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RED-S symptoms: what low energy availability looks like in endurance athletes

What is RED-S? Relative Energy Deficiency in Sport is what happens when you don't eat enough to cover your training for long enough that your body starts cutting back on everything else, from hormones and bones to immunity and recovery. The symptoms to watch for are missed or irregular periods, low libido, stress fractures, picking up every bug going, poor sleep, low mood, and training that stops working however hard you push.

I want to be blunt about who this is for. It is not only an issue for teenage female runners, which is how it often gets framed. It shows up in male cyclists, masters triathletes, swimmers, rowers and ultra runners. Most of them have no idea, because the early signs look like "I'm just tired" or "I need to train harder".

What low energy availability actually means

Energy availability is the energy left over for the rest of your body once training has taken its share. Researchers calculate it as what you eat, minus what you burn exercising, relative to your fat-free mass.

The 2023 International Olympic Committee consensus statement splits low energy availability into two kinds:

  • Adaptable. A short dip with mild, quickly reversible changes. A hard training camp or a few big days in a row can do this, and it's not a problem in itself.

  • Problematic. Low energy availability that goes on too long or goes too deep, causing bigger and potentially lasting disruption to how your body works. This is what leads to RED-S.

That distinction matters. A big week where you can't quite eat enough is normal endurance sport. Months of it is not.

You may have seen a magic number of 30 kcal per kg of fat-free mass per day. That comes from short lab studies in small groups of sedentary women, where around 45 looked adequate and 30 or below caused hormone and bone changes. The IOC statement is clear it was meant as a guide, not a diagnostic line, and that the level at which problems start varies a lot between people, between sexes and between body systems. Men seem to tolerate lower levels before problems show, with symptoms reported somewhere around 9 to 25. In other words, don't try to calculate your way out of this. Look at the signs.

How common is it

More common than most athletes think. The IOC statement reports estimated prevalence of low energy availability or RED-S indicators ranging from 23% to 79.5% in female athletes and 15% to 70% in male athletes, depending on the sport and how it was measured.

One study I find telling looked at 108 competitively trained male endurance athletes. Nearly half, 47.2%, were classed as at risk of low energy availability, and another third were at moderate risk. The cyclists in that group had lower energy availability than the runners. These were not elite pros chasing a race weight. They were ordinary competitive amateurs.

RED-S symptoms to know

These are the signs the IOC lists as health and performance outcomes of problematic low energy availability. No single one proves RED-S, because each has other possible causes, but a cluster of them should get your attention.

AreaWhat it can look likeWhoReproductive hormonesPeriods becoming irregular, more than 35 days apart, or stopping altogetherWomenReproductive hormonesLow testosterone, low sex drive, fewer morning erectionsMenBonesStress fractures, especially repeat ones or in the hip, pelvis or sacrum; low bone densityEveryoneMetabolismLow thyroid hormone (T3), low resting metabolic rateEveryoneImmunityGetting ill more often, more days lost to illness and injuryEveryoneGutBloating, cramps, changes in bowel habitsEveryoneHeartUnusually low resting heart rate or blood pressureEveryoneMood and sleepIrritability, low mood, anxiety, poor sleepEveryonePerformanceTraining stops paying off, poor recovery between sessions, lower strength and power, worse concentration and coordinationEveryone

Two notes on that table. First, if you take hormonal contraception, your bleeding pattern can't be used to judge this, so a missing period may be masked. Second, the low libido and reduced morning erections signal is one of the more useful early clues for men, because it's something you notice before any blood test.

It's not just total calories

One of the more important updates in the 2023 statement is about carbohydrate. Short studies in male endurance athletes found that low carbohydrate intake, under 3g per kg of body weight per day, raised markers of bone breakdown and pushed up hepcidin, the hormone that blocks iron absorption, sometimes even when total energy was adequate. In one three-day study in young women, a low energy, low carb diet raised hepcidin by 264%, against 69% on a diet with the same low energy but more carbohydrate.

This is why I'm wary of the fasted, low-carb everything approach that does the rounds every few years. You can eat "enough" calories and still underfuel the work. If you've read our piece on iron for endurance athletes, the hepcidin link will sound familiar.

Overtraining or under-fuelling

Here's the part that should make every endurance athlete stop and think. Overtraining syndrome and RED-S share a lot of symptoms. When researchers reviewed 21 studies on overtraining, 18 of them showed signs that athletes had ramped up training without eating more to match. So a fair amount of what gets called overtraining may actually be under-fuelling.

If your fitness has gone flat and the instinct is to rest or push harder, check your eating first. A runner adding volume for a marathon, a triathlete stacking swim, bike and run sessions for an Ironman, or a cyclist on a winter base block all burn a lot more in a big week than in an easy one. If your plate doesn't change when your training does, the gap has to come from somewhere.

What actually fixes it

The IOC is clear that the main treatment is restoring energy availability, through changes to diet and training, with enough carbohydrate, protein, fat and micronutrients. Most of the harm appears to come from eating too little rather than training too much, so the food side is usually where the work is.

Practically, that looks like:

  • Eat more on big days. Your intake should rise and fall with your training, not sit at the same number every day.

  • Fuel during long sessions. Carbs on the move take a big bite out of the deficit and protect the session too.

  • Don't run long gaps. Skipping breakfast before a long ride or run, then a late lunch, digs a hole you won't refill at dinner.

  • Keep carbs in. Especially around hard training.

Be patient. Different systems recover at different speeds. In the one 12-month trial that increased food intake in exercising women with RED-S markers, periods returned for some but bone loss was not halted, and over half the participants dropped out. Months of under-fuelling don't undo in a fortnight. If several of the signs above fit you, a proper assessment involves blood tests and sometimes a bone scan, because the IOC's own tool depends on them.

FAQ

Can men get RED-S? Yes. Low testosterone, low libido, bone stress injuries and stalled performance are all recognised signs in men, and the male endurance athletes studied so far show high rates of low energy availability.

Does the pill hide the signs? It can. Hormonal contraception changes your bleeding pattern and hormone levels, so menstrual status can't be used to judge energy availability while you're on it. The other signs still count.

Should I just train less? Sometimes that's part of it, but the IOC notes that most of the harm seems to come from eating too little rather than training too much. For most endurance athletes, eating more is the bigger lever.

How long does it take to develop? There's no fixed timeline. The IOC describes short-term low energy availability as days to weeks and long-term as months to years. The longer and deeper the shortfall, the more likely it becomes a problem.

Is a hard training camp going to give me RED-S? Probably not, if you eat well around it. Short dips are the adaptable kind. The risk is when the dip becomes your normal.

MOVE's daily targets adjust to the training you actually do, so a big week raises your numbers with it instead of leaving you eating the same on a rest day and a five-hour day.

Sources

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