It is not your weight that decides, it is the energy left over after training. The warning signs of under-fuelling — and when it belongs with a doctor.
The honest answer: it is not your body weight that decides, it is how much energy is left for everything else after training. That remainder is called energy availability — the energy your body has for heart, hormones, immune system, bones and repair once training has taken its share.
If that remainder drops too low, problems accumulate. The IOC consensus statement on relative energy deficiency in sport names a widely cited orientation of below 30 kcal per kilogram of fat-free mass per day in women; in men the data are considerably blurrier and the observed range sits lower [1]. The important part of the 2023 version: low energy availability is not a switch but a spectrum from adaptable to problematic [2].
And here is the part that surprises most people: this affects all genders, including people at completely normal or high body weight. Observed markers include bone density, resting metabolic rate, menstrual disturbance, disordered eating, mental health, endurance performance and strength [3]. You cannot see it on anyone — which is exactly why almost nobody talks about it.
Picture your daily energy budget as an account. What you eat goes in; what training burns goes out. What is left over is what your body has to pay for everything else with — heartbeat, temperature, hormone production, immune defence, bone remodelling, concentration.
This quantity is expressed relative to fat-free mass, that is, everything that is not fat tissue. In women a value below roughly 30 kcal per kilogram of fat-free mass per day counts as the orientation threshold beyond which problems accumulate; in men the evidence is thinner and sits lower [1].
Remember: eating too little is not the same as weighing too little.
Two routes lead into low energy availability, and only one of them is obvious. One is deliberately eating less. The other is training more while eating the same — volume grows over weeks, the plate stays as it was.
The second route is the more common and the more invisible one, because body weight can look stable for months while it happens. That is why the scale is a poor instrument for this question. The 2023 framing as a spectrum [2] describes exactly this: there is a zone in which the body still adapts — and one in which it starts presenting bills.
Usually it is not one symptom but a pattern. These signs deserve to be taken seriously [3]:
An absent cycle is not a sign of good shape and not a convenient side effect of training. It is one of the clearest signals that the body is shutting functions down.
Four things that harm nobody and need no diagnosis:
One: do not increase training volume any further right now. Two: cut nothing else — no meal, no food group, no carbohydrates. Three: eat around your sessions instead of doing them fasted and then waiting a long time afterwards. Four: keep protein in the normal 1.4 to 2.0 g/kg range [4] so your muscle mass does not come under additional pressure.
What you should not do: calculate all of this yourself and build a diagnosis out of the result. Fat-free mass cannot be determined properly at home, and the number alone says little anyway [2].
Everything beyond the four points above. The route back after prolonged under-fuelling is a supervised one: a medical assessment (blood work, hormone status, bone density if indicated), plus a qualified dietitian for rebuilding intake, and where disordered eating is suspected, an eating disorder counselling service or a psychotherapeutic contact point.
That is not an escalation, it is the normal path. And it is the only one that addresses the causes rather than just the symptoms.
The fact that the nutrition plan in the Lab has a floor built in exists for exactly this reason: a plan that goes arbitrarily low would not be a plan, it would be a risk.
This guide informs, it does not replace medical or dietetic advice and makes no diagnosis. Nothing here is suitable for establishing or ruling out a condition.
Please seek prompt medical assessment for: an absent or months-long irregular cycle, repeated stress fractures or bone pain under load, persistent exhaustion, recurring infections, heart rhythm disturbances, feeling faint, or strong unintended weight loss.
If eating, weighing or controlling runs your day, if you skip meals out of fear, or if exercise becomes compensation for what you have eaten, contact your doctor, a qualified dietitian, or an eating disorder counselling service. Particular care applies to children and adolescents: while growing they need more energy, not less, and any suspicion belongs in professional hands.
The most honest self-check is not a calculation but a question about the past three months: what has gone up — and what has stayed the same?
Write down two lines. Line one: training hours per week, today and three months ago. Line two: meals and portion sizes, today and three months ago. If line one has grown and line two has not, you have your finding — with no formula at all. That is the moment most people say: "true, I never thought about it that way."
The energy your body has left for all remaining functions once training expenditure is subtracted — expressed relative to fat-free mass. In women a value below roughly 30 kcal per kilogram of fat-free mass per day counts as the widely cited orientation threshold; in men the data are blurrier and sit lower [1]. What matters is the remainder, not body weight.
Yes. The consensus statement describes REDs explicitly across genders, with effects on bone density, resting metabolic rate, hormonal balance, mental health, endurance performance and strength among others [3]. For men the threshold research is thinner and the observed values sit lower [1] — which does not make the problem smaller, only less well researched.
It gets serious when several come together: falling performance over weeks, an absent or irregular cycle, repeated bone or stress reactions, frequent infections, always feeling cold, poor sleep, low mood [3]. Everyone has the odd bad week. A pattern lasting months belongs in a medical assessment.
Not reliably. You would need your fat-free mass and the energy cost of your training, both of which can only be roughly estimated at home, and the IOC classifies low energy availability as a spectrum rather than a threshold anyway [2]. Use the warning signs as your entry point and an assessment as your next step, not the formula.
Yes, that is possible. Energy availability depends on how much is left after training — not on what the scale shows [3]. That is precisely why the most common case develops unnoticed: training volume grows over months, intake stays the same, and weight looks stable.
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