RED-S in Runners: Warning Signs and How to Fuel Properly

RED-S in runners explained: energy availability thresholds, the warning signs in order, and how to recover. Screen yourself with seven honest questions.

RED-S in runners rarely announces itself. It shows up as a stress fracture in a season where nothing changed, a period that quietly stopped, a resting heart rate in the low 40s that everyone congratulates you for, or a training block where the paces just will not come down no matter how much work you do. Relative energy deficiency in sport is what happens when you take in less energy than your training demands for long enough that your body starts shutting down functions it considers optional. Trail and ultra runners are unusually exposed, because the sport quietly rewards being light and normalises enormous training volumes.

What RED-S in runners actually is

The International Olympic Committee describes REDs as a syndrome of impaired health and performance driven by problematic low energy availability, affecting athletes of every gender and competitive level (Mountjoy et al., 2023).

Energy availability is the concept underneath it. It is not calories eaten, and it is not calories burned. It is what remains for everything else once training has taken its share:

Energy availability = (energy intake minus exercise energy expenditure) divided by fat-free mass in kg

The thresholds:

  • Above 45 kcal/kg FFM/day: supports full function and weight gain
  • 30 to 45: suboptimal, with some functions starting to suffer
  • Below 30: low energy availability, and the point where measurable disruption appears

The 30 kcal/kg figure came from controlled work showing that reproductive hormone pulsatility is disrupted below that threshold in regularly menstruating women (Loucks & Thuma, 2003).

A worked example

A 60 kg runner with 18% body fat has 49 kg of fat-free mass. She eats 2,400 kcal and burns 900 kcal on a hilly two hour run.

(2,400 minus 900) divided by 49 = 30.6 kcal/kg FFM

She is sitting right on the threshold, on a day she would describe as eating plenty. Add a bigger long run on Sunday without adding food, and she drops well under. This is how RED-S in runners develops without anyone consciously dieting.

Warning signs, in the order they usually appear

Early, and easy to dismiss

  • Persistent fatigue that a rest day does not fix
  • Feeling cold constantly, especially hands and feet
  • Getting sick more often, particularly recurring upper respiratory infections
  • Poor sleep despite heavy training
  • Irritability, low mood, and loss of motivation for running you used to enjoy

Mid-stage

  • Menstrual changes: cycles lengthening past 35 days, becoming lighter, or stopping. Missing three consecutive periods warrants a medical appointment
  • In men, dropping libido and morning erections, which reflect suppressed testosterone
  • Resting heart rate falling unusually low alongside worsening performance
  • Digestive problems, bloating, and constipation
  • Stalled or reversing performance despite consistent training

Late, and expensive

  • Bone stress injuries, particularly in the metatarsals, tibia, sacrum, and femoral neck
  • Recurring soft tissue injuries that take months rather than weeks
  • Measurable bone mineral density loss
  • Depression and disordered eating patterns

Low energy availability has a large effect on bone injury rates in elite distance athletes, which is why repeated stress fractures should trigger a nutrition review rather than just a longer rest (Heikura et al., 2018).

Why trail runners are exposed

Ultra training makes the arithmetic harder in specific ways. A five hour mountain long run can cost 3,500 to 4,500 kcal, and appetite is suppressed for hours afterward. Runners frequently under-eat on their biggest days without noticing.

Then there is the culture. Climbing is easier when you are lighter, and the sport talks about power-to-weight ratios in ways that make restriction look like professionalism. Add back-to-back long run weekends and travel, and low energy availability becomes structural rather than intentional.

Getting out of it

Stop restricting, immediately. This is not a gradual process. If you recognise several signs above, add food now while you work out the details.

Add energy around training. The gaps that matter most are before, during, and after sessions. Eating 60 to 90 g of carbohydrate per hour on long runs closes a large part of the deficit by itself, and our guide on hourly calorie needs sets those targets.

Reduce training load temporarily. Energy availability has two sides. Cutting a long run in half is as effective as adding 1,500 kcal, and often easier to sustain.

Get tested. Ask for ferritin, vitamin D, a full blood count, thyroid panel, and sex hormones. Women with amenorrhea beyond six months, and anyone with a stress fracture history, should discuss a DEXA scan.

Build a team. A sports dietitian is the highest-value professional here. If disordered eating is part of the picture, a psychologist experienced with athletes matters more than any macro target.

Recovery timelines are slower than people expect

  • Energy and mood: often improve within 2 to 4 weeks
  • Menstrual function: typically 3 to 12 months after energy availability normalises
  • Bone mineral density: 12 months or more, and some loss may be permanent
  • Performance: usually returns and often exceeds the previous baseline, but rarely inside a single season

That last point is the one that persuades people. Runners who fix this almost always end up faster than they were while underfuelled.

Screening yourself honestly

Ask these questions and answer them without negotiating:

  1. Have I lost weight in the last 12 months without intending to?
  2. Have my periods changed, become irregular, or stopped?
  3. Am I cold all the time?
  4. Have I had a bone stress injury, or more than one, in the last two years?
  5. Do I feel guilty or anxious after eating, or about missing a session?
  6. Has my performance stalled or declined despite consistent training?
  7. Do I regularly skip fueling on long runs to keep intake down?

Two or more yes answers means it is worth a conversation with a sports physician or dietitian. The LEAF-Q screening tool, validated for female athletes, formalises this and is available free (Melin et al., 2014).

