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Relative Energy Deficiency in Sport (REDs) was first introduced in 2014 by the International Olympic Committee’s expert writing panel, identifying a syndrome of deleterious health and performance outcomes experienced by female and male athletes exposed to low energy availability (LEA; inadequate energy intake in relation to exercise energy expenditure). Since the 2018 REDs consensus, there have been >170 original research publications advancing the field of REDs science, including emerging data demonstrating the growing role of low carbohydrate availability, further evidence of the interplay between mental health and REDs and more data elucidating the impact of LEA in males. Our knowledge of REDs signs and symptoms has resulted in updated Health and Performance Conceptual Models and the development of a novel Physiological Model. This Physiological Model is designed to demonstrate the complexity of either problematic or adaptable LEA exposure, coupled with individual moderating factors, leading to changes in health and performance outcomes. Guidelines for safe and effective body composition assessment to help prevent REDs are also outlined. A new REDs Clinical Assessment Tool-Version 2 is introduced to facilitate the detection and clinical diagnosis of REDs based on accumulated severity and risk stratification, with associated training and competition recommendations. Prevention and treatment principles of REDs are presented to encourage best practices for sports organisations and clinicians. Finally, methodological best practices for REDs research are outlined to stimulate future high-quality research to address important knowledge gaps.

In the past decade, the study of relationships among nutrition, exercise and the effects on health and athletic performance, has substantially increased. The 2014 introduction of Relative Energy Deficiency in Sport (REDs) prompted sports scientists and clinicians to investigate these relationships in more populations and with more outcomes than had been previously pursued in mostly white, adolescent or young adult, female athletes. Much of the existing physiology and concepts, however, are either based on or extrapolated from limited studies, and the comparison of studies is hindered by the lack of standardised protocols. In this review, we have evaluated and outlined current best practice methodologies to study REDs in an attempt to guide future research.This includes an agreement on the definition of key terms, a summary of study designs with appropriate applications, descriptions of best practices for blood collection and assessment and a description of methods used to assess specific REDs sequelae, stratified as either Preferred, Used and Recommended or Potential. Researchers can use the compiled information herein when planning studies to more consistently select the proper tools to investigate their domain of interest. Thus, the goal of this review is to standardise REDs research methods to strengthen future studies and improve REDs prevention, diagnosis and care.

RED-S is the ‘impaired physiological functioning caused by relative energy deficiency, and includes but is not limited to impairments of metabolic rate, menstrual function (for females), bone health, immunity, protein synthesis and cardiovascular health.’5 Chronic LEA (over many weeks to years) leading to RED-S can limit training adaptation6 and substantially increase injury risk.5 7 8 It cripples performance in both female6 and male athletes.5 It can occur unknowingly without a diagnosed eating disorder.7 There is good news: there are opportunities to mitigate the occurrence and effects of RED-S through system-wide education including prevention/awareness interventions and by optimising energy intake. RED-S-associated diagnostic factors may include: chronic dietary restriction and/or extreme diets, drive for thinness, large changes in body weight or composition in short time periods, training inconsistencies, prolonged fatigue, decreased libido, oligo-amenorrhoea (missing three or more menstrual cycles in 6 months), two or more career bone stress injuries and low bone mineral density for age (Z-score <-1.0).5 Treatment We must ensure athlete access to a referral network of RED-S and eating disorder experts, including sport and exercise medicine physicians, registered sport dieticians and sport psychologists, within each country. There was much discussion about methods to safely improve performance while ensuring athlete physical and mental health.

