RED-S: The Female Runner Health Crisis Nobody Puts a Name On

The name is Relative Energy Deficiency in Sport, or RED-S.

It is one of the most common serious health problems in endurance sport, and one of the least talked about.

What RED-S actually is

RED-S was defined by the International Olympic Committee in 2014.

It describes what happens when a runner burns more energy than she takes in for long enough that her body starts shutting down non-essential systems to protect the essential ones.

The engine of the syndrome is low energy availability. You are still eating. You are still training. The gap between the two is the problem.

The old term for the same pattern was the female athlete triad, which covered menstrual dysfunction, low bone density, and disordered eating.

The updated RED-S framework goes broader.

What gets affected

Bone health drops first. Immune function drops next.

Then your metabolic rate falls, your protein synthesis slows, your reproductive hormones fall off a cliff, and your cardiovascular system starts sending warning signals.

A 2024 case series of NCAA Division I female cross-country runners found reduced ferritin, suppressed resting metabolic rate, and lower vitamin D across a full competitive season.

Why female runners are the highest-risk group

You can develop RED-S at any level.

The elites talk about it more now, but recreational runners with regular training loads are just as vulnerable.

A study of 70 female runners from London running clubs found that 82 percent had a fat-free mass index below the 20th percentile for weight-sensitive sports.

That is a red-flag statistic in a general amateur field.

Iron deficiency stacks on top of it. So does the pressure to stay lean for performance.

The two feed each other in a loop that is hard to see from the outside.

The warning signs you can catch early

Missed periods are the flashing red light. If you are a menstruating athlete and you have gone three months without a cycle, that is a clinical criterion for concern, not a training badge.

The quieter signs matter too. Stress fractures that will not heal. Recurring colds. Poor sleep.

Flat runs that used to feel easy. Mood changes that look like burnout but keep coming back.

What actually helps

Fuel the training you are already doing

The single most effective intervention is boring.

You eat more, especially around your workouts. The average marathon block requires more energy than most runners realize, and the deficit builds up quietly week over week.

Our guide to how to fuel your long runs with real food is a good place to start if you are not sure what enough looks like on paper.

Get bloodwork before you get scared

Ferritin, vitamin D, thyroid, and reproductive hormones are the panel that matters.

Before you assume the worst, get the numbers. You may only need to fix iron deficiency, which is common and treatable.

Rebuild slowly and with a professional

A registered dietitian who works with endurance athletes is worth every penny.

So is a sports medicine doctor who takes menstrual dysfunction seriously.

Do not try to talk yourself out of the diagnosis. Runners are very good at rationalizing a syndrome that is quietly breaking bones and hormones.

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Fuel the training you are already doing continued

A quick way to check yourself is the pre-run and post-run rule.

If you cannot remember what you ate in the hour before a session or the hour after, you are probably running on a deficit without noticing.

Long runs need dedicated carbohydrate during the run and a full meal within an hour of finishing.

Skipping either one for months in a row is one of the fastest routes into low energy availability.

Fix sleep before you fix anything else

Sleep is the underrated recovery lever in the RED-S conversation.

When you are chronically under-fueled, sleep quality is often the first thing to deteriorate, and poor sleep then feeds back into cortisol, appetite, and mood.

Aim for eight hours in a dark, cool room before you touch anything more sophisticated. It is free.

It works.

And it is the intervention most likely to be skipped in favor of a supplement stack that will not fix the underlying deficit.

Male runners are not exempt

The syndrome is most common in female endurance athletes, but the mechanism does not care about biological sex.

Male runners with chronic low energy availability show reduced testosterone, elevated cortisol, and the same downstream problems with bone density and immunity.

Adolescent male and female athletes both show low energy availability rates above 50 percent in some sports.

That is a public health signal, not a rare edge case.

If you are a male runner reading this and dismissing it as a women's issue, you may be the exact person the framework was expanded to cover.

Why this belongs in the running conversation

RED-S is underdiagnosed because the culture around distance running still treats leanness as a proxy for fitness.

Every finisher photo that celebrates a wasted-looking elite reinforces the idea.

The evidence is now clear enough that major sports medicine bodies have moved from suggesting to warning.

You can read the original IOC framework on the Wikipedia summary of Relative Energy Deficiency in Sport for a full breakdown of the model and the affected body systems.

If your training has stopped working and your body has stopped cycling, the answer is almost never more miles or less food. It is more food, sooner, with help.