Relative Energy Deficiency in Sport (REDs): Why Fueling Is a Medical Issue

Relative Energy Deficiency in Sport, or REDs, is the medical condition that develops when an athlete does not take in enough energy to cover both daily living and the demands of training. In its 2023 consensus statement, published in the British Journal of Sports Medicine, the International Olympic Committee defined REDs as a syndrome of harmful health and performance outcomes affecting female and male athletes exposed to low energy availability. The central shift is conceptual: underfueling is framed as a physiological problem with measurable consequences across many body systems, not a matter of willpower, discipline, or simply "not eating enough."

What low energy availability actually means

Energy availability is the energy left over for the body's core functions after exercise is accounted for. When intake is low relative to what training expends, the shortfall is called low energy availability (LEA). The body responds to a sustained deficit the way it would to any energy crisis: it starts economizing. Reproductive signaling, bone remodeling, thyroid activity, and other maintenance processes get throttled back to preserve fuel for immediate survival and movement.

The 2023 framework makes an important distinction here. LEA is described as existing on a spectrum, ranging from adaptable to problematic. A short, deliberate, well-monitored reduction in energy availability differs from a chronic, unrecognized deficit. The condition of concern is problematic LEA that persists and begins to erode health. This nuance matters because it moves the conversation away from a single threshold and toward context, duration, and the individual athlete.

From the female-athlete triad to a broader model

For decades, the dominant model was the female-athlete triad: the interrelationship among low energy availability, menstrual disturbance, and low bone mineral density. That model did real good in raising awareness, but it had two limits. It centered on three outcomes, and it centered on women.

REDs was introduced to widen that lens. The IOC framework retains the triad's core insight, that an energy deficit drives downstream dysfunction, while recognizing that the affected systems extend well beyond bone and menstrual health, and that male athletes are affected too. The triad is best understood as a subset of the larger REDs picture rather than a competing idea.

A multisystem condition

The reason REDs is treated as a medical issue rather than a nutrition footnote is the breadth of systems involved. The consensus statement describes potential effects on energy metabolism, reproductive function, musculoskeletal and bone health, immunity, glycogen synthesis, and cardiovascular and haematological health. The 2023 update also gave more explicit attention to mental health associations and to the added role of low carbohydrate availability alongside overall energy shortfall.

In practical terms, this is why a runner with repeated stress fractures, a cyclist who keeps catching every circulating illness, and an athlete whose performance has plateaued despite harder training may all be describing different windows into the same underlying problem. Bone stress injuries and menstrual disruption are often the most visible signs, but the endocrine and metabolic changes beneath them can be present well before a fracture shows up on imaging.

How clinicians assess it

To move REDs from concept to clinic, the IOC introduced an updated screening and diagnostic tool, the REDs Clinical Assessment Tool version 2 (CAT2). Rather than a single yes-or-no diagnosis, CAT2 stratifies risk across a range of health indicators and sorts athletes into categories often described with a traffic-light scheme, from lower to higher concern, each tied to guidance on training and competition.

The point of this design is severity grading. Not every athlete with a temporary dip needs to stop competing, and not every red flag carries equal weight. A 2024 study applying CAT2 across more than 200 elite athletes reported that those flagged at higher risk levels tended to show markers consistent with the underlying biology, including greater frequencies of menstrual disturbance and bone stress injury and lower levels of certain hormones and bone density, compared with lower-risk athletes. That kind of work is how a consensus framework earns clinical trust: by testing whether its categories track with measurable physiology.

What this means for athletes and those around them

The most useful takeaway is a reframing. Fatigue that does not resolve with rest, recurrent injuries, frequent illness, disrupted or absent menstrual cycles, and stalled performance are not always signs to train harder or eat less. In the REDs model, they can be signals that the energy equation is out of balance. Recognizing that early is the difference between a manageable adjustment and a longer road back.

It also reframes responsibility. Because REDs is multisystem and can be subtle, it is not something an athlete is expected to diagnose alone, nor is it a moral failing. Coaches, families, and clinicians all sit within the picture the IOC drew. Screening tools like CAT2 exist precisely so that concern can be structured and shared rather than left to guesswork.

This article is educational and is not medical advice; any athlete who suspects an energy-availability problem should be evaluated by a qualified clinician who can interpret their full history and testing.

The larger lesson of the REDs framework is that fueling is not the background to sport. It is part of the physiology of sport, and when it goes wrong, the body keeps a careful and eventually costly ledger.