Trailia's readiness scoring watches training load against recovery markers and flags the divergence between rising load and falling readiness, which is often the first objective signal that something is wrong. You can see how it works with the Race Readiness tool.

Key takeaways

  • Energy availability below 30 kcal/kg of fat-free mass per day is where measurable disruption starts
  • The early signs are cold hands, poor sleep, frequent illness, and low mood, not dramatic weight loss
  • Repeated bone stress injuries should trigger a nutrition review, not just more rest
  • Fixing it means adding energy and often reducing load, since both sides of the equation count
  • Bone density recovery takes a year or more, so early action matters far more than perfect diagnosis

Where to go from here

If any of this sounds like your last two seasons, the useful next step is not a spreadsheet. It is eating more this week, particularly around your long runs, and booking an appointment with someone qualified to run the bloods. RED-S in runners is common, under-diagnosed, and highly treatable when caught early. Left alone it takes bone density and years of running.

For the training side, Trailia adapts load week by week against your recovery data rather than pushing a fixed template, which makes the gap between what you are asking of your body and what it can absorb much easier to see.

Citations

  • Mountjoy, M., Ackerman, K. E., Bailey, D. M., et al. (2023). 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 57(17), 1073-1097.
  • Loucks, A. B., & Thuma, J. R. (2003). Luteinizing hormone pulsatility is disrupted at a threshold of energy availability in regularly menstruating women. Journal of Clinical Endocrinology & Metabolism, 88(1), 297-311.
  • Heikura, I. A., Uusitalo, A. L. T., Stellingwerff, T., Bergland, D., Mero, A. A., & Burke, L. M. (2018). Low Energy Availability Is Difficult to Assess but Outcomes Have Large Impact on Bone Injury Rates in Elite Distance Athletes. International Journal of Sport Nutrition and Exercise Metabolism, 28(4), 403-411.
  • Melin, A., Tornberg, A. B., Skouby, S., et al. (2014). The LEAF questionnaire: a screening tool for the identification of female athletes at risk for the female athlete triad. British Journal of Sports Medicine, 48(7), 540-545.
  • Ackerman, K. E., Holtzman, B., Cooper, K. M., et al. (2019). Low energy availability surrogates correlate with health and performance consequences of Relative Energy Deficiency in Sport. British Journal of Sports Medicine, 53(10), 628-633.

Frequently Asked Questions

What is RED-S in runners?

RED-S, or relative energy deficiency in sport, is a syndrome of impaired health and performance caused by taking in less energy than training demands over a sustained period. It affects hormonal function, bone health, immunity, and performance in athletes of all genders. The IOC formalised the definition in its 2023 consensus statement.

How do I calculate my energy availability?

Subtract your exercise energy expenditure from your total energy intake, then divide by your fat-free mass in kilograms. Below 30 kcal per kg of fat-free mass per day is considered low energy availability. A 60 kg runner with 49 kg of fat-free mass eating 2,400 kcal and burning 900 in training sits right on that threshold.

What are the earliest warning signs of RED-S?

The early signs are easy to dismiss: constant cold hands and feet, fatigue that rest days do not fix, frequent colds, poor sleep despite heavy training, and losing motivation for running. Dramatic weight loss is often absent. Most runners notice performance stalling before they notice anything else.

Can male runners get RED-S?

Yes. The 2023 IOC consensus explicitly covers athletes of all genders, and male runners present with suppressed testosterone, reduced libido, poor recovery, and bone stress injuries. Male cases are underdiagnosed largely because there is no obvious signal equivalent to a missing period.

Why are trail and ultra runners at higher risk?

A five hour mountain long run can cost 3,500 to 4,500 kcal, and appetite is suppressed for hours afterward, so under-eating happens without any conscious restriction. The sport also normalises talking about power-to-weight ratios, which makes restriction look professional. Back-to-back long run weekends compound the deficit.

Do repeated stress fractures mean I have RED-S?

Not always, but they should trigger a nutrition review rather than just a longer rest period. Low energy availability has a large effect on bone injury rates in elite distance athletes. If you have had more than one bone stress injury in two years, ask for bloods and a dietitian referral.

How long does it take to recover from RED-S?

Energy and mood often improve within two to four weeks of eating properly, but menstrual function typically takes three to twelve months to return. Bone mineral density recovery takes a year or more and some loss may be permanent. Performance usually returns and often exceeds the previous baseline.

Should I reduce training or just eat more?

Both sides of the equation count, and reducing load is often easier to sustain in the short term. Cutting a long run in half has the same effect on energy availability as adding roughly 1,500 kcal. Most successful recoveries do some of each rather than relying on food alone.

What blood tests should I ask for?

Ferritin, vitamin D, a full blood count, thyroid panel, and sex hormones give a useful picture. Women with amenorrhea lasting more than six months, and anyone with a stress fracture history, should also discuss a DEXA scan for bone density. Bring your training log to the appointment.

Is fueling during long runs enough to prevent RED-S?

It closes a large part of the gap for many runners, since taking 60 to 90 grams of carbohydrate per hour on a five hour run adds well over 1,000 kcal you would otherwise be missing. It is not a complete fix if daily intake is also low. Skipping fueling deliberately to keep intake down is one of the clearest risk behaviours.