Bone Health in Triathletes: Skeletal Loading, Low Energy Availability, and Bone Stress Injury-A Critical Narrative Review

Background and Objectives: Triathlon combines low-impact swimming and cycling with repetitive running and high energy demand. We evaluated triathlon-specific evidence on skeletal adaptation, low energy availability (LEA), Relative Energy Deficiency in Sport (REDs), and bone stress injury (BSI). Materials and Methods: We purposively selected evidence from PubMed/MEDLINE through 11 August 2026 and citation tracking. The search returned 233 records; 25 primary reports representing approximately 20-22 independent cohorts were synthesized, with the consensus literature providing the context. Results: Most studies were small, cross-sectional, and heterogeneous. No uniform bone mineral density deficit emerged; findings varied by site, sex, age, body size, competitive level, race distance, and modality. One four-year cohort of 50 elite short-course athletes found substantial BSI burden and a higher rate in females, but its single-program setting limits generalization. Evidence on female LEA/Triad/REDs relied mainly on questionnaires, dietary assessments, cross-sectional imaging, and case reports; evidence on males and interventions was sparse. Biomarkers, areal bone mineral density, structural imaging, risk screens, and BSI were not interchangeable. Conclusions: Evidence does not support universal imaging, biomarker testing, or supplementation; individualized assessment is preferable. Prospective, preregistered, sex- and age-inclusive studies should align discipline-specific exposure, repeated energy assessment, site-specific skeletal measures, and imaging-verified BSI.