Introduction Although chronic energy deficiency is prevalent in female athletes and linked to health outcomes described as the Female Athlete Triad, the impact on performance in weight bearing sports has not been fully elucidated. The primary purpose of this study was to explore the potential association between key indicators of adaptations to energy deficiency, i.e., the ratio of actual to predicted resting metabolic rate (RMR ratio) and serum total triiodothyronine on run time performance and on VO2max in college age female long-distance runners. We hypothesized that indications of energy deficiency would be negatively associated with running performance in a 5K competitive time trial. Methods We conducted a longitudinal study in 38 collegiate female long-distance runners (17-25 yr; BMI 20.3±0.3 kg/m2) across 10-12 weeks. Energy status, body composition, and 5km time trial (measure of performance on an outdoor course) were assessed pre- and post-competitive season. Runners (n=38) were categorized at baseline based on pre-season RMRratio : metabolically suppressed (SUPP: RMRratio <0.92, n=12), and energy replete (NSUPP: RMR ratio =0.92, n=26). We used repeated measures ANOVA and linear regression to test whether pre-season RMR ratio group and or pre-season TT3 concentrations were significantly associated with the change in run time performance over the competitive season. Results Twenty-one runners completed pre and post-season performance runs. Average body weight was 54.8+ 1.0 kg with 22.8+ 0.7% body fat and average VO2max was 59.7+ 1.2 ml/kg/min. SUPP runners had a slower 5km run times vs. NSUPP (Group effect; 22.5 vs 20.5 min, p=0.04) but the groups exhibited similar body mass, BMR, percent body fat, and percent lean body mass. There was also an effect of group*time for VO¦max, where the increase in VO2max was dependent on group such that the SUPP group demonstrated a smaller increase in aerobic fitness across the season compared to NSUPP group (p<0.001). Controlling for post-season VO2max, pre-season TT3 predicted post-season 5km times (R2=0.614, p=0.001). Conclusion Adaptations to chronic energy deficiency, defined as suppressed resting metabolic rate and or reduced TT3 are associated with poorer running performance and reduced improvements in VO2max across a competitive season.
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