Relative Energy Deficiency in Sport (RED-S)
Evidence: strong
Low energy availability, eating too little for the training done, is the single root cause of a syndrome that damages bone, hormones, immunity and performance. The framework rests on three IOC consensus statements. It affects men as well as women.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
RED-S is what happens when a runner does not eat enough to cover both training and the basic running of the body. The shared root cause is low energy availability: the energy left over after exercise, expressed per kilogram of fat-free mass per day, is too low to support normal function (Mountjoy et al. 2023). When that deficit is chronic, the body downregulates systems it treats as non-essential, and the damage spreads across bone, the endocrine system, the menstrual cycle, immunity and performance itself.
The energy availability threshold
The concept comes from controlled work by Loucks. When energy availability in regularly menstruating women fell below about 30 kcal per kilogram of fat-free mass per day for five days, pulsatile release of luteinising hormone was disrupted (Loucks and Thuma 2003). This 30 kcal/kg figure became the standard reference point, with about 45 kcal/kg treated as the target for healthy function.
The threshold is a useful heuristic, not a hard line. A later critical review argued that the evidence for a single sharp cut-off is weaker than often claimed, and that the dose-response varies between individuals (Areta et al. 2023). The honest reading is that lower energy availability raises risk on a gradient, and the exact number at which any one runner gets into trouble is not fixed.
What it does to the body
Bone is the headline cost for runners
Low energy availability suppresses the hormones that build and maintain bone, lowers bone mineral density, and raises the rate of bone stress injuries (Gallant et al. 2024). For a runner this is often the first hard sign that something is wrong.
The IOC physiological model maps the effects across many systems: impaired bone health and bone stress injury, suppressed reproductive and thyroid hormones, menstrual dysfunction in women and lowered testosterone in men, weakened immunity, poor recovery, and reduced training response and performance (Mountjoy et al. 2023). The bone and endocrine effects are well supported; some of the wider performance claims rest on observational data.
It supersedes the female athlete triad, and it affects men
The older ‘female athlete triad’ linked low energy availability, menstrual dysfunction and low bone density in women. The 2014 IOC statement kept that core but broadened it into RED-S, a wider syndrome affecting many systems and both sexes (Mountjoy et al. 2014).
Men are not exempt. In male endurance athletes, low energy availability tracks with lower bone mineral density (Keay et al. 2018), and screening surveys find a high proportion of competitively trained male endurance athletes, runners included, at risk of low energy availability (Lane et al. 2019). In men the early warning signs are quieter, because there is no menstrual cycle to disrupt, which makes it easier to miss.
Screening and management
The 2018 update added a clinical assessment tool and a risk-stratified return-to-play model, and recommends screening at-risk athletes and managing them through a sports physician, a dietitian and, where needed, mental-health support (Mountjoy et al. 2018). The 2023 statement refined this into the validated IOC REDs Clinical Assessment Tool version 2 (Mountjoy et al. 2023).
The fix is almost always to eat more, not to train less first
The treatment for low energy availability is to raise energy intake to match the training load, restoring availability toward 45 kcal/kg fat-free mass per day. Reducing training can help, but a runner who is underfuelling needs more food before anything else. Disordered eating often sits underneath, so management is rarely just a nutrition spreadsheet.