Markers of hydration status in a 3-day trail running event.

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 (S osm), serum sodium (s[Na]), plasma volume (PV), urine osmolality (U osm), urine specific gravity (U sg), 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 S osm 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 S osm, Uosm, and U sg (n = 60) were nonsignificant (P > 0.05; r = 0.0047, r = 0.0074). There was a significant, but relatively low correlation between changes in S osm 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

Serum osmality values confirm appropriate interstage rehydration. Changes in U osm, U sg, BM, s[Na+], and PV are not closely related to changes in S osm 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.