Female Athlete Triad / RED-S

The Triad. The Female Athlete Triad is the interrelated triad of low energy availability, with or without disordered eating; menstrual dysfunction, most importantly functional hypothalamic amenorrhoea; and low bone mineral density. Each component exists on a spectrum from optimal health to disease, and the three are causally linked.

RED-S. Relative Energy Deficiency in Sport is the broader, more recent IOC concept. It recognises that low energy availability impairs many body systems: resting metabolic rate, menstrual and reproductive function, bone health, immunity, cardiovascular and gastrointestinal function, growth and development, and psychological health. It impairs performance as well, and it applies to both sexes, not only to female athletes.

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The definition. Low energy availability is the central driver of both the Triad and RED-S. Energy availability (EA) is dietary energy intake minus exercise energy expenditure, per kg of fat-free mass per day (kcal/kg FFM/day), the energy left over for normal physiological function after training, and the units matter. The deficit can come from either side of the equation: reduced intake (disordered eating) and/or high exercise expenditure.

The mechanism. Chronically low energy availability suppresses the hypothalamic-pituitary-gonadal axis, and low GnRH and LH lower oestrogen and cause functional hypothalamic amenorrhoea. It also lowers thyroid and metabolic activity, and the two together impair bone health.

The numbers. Each figure is in kcal/kg FFM/day:

• Around 45 is optimal. Roughly energy balance, supporting normal reproductive, metabolic and bone function. The management goal of restoring energy availability means bringing it back toward around 45.
• Around 30 is the disruption threshold. Below it, LH pulsatility and reproductive function are suppressed, metabolic hormones fall (low T3, reduced resting metabolic rate) and bone-formation markers drop, so chronically low energy availability below roughly 30 drives the Triad and RED-S.
• Critically low energy availability, as in the cited case below 10, causes profound disruption.

The limits of the threshold. The 30 kcal/kg figure comes from short-term laboratory studies in small groups of young women, and it describes the point at which measurable disruption appeared in that setting; it was never validated as an individual diagnostic cut-off. Energy availability is also very hard to measure accurately in a free-living athlete, because it compounds the well-known unreliability of self-reported dietary intake with an estimate of exercise energy expenditure and a body-composition measurement. A calculated EA therefore carries a wide margin of error.

The 2023 IOC model. The 2023 IOC consensus moves away from a single threshold toward a physiological model that distinguishes adaptable from problematic low energy availability, modified by individual factors. The same calculated EA may be benign in one athlete over a short period and damaging in another sustained for months.

In practice. A calculated EA above 30 does not exclude the diagnosis in an athlete who has stopped menstruating and is breaking bones. Treat the number as a way of understanding the mechanism and framing the dietary conversation, not as a rule-in or rule-out test.

The orthopaedic consequence. Low energy availability and low oestrogen reduce bone mineral density, cause bone stress injuries and stress fractures, and impair healing. Not every stress fracture points to an energy deficit, though, and knowing which ones do is what makes the screening decision practical.

Trabecular sites. Bone stress injuries at trabecular (cancellous) sites, the sacrum, pubic rami, femoral neck and calcaneus, are the ones most associated with low bone mineral density and hormonal disruption. Trabecular bone has a high surface area and turns over fast, so it registers oestrogen deficiency early. A sacral or femoral neck stress fracture in a young female endurance athlete should therefore trigger a full RED-S assessment almost reflexively.

Cortical sites. Injuries at predominantly cortical sites, the tibial shaft and metatarsals, are more often explained by training load, surface, footwear and biomechanics. A second metatarsal stress fracture in a runner who has just doubled her mileage may be exactly what it looks like.

A low threshold. Any female athlete with a bone stress injury or stress fracture should be assessed for the Triad or RED-S, and some presentations warrant the workup regardless of site:

• Multiple or recurrent stress injuries at any site
• Any stress injury in an athlete with menstrual disturbance; amenorrhoea plus a stress fracture is a major red flag

A different axis. The familiar high-risk versus low-risk classification of stress fractures describes the risk of non-union and the need for aggressive treatment rather than the likelihood of an underlying energy deficit. See Stress Fractures.