Low energy availability (LEA) occurs when energy expenditure from athletic training and bodily functions exceeds caloric intake. This imbalance results in declines in athletic performance and increases the risk of injury. Relative energy deficiency in sport (REDs) is a condition that occurs when the energy deficit is severe enough to cause alterations to metabolic rate, menstrual function, immune function, bone health, protein synthesis, and cardiovascular function. Many athletes, particularly those competing in endurance, aesthetic, or weight-class sports, are adversely impacted by this condition. This study aims to determine the prevalence of LEA and REDs among athletes and present the first secondary analysis of the impacts of these phenomena on sports performance and risk of injury. This systematic review was registered on PROSPERO (CRD42023469253). Literature searches were performed following PRISMA guidelines using PubMed, Embase, and Cochrane online databases. Inclusion criteria were articles discussing the prevalence of LEA or REDs, the impact of LEA or REDs on athletic performance, or the impact of LEA or REDs EA on injury. A total of 59 studies met the inclusion criteria for this meta-analysis, and 2737 of 6118 athletes (44.7%) in 46 different studies were determined to have LEA, including 44.2% of female athletes and 49.4% of male athletes. In addition, 460 of 730 athletes (63.0%) in eight different studies were determined to be at risk of REDs. Athletes with LEA were found to have decreased run performance, training response, endurance performance, coordination, concentration, judgment, explosive power, and agility relative to athletes with normal energy availability, as well as an increased likelihood of absence from training due to illness. Studies had mixed results as to whether LEA increased the risk of injury in general. However, most studies concluded that athletes with LEA have impaired bone health and a higher risk of bone stress injuries. To our knowledge, this is the first systematic review analyzing the impacts of LEA and REDs on athletic performance and risk of injury. Due to the high estimated prevalence of LEA among athletes, coaches may want to consider employing surveys such as the low energy availability in females questionnaire (LEAF-Q) to identify athletes at risk for LEA, as early identification and correction of LEA can prevent the development of symptoms of REDs, reduce the risk of impaired bone health and bone stress injuries, and help athletes optimize the performance benefits from their training.

Background/Objectives: Energy availability (EA), defined as the dietary energy remaining after exercise energy expenditure (EEE), is a central determinant of both health and performance in athletes. Chronic insufficient EA leads to low energy availability (LEA), which is an underlying mechanism of Relative Energy Deficiency in Sport (REDs). This narrative review critically explores the conceptual evolution of EA and LEA, summarizes current physiological evidence, and discusses methodological and practical challenges in their assessment and application in free-living athletes. Methods: Evidence from experimental and observational studies was reviewed to describe the hormonal, metabolic, and performance outcomes associated with LEA. Screening tools, including the Low Energy Availability in Females Questionnaire (LEAF-Q) and the Low Energy Availability in Males Questionnaire (LEAM-Q), were also evaluated for their validity and applicability in different sports contexts. Results: LEA is associated with alterations in thyroid and reproductive hormones, which, in turn, contribute to reduced resting metabolic rate, lower bone mineral density, and delayed recovery. While screening questionnaires can help identify athletes at risk, their accuracy varies by sport and individual characteristics. Incorporating hormonal and metabolic biomarkers provides a more direct and sensitive method for detecting physiological stress. Measuring dietary intake, EEE, endocrine balance and body composition in real-world settings remains a major methodological challenge. Combining hormonal, metabolic, and behavioral indicators may improve the identification of athletes experiencing LEA. Conclusions: EA plays a central role in the interaction between nutrition, exercise, and athlete health, but methodological limitations in its assessment may compromise accurate diagnosis. Improving measurement techniques and adopting integrated monitoring strategies are essential to improve early detection, guide individualized nutrition, and prevent RED-related health and performance impairments.

Background/Objectives: The primary aim of this study was to characterize athletes approaching an outpatient interdisciplinary and multidisciplinary consultation structure for athletes with a suspected relative energy deficiency in sport (REDs) cross-sectionally and longitudinally to prove treatment efficacy. Methods: Data of 58 athletes suspected of REDs were collected at the onset (t0) and completion (t1) of interdisciplinary and multidisciplinary REDs treatment (clinical practice) between January 2019 and December 2022. The data included extracted information from medical records, anthropometric characteristics, physical performance diagnostics, laboratory values, dietary records, and partially gynecological and psychosomatic diagnostics. Results: The sample primarily consisted of female athletes (97%) under 18 years of age (66%) who were underweight with a body mass index (BMI) < 18.5 kg/m2 and BMI percentile below the 10th percentile for their age and gender-specific norms (59%), and experienced menstrual disorders (93%). The dietary behavior is characterized by plant-based and low-energy-dense foods. Eating disorders (anorexia nervosa and bulimia nervosa) were diagnosed in 40% of the athletes according to the International Statistical Classification of Diseases and Related Health Problems 11th revision criteria. During the program, 64% of the athletes exhibited a mean weight gain of 7 (±6) kg (p < 0.001), excluding those still undergoing treatment (36%). Conclusions: The proposed interdisciplinary and multidisciplinary treatment approach proved effective and holds promise for future evidence-based developments in REDs treatment.

