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.