What are the ‘relative energy deficiency in sport’ (RED-S), the ‘athlete triad’ and low energy availability? Definition of concepts and history.
What is RED-S?
The relative energy deficiency in sport (RED-S) is currently a very hot topic in sports nutrition, and for a good reason. The RED-S is a theoretical model originally defined in 2014 and it is thought to have the potential to explain the origin of a wide range of signs and symptoms that may negatively affect the health and performance of physically active individuals (both males and females). These effects include endocrine, physiological, metabolic, and psychological dysregulations, which are thought to ultimately affect physical capacity and performance.
RED-S explains the origin of a wide range of signs and symptoms that may negatively affect the health and performance of physically active individuals (both males and females)
It is typically said that someone suffers from ‘RED-S’ when a range of these systems shows altered function. The cause of these alterations is thought to be ‘energy availability’ or —specifically— ‘LOW energy availability’ (or LEA, see explanation below).
What is The Triad?
Interestingly, there is another — anteceding — model that also provides a theoretical framework to understand negative effects of LEA, which is the ‘triad’ model. Originally called the ‘female athlete triad’, this model was first mentioned in the literature in 1993 and formally established by a group of experts in 1997. The recognition came in an effort to find a solution to the large incidence of female athletes showing a coexistence of the ‘triad’ of symptoms of menstrual dysfunction, osteoporosis (weakened bones) and eating disorders (reduced energy availability).
Decades of research have provided strong support to this model, and thanks to this we have now a clearer understanding of how to diagnose this condition and what are the typical signs and symptoms. More recently, in the last couple of years, parallels have been drawn between this condition and observations of bone and reproductive system dysregulation in exercising males too in the ‘male athlete triad’.
Are the RED-S and triad models different?
The RED-S model was created by a separate group of researchers/practitioners and its foundations are based on the triad but brings to the forefront more prominently two theoretical statements: 1) the fact that males, and not just females, may also be affected, and 2) that it may affect a larger number of systems/areas in addition to the reproductive system and bone. The triad model, due to its legacy, originally focused more on females and the majority (but not all) of the effects were reported around reproductive function and bone.
Our understanding of both models is currently developing and there are researchers world-wide trying to provide evidence for these. Regardless of the model, however, low energy availability sits at the core of these two important models and is thought to be the key origin of all the signs and symptoms they report.
Energy availability, what is it?
Energy availability is broadly defined as the ‘dietary energy available to maintain normal physiological function’ and mathematically is defined and calculated as:
This is, dietary energy intake minus the energy expended during exercise, normalised to fat free mass (the most metabolically active component of our body mass). The result is the dietary energy ‘available’ (in kcal/kg FFM/day) to maintain normal function of tissues and organs (the energy necessary to maintain all the cells in your body functioning normally).
The result is the dietary energy ‘available’ (in kcal/kg FFM/day) to maintain normal function of tissues and organs
Key research in humans using this calculation led by Prof. Anne Loucks, was set to define if it was exercise stress per se the origin of endocrine responses associated to the female triad, or the amount of energy available (after factoring in exercise energy expenditure). This research showed that it was not exercise stress, but low energy availability what was causing hormonal dysregulations.
Ok, understood, but what is LOW energy availability?
There are different iterations of the calculation of energy availability, and the concept of energy availability is similar to the concept of energy balance, but it is different. The explanation of how these are different is a bit longer that a length of a blogpost allows, but it is important to know that an energy availability of ~45 kcal/kg/FFM day, is roughly equivalent to an energy balance of zero (no net loss or gain of body weight through time) —some people call this normal or adequate energy availability—.
It is important to know that an energy availability of ~45 kcal/kg/FFM day, is roughly equivalent to an energy balance of zero
Instead, low energy availability has been defined at or under a threshold of daily 30 kcal/kg FFM/day based on the research by Prof Loucks, showing that there is a clear effect on hormonal markers of reproductive system and bone metabolism in females under this value. However, this all-important research was conducted mainly on sedentary females in laboratory-based studies of 3-6 days of duration. Whether this threshold can be extrapolated to fieldwork and to both sexes is questionable (see: is there a universal threshold of low energy availability of 30 kcal/kg FFM/day?).
Areta JL, Taylor HL, Koehler K. Low energy availability: history, definition and evidence of its endocrine, metabolic and physiological effects in prospective studies in females and males. Eur J Appl Physiol. 2021;121:1–21.
Mountjoy M, Sundgot-Borgen J, Burke L, Ackerman KE, Blauwet C, Constantini N, et al. International Olympic Committee (IOC) Consensus Statement on Relative Energy Deficiency in Sport (RED-S): 2018 Update. International Journal of Sport Nutrition and Exercise Metabolism. 2018;28:316–31.
De Souza MJ, Nattiv A, Joy E, Misra M, Williams NI, Mallinson RJ, et al. 2014 Female Athlete Triad Coalition Consensus Statement on Treatment and Return to Play of the Female Athlete Triad: 1st International Conference held in San Francisco, California, May 2012 and 2nd International Conference held in Indianapolis, Indiana, May 2013. British Journal of Sports Medicine. 2014;48:289–289.
Loucks AB. Exercise Training in the Normal Female: Effects of Low Energy Availability on Reproductive Function. In: Hackney AC, Constantini NW, editors. Endocrinology of Physical Activity and Sport. Cham: Springer International Publishing; 2020 [cited 2020 Mar 31]. p. 171–91. Available from: http://link.springer.com/10.1007/978-3-030-33376-8_11