What is RED-S? Relative energy deficiency in sport, signs and risks

Short answer

RED-S (Relative Energy Deficiency in Sport) is a multi-system health and performance problem that arises when an athlete does not fuel enough for the energy they expend in training. At its core lies "low energy availability": when the energy left for the body's basic functions falls short after training, hormonal, skeletal, immune and mental balances are disrupted.

This picture used to be called the "female athlete triad" and was described only in women. Today we know it also occurs in men, and its effects are not limited to periods and bone. A drop in performance, recurrent stress injuries, menstrual disturbance and frequent illness are important signs. A "eat less, train more" culture is the main ground for this risk.

Contents

What is RED-S, and where does it come from?
How it differs from the old "female athlete triad"
Signs: what to watch for
low energy availability: the root of the problem
Screening and approach
The risk of the "eat less, train more" culture
For clinicians

What is RED-S, and where does it come from?

The concept was first introduced by the International Olympic Committee (IOC) in its 2014 consensus statement (Mountjoy et al., BJSM), defined as "beyond the female athlete triad." It was updated in 2018; its most comprehensive form is the 2023 IOC consensus statement (again Mountjoy et al., BJSM). These statements stress that the core problem of the condition is low energy availability.

Energy availability is simply this: when you subtract what you expend in training from what you take in, how much is left for the body's basic functions such as the heart, bone, hormone production and immunity? If this amount stays persistently low, the body switches to a "saving mode" and cuts back non-vital functions.

How it differs from the old "female athlete triad"

The earlier framework was the female athlete triad, defined in the 2007 ACSM statement: low energy availability, menstrual dysfunction and low bone density. This triad is still valid and important; but RED-S extends it in two directions:

Sex: The problem is not specific to women. In male athletes, low energy availability can lead to reduced testosterone, impaired bone health and loss of performance.
Scope: The effects are not limited to periods and bone. RED-S is a multi-system picture that also includes metabolism, immunity, protein synthesis, cardiovascular health, gastrointestinal function and mental state.

So the triad is an important subset within RED-S; RED-S is the broader umbrella.

Signs: what to watch for

RED-S progresses insidiously and is often noticed as the complaint "I'm training well but getting no results." The main signs:

Drop in performance: Unexplained loss of endurance, failure to recover, no response to training.
Menstrual disturbance or absence (in women); reduced libido and low-testosterone signs in men.
Recurrent stress injuries / stress fractures — one of the most concrete signs of impaired bone health (see bone stress injury).
Frequent illness: Weakened immunity, recurrent infections.
Mental signs: Fatigue, irritability, difficulty concentrating, low mood; sometimes disordered eating.
Gastrointestinal problems, feeling cold, sleep disturbance.

No single sign makes the diagnosis; but the combination of "performance drop + menstrual disturbance + recurrent injury" is a strong warning and requires assessment.

low energy availability: the root of the problem

At the centre of RED-S is low energy availability. This is not always a conscious eating disorder; often the athlete, having not raised their nutrition enough to match increased training load, unknowingly slips into an energy deficit. The study by Loucks and Thuma, published in the Journal of Clinical Endocrinology & Metabolism in 2003, showed that in regularly menstruating women the release of reproductive hormone (luteinising hormone) is disrupted below a certain energy availability threshold, establishing the physiological basis of this mechanism.

An important point: low energy availability is not always synonymous with thinness. An athlete at a normal-looking weight can also be in energy deficit if they under-fuel relative to their expenditure. So the assumption "weight is normal, no problem" is misleading.

Screening and approach

When RED-S is suspected the approach is holistic and cannot be reduced to a single blood test. The IOC statements recommend risk-stratification tools that sort health and performance risks into categories. In practice, assessment can include:

Nutrition and training history, evaluation of weight/energy balance.
Menstrual history (in women), hormonal assessment.
Bone health assessment (DXA, especially with recurrent stress fracture).
Screening of mental state and eating behaviour.

