The Female Athlete Triad and RED-S: How to Recognize and Avoid It

A woman training hard, eating what she thinks is enough, and no longer having periods is usually told two things: that it is normal for athletes, and that it is convenient. Neither is true. The missing period is a downstream readout of an energy problem, and by the time it appears the body has already been rationing for months. The condition has a name, a well-described mechanism, and a fix that is the opposite of what most people in it are willing to do.

What RED-S actually is

Relative Energy Deficiency in Sport is the current name for what was previously called the Female Athlete Triad. The underlying variable is energy availability: what is left for basic physiological function after the cost of training comes out of what was eaten. It is not calorie intake and it is not body fat. A woman can eat a substantial number of calories and still have very little availability if her training load is high enough — which is why this is missed in people who look like they are eating plenty.

When availability stays low, the body funds essential operations and defunds the rest. Reproduction is cut first, because it is expensive and not required for survival. Thyroid output falls to lower the running cost of everything else. Bone remodelling shifts toward breakdown (Mountjoy et al., Br J Sports Med 2023). It is a sensible adaptation to famine, and a problem only because this famine is voluntary and open-ended.

This is not confined to elite sport. Recreational runners, CrossFit athletes, lifters cutting for a photo, and women simply dieting hard while training hard all develop it. Estimates suggest 30-60% of active women have at least one component.

The mechanism, in order

The chain starts in the hypothalamus. Low energy availability suppresses the pulsatile release of GnRH — and the key word is pulsatile, because the pituitary responds to the rhythm of that signal, not just its presence. When pulse frequency falls, LH secretion becomes disordered, FSH follows, the ovarian follicle is not driven properly, oestrogen production falls, ovulation fails, and the cycle becomes irregular or stops. Controlled work restricting energy availability in regularly menstruating women disrupted LH pulsatility within days, which established that the trigger is energy, not exercise itself (Loucks & Thuma, J Clin Endocrinol Metab 2003).

That point deserves emphasis. Training volume is not the cause; training volume that is not paid for is. Two women doing identical sessions can sit on opposite sides of the line depending on what they eat.

Alongside this, conversion of T4 to active T3 falls, reverse T3 rises, and cortisol climbs. The endocrine picture is a low-output reproductive axis, a downregulated thyroid and a raised stress hormone at once. Nothing in that list is broken. It is a system doing what it was built to do.

What it looks like from the inside

The presentation is a collection of things that each seem minor and have each been explained away.

• Menstrual change — absent periods, short or long cycles, unusually light bleeding, or cycles that appear normal but are anovulatory
• Fatigue that rest does not fix, and performance that declines despite consistent training
• Feeling cold constantly, and an unusually low resting heart rate mistaken for fitness
• Hair thinning, dry skin, brittle nails
• Low libido, low mood, irritability or anxiety
• Recurrent stress fractures or injuries that will not resolve
• Constant food preoccupation, rigid eating rules, or a history of disordered eating
• Poor sleep, often with early waking

The performance decline is the signal most often misread. It looks like undertraining, so the response is more training, which deepens the deficit — how a manageable problem becomes a two-year one, and why recovery as the limiting factor is the frame that resolves it.

What the labs show

The shape matters more than any single value: everything that should be high is low, and the stress marker is up. Low LH with low oestrogen distinguishes this from primary ovarian problems, where LH would be high — and it is why ferritin and thyroid markers belong on the same panel rather than being chased separately.

The bone cost, and why it is the urgent part

Oestrogen is the dominant restraint on bone resorption. Remove it and resorption outpaces formation continuously — the same process that drives postmenopausal bone loss, except occurring in a woman in her twenties or thirties who feels fine and has no symptoms until something breaks (Ackerman et al., Bone 2019).

Timing makes it worse. Peak bone mass is built through the late teens and twenties and is essentially banked for life; bone not laid down in that window is forgone, and it lowers the starting point for every subsequent decade. Cycles usually return with energy restoration. Bone recovers slowly and often incompletely. That asymmetry is the argument for treating this urgently rather than after the season — the same logic behind hormone therapy for bone and the oestrogen-bone relationship.

One correction that comes up constantly: a hormonal contraceptive producing a monthly bleed does not mean the problem is solved. Withdrawal bleeding is a response to the medication, not evidence that the hypothalamic axis has recovered — it removes the most visible warning sign while the underlying energy deficit continues (De Souza et al., Female Athlete Triad Coalition consensus).

The recovery protocol, and what it feels like

The intervention is more energy and less training. It is simple to state and genuinely difficult to do, because it asks a woman who is afraid of weight gain to eat more, and a woman who identifies with training to train less.

1. Increase total intake, often by 300-600 calories a day above current intake, and treat that as a floor rather than a target to negotiate down.
2. Increase carbohydrate specifically, in the region of 3-5 g/kg/day. Very low-carbohydrate approaches worsen this — carbohydrate availability is one of the signals the hypothalamus reads.
3. Reduce training volume by 20-30%, temporarily. Intensity can often be preserved; total load cannot.
4. Protein around 0.8-1.0 g per pound of goal weight, which supports the tissue being rebuilt without displacing the carbohydrate.
5. Correct low ferritin if present, and address sleep and psychological load, both of which raise the effective cost of the same training.
6. Re-test at three to six months, and treat return of ovulatory cycles as the endpoint rather than return of any bleeding.

What to expect, in order: warmth, mood and sleep improve first, often within weeks; training quality follows; cycles are slowest, commonly returning within 6-12 months. Some weight gain is part of the mechanism, not a side effect — the body will not restart an expensive system while it believes supply is short. Bone recovers last and least.

Bottom line

RED-S is common, under-diagnosed and mechanistically straightforward: insufficient energy availability suppresses hypothalamic pulsatility, which shuts down the reproductive axis, throttles thyroid output and tips bone toward loss. Each sign alone looks like something else, and the performance decline drives exactly the wrong response. The fix is more food — carbohydrate in particular — and less training volume, held long enough for the axis to restart. Cycles come back. Bone largely does not, which is why this belongs on the radar before any other hormone intervention.

Educational content, not medical advice. Laboratory interpretation and any treatment decision are made by a licensed physician after individual evaluation. Individual results vary.