The short answer
- Relative Energy Deficiency in Sport is what happens when the energy you take in does not cover both your training and the ordinary business of keeping you alive.
- It is usually accidental. Training goes up, eating stays where it was, and the gap opens quietly over months.
- It affects men. In one study of collegiate cross country runners, 42% of the men and 29% of the women were below the research threshold for low energy availability.
- The signs are scattered: bone injuries that keep coming back, recovery that stopped working, frequent illness, a season going backwards, periods stopping, feeling cold and flat.
- Eating more is the treatment. If eating more is the part that feels impossible, that is the thing to get help with, and it is not a training problem.
Most of what we write about here is one problem in one place. This one is the opposite. It is a single underlying cause that presents as four or five unrelated complaints, which is exactly why it goes unnamed for years while each complaint gets treated on its own.
What it is
Your body spends energy on two things: the training you do, and everything else it has to run in the background. Bone turnover, hormones, immune function, repair, temperature, mood. When what you eat does not cover both, the background work is what gets cut.
That state is called low energy availability, and the syndrome of health and performance effects that follows is Relative Energy Deficiency in Sport.
Two words in there matter more than the rest. Relative: this is not about being thin or heavy, it is about intake relative to what you are asking your body to do. And syndrome: it is a cluster, not a single symptom, which is why no one test finds it.
It is usually an accident
The version people picture is deliberate restriction. That version exists and matters. But the common version in runners is simpler and duller: the training went up and the eating did not follow.
A marathon block adds several hours of running a week. Appetite does not reliably rise to match it, and heavy endurance training can blunt appetite rather than increase it. Add a busy job, a skipped lunch, an evening run before a late dinner, and the gap opens without a single decision being made.
It is not a women's problem
This is the part most runners have never been told, and it is the reason the condition gets missed in half the people who have it.
More of the men than the women, in that sample. The IOC statement specifically flags emerging data on the effects of low energy availability in males as one of the developments since its previous version1.
The reason it still gets framed as a women's problem is that one of the clearest early signs, periods stopping or becoming irregular, only exists in half the population. Men lose the same background functions without a visible marker, so it takes longer to notice.
What it looks like from the inside
None of these is diagnostic on its own. Several of them together, over months, is the pattern:
- Bone injuries that keep returning, or a stress injury that will not settle
- Training hard and getting slower, or a season that goes backwards despite the work
- Recovery that used to take a day now taking three
- Colds and minor illnesses arriving more often than they used to
- Periods becoming irregular or stopping
- Feeling cold most of the time
- Low mood, poor sleep, low motivation for training you normally enjoy
- Nagging soft tissue problems in several places at once
The bone one is the strongest single signal in runners.
Why we do not give you a number to hit
The research uses a threshold expressed as energy per kilogram of fat free mass per day, and you will find it quoted all over the internet. Studies in runners use it to classify groups, including the ones above2,4.
We are deliberately not turning that into a self-test here.
Calculating it requires a measurement of fat free mass you do not have, and an estimate of the energy cost of your training that is rough at best. More importantly, a number that you track daily turns fuelling into a target to be hit or missed, and for some people that is the start of the problem rather than the end of it. The IOC's own approach is a clinical assessment across many domains, not a single figure1.
What is worth knowing without any arithmetic: carbohydrate availability around training does specific work that total calories alone does not cover, and it is one of the areas the 2023 statement singles out as having grown since the last version1.
What actually fixes it
Eating more, for long enough. Often training less for a while as well, so the gap closes from both ends.
That sentence is short, and the doing of it is not. The change usually has to be large enough to feel uncomfortable and held for months rather than weeks, and the first thing that returns is rarely performance. Warmth, sleep, mood and appetite tend to come back before the racing does.
Bone takes longest. If bone density has fallen, rebuilding it is measured in years, not seasons, which is the strongest argument for catching this early rather than proving it with a third fracture.
When to get it looked at
The last one is the one people leave out, and it is the one that most changes what kind of help is useful. It is worth saying to someone even if nothing else on the list applies.