ObjectiveThis cross-sectional retrospective and prospective study implemented the 2023 International Olympic Committee Relative Energy Deficiency in Sport (REDs) Clinical Assessment Tool version 2 (CAT2) to determine the current severity of REDs (primary outcome) and future risk of bone stress injuries (BSI, exploratory outcome) in elite athletes.MethodsFemale (n=143; 23.3±4.3 years) and male (n=70; 23.1±3.7 years) athletes (performance tier 3 (52%), tier 4 (36%), tier 5 (12%)) participated in a baseline CAT2 (with minor modifications) assessment, including a self-report questionnaire (menstrual function (females), BSI, Eating Disorder Examination Questionnaire (EDE-Q)), bone mineral density (BMD via DXA) and fasted blood analysis (triiodothyronine (T3), testosterone, cholesterol). Athletes were assigned a green, yellow, orange or red light via CAT2. The prospective risk of new self-report of physician-diagnosed BSI was assessed over a subsequent 6–24 months.ResultsREDs prevalence was 55% green, 36% yellow, 5% orange and 4% red light. The CAT2 identified a greater prevalence of amenorrhoea and BSI and lower T3, testosterone and BMD (p<0.01) in red, orange and yellow (those with REDs) versus green light. ORs for a prospective self-reported BSI (majority physician diagnosed) were greater in orange vs green (OR 7.71, 95% CI (1.26 to 39.83)), in females with severe amenorrhoea (OR 4.6 (95% CI 0.98 to 17.85)), in males with low sex drive (OR 16.0 (95% CI 4.79 to 1038.87)), and athletes with elevated EDE-Q global scores (OR 1.45 (95% CI 0.97 to 1.97)).ConclusionThe CAT2 has high validity in demonstrating current severity of REDs, with increased future risk of self-reported BSI in athletes with a more severe REDs traffic light category.

Preferentially energy derived from dietary intake covers the demands of training, and the remaining energy, EA, is quantified in kcal/kg of fat free mass.2 In figure 1, the central bar illustrates adequate EA in an athlete where energy intake is sufficient to cover the demands of training and fundamental life processes to maintain health. LEA occurs in the case of high training loads, relative to nutritional intake. [...]this figure reinforces the important point in the IOC statements on RED-S that psychological factors which determine these behaviours are key to the development, continuation and management of RED-S.2 3 Figure 2. Hormones are key to health and to drive positive adaptations to exercise, to improve athletic performance. [...]hormones can be informative in tracking the response of an individual to these three input variables.

Low energy availability (LEA) is a recognized risk factor that affects the health and performance of athletes. This narrative review summarizes current evidence on the relationship between LEA, resting metabolic rate (RMR), and bone mineral density (BMD). It focuses on the applicability of the RMR ratio as an indicator of metabolic adaptation to energy deficiency and analyzes the associations between energy availability and skeletal health outcomes. This narrative review demonstrates that reduced energy availability is related to a decrease in the RMR ratio and hormonal alterations characteristic of Relative Energy Deficiency in Sport (RED-S). Furthermore, prolonged LEA has been associated with impairments in bone metabolism and lower Z-scores, particularly among endurance and aesthetic athletes. However, the findings also suggest that the impact of LEA on BMD may be modulated by sport-specific loading patterns and additional individual factors. Considerable methodological heterogeneity between studies limits the direct comparability of the results, highlighting the need for standardization in the evaluation of EA, RMR, and BMD. This review emphasizes the importance of comprehensive screening strategies combining nutritional, metabolic, hormonal, and skeletal markers for early identification of the risk of RED-S. Future research should prioritize longitudinal designs to better understand the dynamics of metabolic and skeletal changes in response to fluctuations in energy availability.

Correspondence to Dr Nicola Keay, Durham University, Durham DH1 3LY, UK; nickykeayfrancis@googlemail.com Relative energy deficiency in sport (RED-S)1 is a clinical syndrome encompassing adverse health and performance (figure 1) consequences of low energy availability (LEA)2 in male3 and female exercisers of all ages and all levels from recreational to elite. LEA is a situation where energy intake is insufficient to cover the combined energy demands of training and baseline physiological processes to maintain health. [...]early identification of those at risk of LEA is essential in preventing the clinical consequences of RED-S.