RED-S in Endurance Sport: Spotting and Fixing Low Energy

You are training more than ever, sleeping badly, catching every cold going around, and losing ten seconds per kilometre in your intervals. The plan is sound, the gear is sound, your legs are not. In very many cases the cause is not the training but the energy missing from it — a state sports medicine has grouped under the name RED-S since 2014: Relative Energy Deficiency in Sport.

What RED-S means — and what it does not

RED-S describes the consequences of low energy availability: over a sustained period, the body receives less energy than basal metabolism plus training consumes. The crucial word is "relative". Energy can be missing even when you eat normally — namely when training volume has grown so much that intake no longer keeps pace with expenditure. It is therefore not a nutrition problem in the narrow sense but a balance problem.

The second misunderstanding concerns body weight. For years RED-S was described as a female issue in elite sport with low body fat. Today it is clear that it affects men just as much, recreational athletes just as much, and also athletes at perfectly unremarkable weight when intake is chronically insufficient. The trigger is rarely a deliberate diet — far more often it is several ambitious blocks back to back, a training camp, a cold with loss of appetite, or simply the fact that at 15 hours of training per week you still eat as you did at eight.

The warning signs: a pattern, not a single symptom

RED-S is deceptive because every individual sign looks harmless. Only the combination is meaningful. Collect these observations over two to four weeks before drawing conclusions:

• Performance dropping with no training reason. Intervals get slower even though you work harder, and easy runs feel laboured.
• Resting heart rate stays high. After a hard block it should come back down on rest days — if it does not, recovery is not complete. On reading it correctly, see Resting heart rate and recovery.
• Sleep that no longer carries you. You sleep too little or wake after a few hours and cannot get back down. More in Sleep and endurance performance.
• Constantly feeling cold. A lowered metabolic rate, especially cold hands and feet and a chill that does not match room temperature or clothing.
• Evening cravings for sugar. The body tries to catch up on energy, usually in the second half of the day.
• Frequent infections. Three or more colds within one training block is not bad luck but an immune signal.
• Missed or irregular periods. In women this is one of the earliest and most reliable markers. Men often notice reduced libido and morning erections.
• Gastrointestinal problems under load. With low energy availability gastric emptying slows, which aggravates discomfort during running.
• Stress fractures and tendon problems. Rising bone and tendon complaints without an obvious training mistake. On prevention, see Injury prevention for runners.
• Mood and motivation. Irritability, anhedonia and a drop in motivation you did not know before.

A single one of these signs carries little weight. Four or five at once — and sustained over weeks — are an urgent indication that the energy balance is wrong.

Energy availability: the number behind the feeling

Instead of counting calories all day, sports science uses a more robust figure: energy availability. It describes how much energy remains for all other body functions after training costs are subtracted — expressed per kilogram of fat-free mass per day.

The formula is: (energy intake in kcal − training expenditure in kcal) ÷ fat-free mass in kg. To estimate training cost: running burns roughly 1 kcal per kilogram of body weight per kilometre, cycling about 0.3 to 0.4 kcal per kilogram per kilometre, swimming around 0.8 to 1.0 kcal per kilogram per kilometre — depending on pace, wind and technique. You roughly know your fat-free mass if you have estimated your body fat percentage.

As a guide, values below 30 kcal per kilogram of fat-free mass per day count as critical, 30 to 45 as borderline to sufficient, above 45 as safe. Concretely, for an athlete with 50 kg of fat-free mass and an average training load of 700 kcal per day, intake should sit around 2,950 kcal so that 45 kcal per kilogram remain. For an athlete with 62 kg of fat-free mass and 900 kcal of training per day, that is around 3,690 kcal. These numbers surprise many people — which is precisely the core of the problem.

What happens in the body when energy is short

The body operates a clear priority list. Reproduction, bone building, digestion and immune function are expensive but not essential to survival — they are the first to be shut down when energy runs short. That explains why RED-S affects so many systems at once:

• Hormonal system. The thyroid produces less active T3, metabolism drops, and the body downregulates sex hormones. Periods stop in women, testosterone falls in men. Both noticeably reduce recovery and muscle building.
• Bones. Bone remodelling is an energy-intensive process. Together with the hormonal picture this explains why RED-S stress fractures appear even in young, athletic people without a fall or a training error. For context: Stress fractures in runners.
• Metabolism. Metabolic rate falls while blood sugar starts to swing. The body gets better at surviving and worse at sport.
• Immune system. Infection frequency rises because immune defence gets fewer resources. That is a performance issue, not just a health one.
• Muscle and performance. Glycogen stores fall, and the capacity to set and absorb hard stimuli drops. Part of the training stimulus is lost even though the volume is right.
• Gastrointestinal tract. Gastric emptying slows, which sharply worsens the problem of fuelling under load. See Gut training.

