What we are talking about
For a long time the subject was summarized in the so-called female athlete triad: low energy availability, menstrual cycle changes and loss of bone density. In 2014 the International Olympic Committee proposed a broader concept, RED-S (relative energy deficiency in sport), updated in the 2023 consensus led by Margo Mountjoy and colleagues in the British Journal of Sports Medicine. The central idea is simple and uncomfortable: when food does not cover the cost of training, the body cuts functions it considers "optional".
The root of everything is low energy availability, that is, the energy left over after subtracting what you burned training. Below a certain point, the organism goes into economy mode. And it does not announce it politely.
Why this affects women so much
RED-S hits men and women, but the female body sends more visible and earlier signals. One of them is the menstrual cycle. Classic studies by Anne Loucks showed that, when energy availability falls below roughly 30 kcal per kilo of fat-free mass per day, the pulsatility of the hormones that regulate the cycle already changes within days. An important warning is due: that number became a reference, but the 2023 consensus itself recognizes that there is large individual variation and that low energy is a spectrum, from adaptable to problematic. It is not a magic line the same for everyone.
When estrogen falls chronically, bone loses part of the stimulus that keeps it strong. That is why amenorrhea (absence of menstruation) is not "the luck of not menstruating while training". It is an alarm.
Bones: damage that does not hurt at first
Bone density is built mostly in youth and depends on mechanical stimulus plus energy and hormones in order. With low energy and an interrupted cycle, the math does not close. The result appears as a stress fracture, that injury that keeps coming back in the same place. Sports science reviews indicate that athletes with menstrual dysfunction have much higher prevalence and risk of bone stress injury over their careers than those who menstruate normally, with estimates that usually sit around two to five times depending on how many triad components are present.
The problem with bone is that it does not warn early. You train, feel tired, think it is normal, and the loss happens silently until it becomes pain.
Iron and anemia: the invisible fuel
A woman who trains has two fronts of iron loss at once: menstruation and the impact of long-duration exercise itself. That is why iron deficiency is much more common in them. Surveys in endurance athletes put iron deficiency prevalence between 20% and 50% of women, versus 0% to 17% of men. In a study with runners from the Detroit marathon and half marathon, clinical iron deficiency appeared in about 50% of women and 15% of men, and the severe form in about 15% of women and 3% of men.
Low iron drags down the capacity to carry oxygen. Translating to training: unusual fatigue, shortness of breath at an easy pace, dizziness, hair falling out, unexplained performance decline. It is neither fussiness nor lack of will.
Warning signs to take seriously
- Menstruation disappeared or became irregular after training increased.
- Repeated stress fractures or bone pain that does not go away.
- Disproportionate fatigue, shortness of breath on light effort, dizziness.
- Frequent colds and flu, slow recovery between sessions.
- Loss of performance even training the same or more.
None of these signs closes a diagnosis alone. But two or three together are reason to talk to a professional, not to tighten the belt further.
The principle, not the dose
Here lives the most important lesson. Many elite athletes live with narrow energy margins for specific seasons. That is not a model to copy. The elite is not the same as the amateur. They have rare genetics, years of base, enormous training volume, and an entire team behind them: dietitian, sports physician, physical therapist, routine testing and full-time recovery. Their life is to train, eat and sleep under supervision.
We mortals have work, family, broken sleep and no laboratory checking our iron every week. Copying the dose of restriction from a professional, without her structure, is a recipe for injury. Take the principle (eat enough for the load, respect the body's signs) and ignore the dose.
The basics that work
The strongest defense against RED-S is boringly simple: eat enough for the training load. If volume went up, food goes up with it. Regular cycle, stable energy, good sleep and progressing performance are good signs that the energy account is closing.
And most important: this is not medical advice. Nothing in this text replaces individual evaluation. Women's health in sport, cycle changes, suspected anemia or menopause call for follow-up from a physician, gynecologist and dietitian. Darwin helps organize training and record signs over time, but the one who diagnoses and prescribes is the health professional. The role of technology is to give you context for the conversation, not to replace the consultation.
