Relative Energy Deficiency in Sport (REDs) and Body Composition

Background: Relative energy deficiency in sport (REDs) is a multi-system syndrome precipitated by sustained low energy availability (LEA) and known to perturb bone, muscle, adipose, endocrine, and fluid–electrolyte homeostasis in athletic populations. Research question: How does chronic exercise-induced LEA modify the four principal compartments of body composition—bone mineral density, skeletal muscle mass, adipose (total and visceral) tissue, and hydration status—and to what extent do these modifications differ across endurance, strength, and aesthetic sporting disciplines? Objective: The objective of this narrative review was to synthesise the post-2000 peer-reviewed literature addressing this question, to delineate sex- and sport-specific patterns of compositional change, and to appraise the evidence base underpinning current nutritional and screening recommendations. Methods: A structured search of PubMed, Web of Science, Scopus, and Google Scholar (January 2000–June 2026) was performed; 85 sources specific to REDs, body composition, and athletes met the pre-specified inclusion criteria and informed the synthesis (104 references are cited in total, including background and methodological sources), the reporting of which was guided by the SANRA framework. Scope: The review integrates physiological, endocrine, and nutritional evidence across both sexes, contrasts REDs risk across endurance, ultra-endurance, weight-category/aesthetic, speed–power, and team disciplines and addresses practical implications for screening, interdisciplinary care, and future research.