Endocrine markers of energy deficiency and within-day energy balance metrics in elite athletes.

Introduction: This cross-sectional study aimed to determine the prevalence of Relative Energy Deficiency in Sport (REDs) among Swedish national-team athletes across multiple sports and to examine selected endocrine and metabolic markers in relation to within-day energy balance (WDEB) metrics. Methods: REDs severity was retrospectively classified in 43 athletes (60.5% females) using the IOC REDs Clinical Assessment Tool 2 framework. Assessments included DXA-derived body composition, resting metabolic rate (RMR) by ventilated-hood indirect calorimetry, blood markers, validated questionnaires, clinical eating disorder interview. 4-day dietary intake, energy availability (EA), EB, and WDEB metrics were assessed in a subsample ( Results: Twenty athletes (46.5%) were classified as controls (no/low risk; 45.0% female), whereas 23 (53.5%) were classified with REDs (73.9% female). Most REDs cases were mild, however 61% exhibited ≥2 additional potential REDs indicators, a pattern also observed in 50% of controls. Athletes classified with REDs had higher cortisol and cortisol-to-insulin ratio, lower RMR ratio, lower energy intake and 24 h EA, and greater exposure to negative WDEB patterns. In unadjusted analyses, insulin was associated with both 24 h EB and EA. Exploratory analyses in females identified associations between cortisol-to-insulin ratio and negative WDEB metrics, and between Exercise Addiction Inventory score and 24 h EB; however, these associations were no longer significant after Benjamini-Hochberg false discovery rate correction. Conclusion: More than half of the elite athletes in this cohort, were classified with REDs, with a higher prevalence among females. REDs classification was associated with lower EA, greater exposure to negative WDEB patterns, and additional endocrine and metabolic alterations consistent with energy deficiency. Notably, a similar pattern was also observed in 50% of controls, supporting a continuum rather than discrete risk categories. Although WDEB and dietary metrics may provide useful contextual information they should be interpreted cautiously and integrated with comprehensive multi-marker clinical REDs evaluation.