What is Relative Energy Deficiency in Sport (RED-S)? 

Relative Energy Deficiency in Sport (RED-S) describes a condition in which an imbalance between energy intake and energy expenditure leads to impaired physiological function across multiple body systems [1]. In simple terms, it occurs when the body is not sufficiently fuelled to meet the combined demands of the body’s normal physiological processes and added training. Over time, this lack of energy can compromise an athlete’s health and performance. 

Causes

Athletes who are affected by RED-S are described as having a low energy availability (LEA) which is the core issue that ‘causes’ RED-S. Energy availability is easiest to understand as an equation: energy intake (kcal) minus exercise energy expenditure (kcal) divided by fat-free mass (kg) [2].  When this balance is too low, the body does not have sufficient energy to support the essential physiological functions of the body and the exercise demands placed upon it. As a result, processes such as metabolism, bone health, immune function, hormonal regulation and more can become compromised. 

A LEA can occur as a result of a number of reasons but here are some: 

1. Undereating: 

• This may be intentional or unintentional. Some individuals deliberately reduce their intake to achieve physique goals through ‘dieting’. Others may unintentionally underfuel through poor nutrition knowledge or underappreciating how many calories their training truly burns. 

2. High training load 

• A sudden increase in training volume or intensity is not always matched by an increase in energy intake. When nutrition does not increase with training demands, a significant energy deficit can occur. 

3. Eating disorders

• Some athletes unfortunately experience restrictive eating patterns and fears of weight gain and so do not allow themselves to fuel adequately. 

4. Sporting pressure & culture 

• Athletes often face pressure whether this is from coaches, judges or social media. In certain sports some pressures place an expectation to maintain a lean physique particularly in gymnastics and distance running which can contribute to underfuelling. 

Risk Factors

RED-S was originally coined as the “Female Athlete Triad” and so was thought to only affect female athletes [1]. However after more research it has been shown to affect athletes of any gender, age or ability level. 

As with many conditions, certain populations are at greater risk. In particular, endurance athletes are considered to be one of the highest-risk groups due to their consistently high energy expenditure. This includes long distance running, cycling and rowing [3]. 

It is important to note that RED-S is not limited to just elite sport – recreational athletes and the highly active individuals within the general population may also be affected. 

Signs, Symptoms and Consequences

RED-S can present with a wide range of signs and symptoms, often developing gradually and can easily go unnoticed. Early indicators include persistent fatigue, frequent illness, reduced performance, poor recovery and recurrent injuries especially stress fractures. Hormonal disturbances are another key feature. Females often present with absent or irregular menstrual cycles alongside reduced FSH and LH pulsatility; whilst males show reduced testosterone levels [4]. This affects reproductive function in the short term, with long term effects still not fully understood. 

If left unaddressed, RED-S can have significant long-term health consequences. Prolonged LEA negatively affects the body’s metabolism which in hand can lead to decreased bone formation and decreased bone mineral density, heightening the risk of fractures [5]. LEA also impacts the endocrine system with direct effect on insulin, growth hormone, thyroid hormones and cortisol which causes widespread effects on physiological function [6]. Studies also suggest that RED-S can impair cardiovascular function and increase the risk of coronary vascular disease [1]. 

Early recognition of these signs is essential to prevent more serious and lasting complications. 

Management and Treatment Options

RED-S is a condition that can be reversed and so the management focuses on restoring adequate energy availability with an individualised and multidisciplinary approach. The primary aim is to rebalance energy intake and expenditure often achieved by reviewing calorie intake with input from a sports dietitian who can provide fueling strategies and ensure nutritional needs are met. Medical input from doctors may also be required to monitor metabolic and hormonal health as well as bone density. 

Physiotherapists play an important role in managing any associated injuries by modifying training and supporting a safe return to activity. Psychologists [7] also provide input for cases in which psychological factors or eating disorders are present. Education is a key component – for athletes, coaches, parents and all staff to understand the importance of adequate fueling for performance but also the athlete’s long-term health [5,8] 

Practical Tips 

Here are some key tips to reduce the risk of RED-S: 

1. Prioritise your nutrition plan as much as your training plan 
2. Fuel before and after sessions 
3. Monitor performance trends 
4. Track your menstrual health 
5. Listen to early warning signs – and seek help if these arise! 

Conclusion

Relative Energy Deficiency in Sport is becoming a more common yet overlooked condition that affects athletes. From a physiotherapy perspective, we are in a key position to recognise early warning signs of RED-S through patterns of recurrent injury, delayed recovery or unexplained drops in performance, and initiate appropriate referrals when necessary. 

Effective management relies on a collaborative approach with a multidisciplinary team to ensure energy imbalances are altered so that long-term health consequences do not arise. By increasing the awareness of RED-S, we can protect athlete’s health and performance. 

Blog By: Jess Simmons (Musculoskeletal Physiotherapist at Crouch Physio).

References

1. Dave, S.C. and Fisher, M. (2022) ‘Relative energy deficiency in sport (red – S)’, Current Problems in Pediatric and Adolescent Health Care, 52(8), p. 101242. doi:10.1016/j.cppeds.2022.101242. 
2. Jeukendrup, A.E. et al. (2024) ‘Does relative energy deficiency in sport (Reds) syndrome exist?’, Sports Medicine, 54(11), pp. 2793–2816. doi:10.1007/s40279-024-02108-y. 
3. Wasserfurth, P. et al. (2020) ‘Reasons for and consequences of low energy availability in female and male athletes: Social Environment, adaptations, and prevention’, Sports Medicine – Open, 6(1). doi:10.1186/s40798-020-00275-6. 
4. Dipla, K. et al. (2020) ‘Relative energy deficiency in sports (red-S): Elucidation of endocrine changes affecting the health of males and females’, Hormones, 20(1), pp. 35–47. doi:10.1007/s42000-020-00214-w. 
5. Fortuna, M. et al. (2025) ‘Relative energy deficiency in sport (red-S): A systematic overview of mechanisms, effects, and clinical implications’, Quality in Sport, 42, p. 60506. doi:10.12775/qs.2025.42.60506. 
6. Elliott-Sale, K.J. et al. (2018) ‘Endocrine effects of relative energy deficiency in sport’, International Journal of Sport Nutrition and Exercise Metabolism, 28(4), pp. 335–349. doi:10.1123/ijsnem.2018-0127. 
7. Gould, R.J., Ridout, A.J. and Newton, J.L. (2022) ‘Relative energy deficiency in sport (red-S) in adolescents – A practical review’, International Journal of Sports Medicine, 44(04), pp. 236–246. doi:10.1055/a-1947-3174. 
8. Grabia, M. et al. (2024) ‘Female athlete Triad and relative energy deficiency in sport (reds): Nutritional Management’, Nutrients, 16(3), p. 359. doi:10.3390/nu16030359.