RED-S and underfuelling: when discipline becomes a problem
Not eating enough for your training load has a name and consequences that reach well beyond performance. The signs worth taking seriously, and why a missing period is one of them.
Table of contents
This is the most important article on this site, and the least fun.
Relative Energy Deficiency in Sport (RED-S) describes what happens when you do not take in enough energy to cover both your training and the basic business of keeping your body running. The body responds by shutting down what it considers non-essential — and reproduction, bone building and immune function are all on that list.
This page is not a diagnosis. If any of it lands, that is a conversation with a doctor, ideally one who understands athletes.
Why it is common in runners
Several things push the same direction: running is high volume, appetite is often suppressed after hard sessions, running culture treats being lighter as being better, and the deficit can be entirely unintentional. Most people with low energy availability are not trying to restrict — they are just eating a normal amount while training a lot more than normal.
The signs
Your period stops or becomes irregular. This is the clearest signal and the most commonly misread one. It is not a badge of training hard, and it is not harmless. The hormonal state behind it directly affects bone density, and low bone density in your twenties and thirties is not something you get to fix later.
Others worth noticing together rather than individually:
- Stress fractures, or repeated bone-related injuries
- Being cold all the time
- Getting every cold going around, and taking longer to shake them
- Poor sleep despite being exhausted
- Performance flat or declining despite consistent training
- Low mood, irritability, poor concentration
- Digestive problems
Any one of these has many explanations. Several together, in someone training hard, is a pattern.
The performance argument, since it sometimes lands better
Underfuelling does not make you faster. It reduces your ability to absorb training, blunts recovery, and increases injury risk — and time off is the thing that actually costs you fitness.
Runners who fuel adequately train more consistently, and consistency is what produces results over a season.
What to do
Eat more, particularly carbohydrate, particularly around training. The most common practical gap is not eating enough after sessions, when appetite is suppressed and the meal gets skipped or shrunk.
Do not simultaneously increase training and reduce intake. If mileage goes up, food goes up.
Get bloods and, if indicated, a bone density scan — through a doctor.
If your period has stopped, take that seriously now rather than in a year. Time matters for bone.
Where the culture makes it harder
Running has a persistent story that lighter is faster. It is true over a narrow range and enormously destructive as a general principle — plenty of careers have ended with stress fractures acquired chasing a number on a scale.
If you find that the amount you eat is something you think about a great deal, or that eating enough feels genuinely difficult rather than just inconvenient, that is worth talking to someone about. It is a common experience among people who run, and it is not a character failing.
Related
What to eat before, during and after a run covers the practical fuelling side. Iron and ferritin covers a deficiency that often travels alongside this one, and running and your cycle covers what a normal cycle looks like around training.
