OBJECTIVE: To examine the relationships between changes in static prestage and poststage measures of commonly used hematological and urinary markers of hydration status and body mass (BM) in participants in a 3-day trail run. DESIGN: Descriptive field study. SETTING: Three Cranes Challenge trail run, South Africa. PARTICIPANTS: Twenty (6 men and 14 women) amateur runners. INTERVENTIONS: In stage 1 (S1), 29.3 km and 37.9 km in stage 2 (S2) and 27.8 km in stage 3 (S3). MAIN OUTCOME MEASURES: Prestage and poststage individual changes in serum osmolality (Sosm), serum sodium s[Na], plasma volume (PV), urine osmolality (Uosm), urine specific gravity (Usg), and BM. RESULTS: Consistently, mild environmental conditions were experienced on the 3 days of the race (ambient temperature range, 11.5-22.8°C). Mean Sosm increased by 5 ± 6, 7 ± 9, and 3 ± 4 mOsm/kg during S1, S2, and S3, respectively, and returned to baseline pre-S2 and pre-S3. The correlation between individual prestage and poststage changes in Sosm, Uosm, and Usg (n = 60) were nonsignificant (P > 0.05; r = 0.0047, r = 0.0074). There was a significant, but relatively low correlation between changes in Sosm and percentage reduction in BM (r = 0.35; P < 0.01) and prechange and postchange in s[Na] (r = 0.45; P < 0.001). CONCLUSIONS: Sosm values confirm appropriate interstage rehydration. Changes in Uosm, Usg, BM, s[Na], and PV are not closely related to changes in Sosm as markers of hydration assessment in multiday events in which single static measures of hydration status are required. These measures of hydration station status are therefore not recommended in this field setting.
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