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

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 (20 km time‐trial [ TT ]). Sub‐elite athletes were allocated to one of two heat training groups (90 min/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 ( T c ). A euhydrated mild control group (22°C; CON ; n = 9) was later added, with training clamped to the same relative heart rate (~75% HR max ) as elicited during DEH and EUH ; thus all groups experienced the same internal training stress (% HR max ). 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 (%W max ) across all groups ( DEH : 24 ± 6%; EUH : 34 ± 9%; CON : 48 ± 8%W max ) during training required to elicit the same % HR max (77 ± 4% HR max ). 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 5 days 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.