Screening. The assessment of an at-risk athlete draws on:

• A history of disordered eating, menstrual status (amenorrhoea or oligomenorrhoea) and stress fractures
• Validated questionnaires: the Low Energy Availability in Females Questionnaire (LEAF-Q) and the Low Energy Availability in Males Questionnaire (LEAM-Q)
• DXA for bone mineral density
• Hormonal and metabolic biomarkers, where useful

A diagnosis of exclusion. Functional hypothalamic amenorrhoea is a diagnosis of exclusion. Labelling an athlete's missing periods as "just her training" without testing is a real clinical error, because it delays the diagnosis of conditions that need entirely different treatment. Exclude:

• Pregnancy, always first, with a beta-hCG. It is the commonest cause of secondary amenorrhoea in a woman of reproductive age, athlete or not.
• Thyroid disease and hyperprolactinaemia, with TSH and prolactin. A prolactinoma presents exactly this way and will not respond to eating more.
• Polycystic ovary syndrome, which is common and can coexist with athletic training; clinical hyperandrogenism and testosterone help separate it. The distinction matters because PCOS is not a state of oestrogen deficiency and does not carry the same bone risk.
• Premature ovarian insufficiency. An elevated FSH points away from a hypothalamic cause, since functional hypothalamic amenorrhoea produces low or inappropriately normal LH and FSH with low oestradiol.
• Coeliac disease and inflammatory bowel disease, which cause inadvertent energy deficiency through malabsorption rather than through under-eating.

The other bloods. Alongside these, check full blood count, ferritin, vitamin D, calcium and renal function, and consider T3: a low T3 is one of the more useful markers of ongoing energy deficiency.

When to request DXA. DXA is indicated in any of these situations:

• After six or more months of amenorrhoea
• After a bone stress injury
• With disordered eating
• At a very low BMI

Use the Z-score. In premenopausal women, children and adolescents, and men under 50, the age- and sex-matched Z-score is the correct measure. The T-score, which compares with the young-adult peak, is for postmenopausal women and older men and is covered in the Osteoporosis topic.

The athlete caveat. Weight-bearing and impact athletes normally have higher-than-average bone mineral density, so a Z-score that looks "normal" may be abnormally low for an athlete. Hence the Female Athlete Triad/ACSM definition of low BMD as a Z-score below -1.0 in an athlete with secondary risk factors (low energy availability, amenorrhoea, prior stress fracture), rather than the general -2.0 cut-off.

Terminology. In this young population a Z-score of -2.0 or lower with secondary clinical risk factors supports "osteoporosis". Otherwise "low bone mass" or "below the expected range for age" is the preferred term.

Sport matters. Athletes in non-weight-bearing sports, such as cyclists and swimmers, can have low bone mineral density even without the Triad. DXA is also used to monitor the response to restoring energy availability over time.

The team. Management is multidisciplinary: a sports physician, a dietitian and a psychologist or psychiatrist to treat any disordered eating, working with the coach and the athlete.

The cornerstone. Restore energy availability by increasing energy intake and/or reducing exercise energy expenditure. This restores menstrual function and improves bone health, and the condition is reversible when energy balance is restored. The return of spontaneous menstruation is the endpoint that tells you it has worked; everything else is adjunctive.

The contraceptive trap. The point most likely to be tested, and most often got wrong, is that the combined oral contraceptive does not treat this condition. Prescribing it to an amenorrhoeic athlete is intuitive and wrong for two reasons.

It does not restore bone. The pill has not been shown to restore bone mineral density in functional hypothalamic amenorrhoea, because the problem is not simply absent oestrogen. Low energy availability independently suppresses bone formation through low IGF-1 and low T3, and no amount of exogenous oestrogen corrects an energy deficit.

It hides recovery. More insidiously, the pill produces a withdrawal bleed that looks like a period, which masks the single best clinical marker of recovery: the spontaneous return of menses. An athlete on the pill can appear to be improving while her energy deficit continues untreated.

Supporting bone. The adjuncts to restoring energy availability:

• Calcium and vitamin D to recommended intakes, correcting a measured deficiency.
• Transdermal 17-beta-oestradiol with cyclic progestogen, where pharmacological oestrogen is judged necessary, typically for a persistently low BMD with continuing bone stress injuries despite genuine attempts at nutritional restoration. It is preferred over the oral pill because it avoids the hepatic first-pass effect that suppresses IGF-1, and therefore does not blunt bone formation in the way oral ethinyl oestradiol does.
• Bisphosphonates are generally avoided in young women of childbearing potential. The evidence in this population is thin, and their very long skeletal half-life raises unresolved concerns about a future pregnancy.

Contraception in its own right. If the athlete needs contraception for contraception's sake, that is a legitimate separate indication. It should be prescribed knowing that it neither treats the bone nor allows menses to be used as a recovery marker.

Return to sport. Risk stratification guides return-to-sport decisions, both clearance and ongoing participation.