Treatment generally rests on increasing energy availability: either raising intake or adjusting training load. So RED-S management is often the work of a team — a doctor, a sports dietitian and, where needed, a mental health professional.

This article does not recommend a specific calorie or "how many kcal/kg" target. Energy needs vary by person, sport and phase; the right target can only be set through assessment by a doctor and sports dietitian.

The risk of the "eat less, train more" culture

In many sports the belief that "lighter = faster/better" is widespread, and in young athletes this pressure is a serious risk. In the short term, weight loss may appear to improve performance; but a persistent energy deficit results in bone loss, hormonal disruption, recurrent injury and long-term decline in performance. This risk is even more important in athletes who are still growing — see youth and adolescent athletes.

The healthy approach is not "restrict" but "fuel enough and load intelligently." Energy balance is the foundation of sustainable performance.

For clinicians

RED-S is a multi-system syndrome with low energy availability as its aetiological core (IOC consensus statements 2014, 2018, 2023). The 2023 statement broadens the conceptual model and offers an indicator/risk-stratification framework supporting clinical diagnosis (primary and secondary indicators; green/yellow/red-light sport participation decisions). The mechanistic basis is robust from Loucks and Thuma's (2003) threshold data: luteinising hormone release is disrupted below ~30 kcal/kg fat-free mass/day; this value is a reference rather than a clinical cut-off, with high individual variability. In men too, low energy availability is linked to testosterone suppression and bone mineral density loss. The female athlete triad (ACSM 2007) remains valid within RED-S but is narrower. In clinical practice, consider RED-S in the context of an unexplained performance plateau, functional hypothalamic amenorrhoea, recurrent/multiple stress injury, low bone mineral density or disordered eating; management should be multidisciplinary (doctor, sports dietitian, mental health) and focus on restoring energy availability rather than on a fixed calorie prescription.

Frequently asked questions

Does RED-S only occur in female athletes?

No. It used to be defined only in women as the "female athlete triad," but RED-S also occurs in men. In men, low energy availability can lead to reduced testosterone, impaired bone health and loss of performance. The problem depends not on sex but on energy balance.

Can I have RED-S if my weight is normal?

Yes, you can. The root of RED-S is not thinness but under-fuelling relative to energy expended (low energy availability). An athlete at a normal-looking weight can be in energy deficit if they are not eating enough to meet training load. So looking only at weight is misleading.

What are the signs of RED-S, and when should I be suspicious?

Unexplained performance decline, menstrual disturbance/absence (in women), recurrent stress fractures, frequent illness and fatigue/mood changes are the main signs. If these occur together, especially if you are an athlete training intensively, it is advisable to see a doctor.

How do I learn how much I need to eat in RED-S?

There is no single calorie target that fits everyone; needs vary by person, sport and training phase. The right energy target is set not by online calculators but by a doctor and sports dietitian who assess you. The aim is not to restrict but to eat enough and in balance to meet your expenditure.

Scientific references

Other references:
Mountjoy M et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med 2023;57(17):1073-1097.
Mountjoy M et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. Br J Sports Med 2018;52(11):687-697.
Mountjoy M et al. The IOC consensus statement: beyond the Female Athlete Triad—Relative Energy Deficiency in Sport (RED-S). Br J Sports Med 2014;48(7):491-497.
Loucks AB, Thuma JR. Luteinizing hormone pulsatility is disrupted at a threshold of energy availability in regularly menstruating women. J Clin Endocrinol Metab 2003;88(1):297-311.
Nattiv A et al. American College of Sports Medicine position stand: the female athlete triad. Med Sci Sports Exerc 2007;39(10):1867-1882.

This information is for general guidance only; it does not replace an examination, a diagnosis, or your physician's individual advice. Please consult a physician for your symptoms. About the author: Doç. Dr. Oğuz Yüksel — Academic Profile.