Effects of hydration status during heat acclimation on plasma volume and performance.

Abstract

The impact of hydration status was investigated during a 5-day heat acclimation (HA) training protocol vs mild/cool control conditions on plasma volume (PV) and performance (20km time-trial [TT]). Sub-elite athletes were allocated to one of two heat training groups (90min/day): (a) dehydrated to ~2% body weight (BW) loss in heat (35°C; DEH; n=14); (b) euhydrated heat (35°C; EUH; n=10), where training was isothermally clamped to 38.5°C core temperature (Tc ). A euhydrated mild control group (22°C; CON; n=9) was later added, with training clamped to the same relative heart rate (~75% HRmax ) as elicited during DEH and EUH; thus all groups experienced the same internal training stress (%HRmax ). Five-day total thermal load was 30% greater (P<0.001) in DEH and EUH vs CON. There were significant differences in the average percentage of maximal work rate (%Wmax ) across all groups (DEH: 24±6%; EUH: 34±9%; CON: 48±8%Wmax ) during training required to elicit the same %HRmax (77±4% HRmax ). There were no significant differences pre-to post-HA between groups for PV (DEH: +1.7±10.1%; EUH: +4.8±10.2%; CON: +5.2±4.0%), but there was a significant pooled group PV increase, as well as a 97% likely pooled improvement in TT performance (DEH: -1.8±2.8%; EUH: -1.9±2.1%, CON; -1.8±2.8%; P=0.136). Due to a lack of between-group differences for PV and TT, but pooled group increases in PV and 97% likely group increase in TT performance, over 5days of intense training at the same average relative cardiac load suggests that overall training stress may also impact significant adaptations beyond heat and hydration stress.