Relative Energy Deficiency in Sport (REDs) in Ultra-Trail Running: Implications for Performance, Injury, and Long-Term Health.

Ultra-trail running has experienced substantial global growth, yet the nutritional and physiological consequences of prolonged participation remain incompletely understood. Relative energy deficiency in sport (REDs), resulting from problematic low energy availability (LEA), has emerged as an important concern within endurance sports; however, evidence specific to ultra-trail athletes remains fragmented. This narrative review synthesises current evidence relating to the development, manifestations, and consequences of REDs in ultra-trail running. Available literature suggests that the combination of high training volumes, prolonged competition, inadequate energy intake, gastrointestinal limitations, sleep disruption, and environmental stressors may place ultra-trail athletes at heightened risk of chronic LEA. Consequences extend beyond impaired performance and recovery to include endocrine dysfunction, compromised bone health, increased injury susceptibility, immune disturbance, and adverse psychological outcomes. Despite growing recognition of REDs within endurance sport, prevalence estimates in ultra-trail populations remain poorly defined, and validated sport-specific screening and management strategies are lacking. Greater awareness among athletes, coaches, and healthcare professionals is required to facilitate earlier recognition and intervention. Future research should prioritise prospective studies, sport-specific diagnostic approaches, and targeted prevention strategies to better support athlete health, performance, and long-term participation in ultra-trail running.