The result is a paradox: the athlete who trains with the most discipline loses the most performance, because the body lacks the resources to process training as a stimulus.

The most common causes in recreational sport

Let us talk reality: almost nobody has RED-S because of a conscious decision to undereat. It almost always develops gradually.

• Volume rises, intake stays the same. The classic: marathon volume goes up 40 percent and you eat as before. On the logic of that relationship, see Increasing training volume.
• The wish for a lighter race weight. Every kilo counts uphill, a legitimate consideration. Combined with high volume, it tips. See Race weight in endurance sport.
• Fasted runs without refuelling. Occasional fasted runs are a sensible stimulus. Fasted runs at high volume without full energy intake afterwards are a different matter.
• Parallel disciplines. In triathlon training several sports sit side by side — expenditure is systematically underestimated at the table.
• Loss of appetite after hard work. After a long run or an intense block the system often will not eat, because cortisol release dampens appetite and gastric motility.
• Quietly extending training. If you do not log what you actually do, the balance is wrong — but you cannot see it.

The way out: in four steps

RED-S is not solved with a supplement but by inverting intake and load.

• Step 1: Reduce training. Deliberately cut volume for two to three weeks and push hard sessions back. You lose less fitness than you think, because you give a loaded body the processing pause it needs. On structuring this, see The deload week.
• Step 2: Increase intake. Not with one giant meal, but with regular meals across the day and consistent refuelling after every session. The first goal is not a surplus above training expenditure but a guaranteed absence of deficit.
• Step 3: Protect carbohydrate around the session. The single best lever is carbohydrate before and after every session. Details in Post-workout nutrition and High-carb fuelling.
• Step 4: Measure the pattern, do not feel it. Log resting heart rate, sleep, weight trend and how performance feels. RED-S is a trend diagnosis, not a snapshot. If your log shows no pattern, you cannot see one.

A note on order: step 1 before step 2. If you raise intake but keep training at full volume, you only shift the problem. Without unloading training, the system stays in deficit because expenditure eats the increase.

How RED-S differs from overtraining

RED-S and overtraining are often lumped together, but they are different mechanisms with overlapping symptoms. In overtraining the training stimulus is the problem — too much intensity, too much volume, too little recovery at adequate energy. In RED-S energy is the bottleneck, and training volume can be objectively perfectly reasonable. The distinction matters practically, because the countermeasures bite differently.

In practice there are three constellations. First: overtraining without low energy — unloading helps, intake stays the same. Second: RED-S without overreaching — intake must rise while training is adjusted only slightly. Third, the most common case in recreational sport: both together, high volume and chronically too little energy. Here both measures apply, and unloading is the precondition for the extra energy to work at all. The warning signs of overtraining are in Overtraining: 10 warning signs, and the load-management overview is in the Peakora training hub.

When medical assessment is needed

Self-observation does not replace diagnosis. There are situations where you should not start with training and eating adjustments, but with a doctor's appointment:

• Stress fracture or multiple bone injuries in a short period without a clear accident.
• Missed periods for more than three months or a prolonged absence at normal training volume.
• Unintentional, significant weight loss or eating behaviour you experience as problematic — that needs a specialist team, not just a training plan.
• Persistent cardiac symptoms such as a sense of arrhythmia, dizziness or unusual breathlessness on exertion.
• Persistent exhaustion beyond four weeks that does not improve even with less training.

Blood markers frequently checked in this context are ferritin and iron status, thyroid values, vitamin D and, in women, sex hormones. On iron status as a performance topic in its own right: Iron deficiency in endurance athletes. The Peakora glossary explains the key terms around load, recovery and energy for quick reference.

How Peakora makes the pattern visible

The Peakora engine cannot diagnose RED-S — only a doctor can. But it can make visible the pattern in which RED-S typically shows up: a 24-hour recovery score built from resting heart rate, HRV and sleep that no longer returns to baseline over weeks, combined with a TSB that stays negative despite reduced load.

If you also log your weight, the recovery monitor shows you two independent trend lines side by side: load is falling, recovery is not coming. That exact combination is the picture sports physicians look for when RED-S is suspected. The coach responds by cutting volume and shifting intensity — not by pushing load further, as a rigid volume-percentage template would.

The most important step, though, remains the simplest: if you have trained hard for weeks and feel worse rather than better, eat more and train less — not the other way round. Performance comes back through recovery, not through extra hardness.

Frequently asked questions

Note: This article is training information and does not replace medical assessment. With persistent complaints, load-related bone or tendon pain, or unusual cardiac symptoms, please see a doctor.