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Relative Energy Deficiency in Sport (RED-S)^[1] is a syndrome of: "impaired physiological functioning caused by relative energy deficiency and includes, but is not limited to, impairments of metabolic rate, menstrual function, bone health, immunity, protein synthesis, and cardiovascular health^[2]". The term was introduced by the International Olympic Committee (IOC) in 2014, yet recognition and diagnosis of RED-S among healthcare professionals, coaches and athletes remains poor.

RED-S is characterised by a shortage of available energy to keep up with the demands of exercise, on top of essential daily functions, such as growth, respiration and digestion. When the balance between energy intake and expenditure is tipped in the direction of low energy availability (LEA) over a period of time, wide-ranging adverse effects on all bodily systems can seriously compromise long term health and performance. There are calls for increased understanding of such consequences among all athletes, particularly male athletes, para athletes, and those of various races.

RED-S can occur in any athlete, dancer or exerciser of any age, gender, level, or sport. While the condition can occur among both genders, the prevalence of LEA is purportedly higher among females than males, although precise differences are unknown^[3]. Athletes training and competing in endurance sports (such as long-distance running) or activities which promote a low body weight (i.e. for aesthetics or performance, such as ballet) tend to be at the greatest risk for RED-S for two main reasons.

1) Endurance sports, by nature, demand a lot of energy which can easily be underestimated in the case of inadvertent under-eating

2) Where there are perceived advantages to achieving a certain body composition or aesthetic, athletes are more likely to manipulate food intake in order to gain a perceived performance advantage

Causes of LEA, and subsequently RED-S, can be viewed along a spectrum from inadvertent under-eating (unintentionally consuming insufficient calories for one's activities) on one end, to a clinical eating disorder on another. Since eating and exercise behaviours among sportspeople often differ from that of non-athletes, disordered eating and overtraining may more easily go unnoticed. Disordered eating is 20% more prevalence among both male and female athletes than non-athletes^[4].

While the basic concept of an energy imbalance is relatively simple one, screening and diagnosis of this condition can be challenging. Early warning signs of RED-S can be relatively subtle in isolation and are not all applicable to every individual. Since many adverse health and performance consequences of the condition are not unique to LEA, diagnosis of RED-S is a process of exclusion. Medical assessment is required to rule out other health conditions and to quantify parameters such as hormone and nutritional markers reflecting the severity of an energy deficit. To date, there is no standardised screening method or reference protocol for determining an individual's energy availability. While some symptoms of RED-S often overlap with those of the Female Athlete Triad, the consequences extend to a far broader range of physiological and psychological components across both males and females.

Treatment for RED-S requires a multidisciplinary team including medical, dietary, and often, mental health support. In the case of an eating disorder, clinical dieticians and psychiatric support is paramount. An individual’s reluctance to undergo treatment usually increases with the severity of the problem^[5], especially when an eating disorder is present^[6]. Return to sport can be a powerful motivator for athletes and exercisers to comply with treatment protocols^[7]. Athletes, dancers, exercises and their coaches of athletes will need to carefully tailor their training load inline with nutritional modifications and often, a gradual re-introduction of training will follow a period of complete rest. Medical parameters such as hormonal markers can be combined with training metrics such as training volume/intensity, heart rate, heart rate variability can be used to track recovery strategies aimed at returning to optimal health and performance.