Effect of varying levels of hypohydration on responses during submaximal cycling

The effect of varying levels of hypohydration on hemodynamic, cardiorespiratory, and metabolic responses to progressive incremental submaximal cycling was examined in nine male subjects. Subjects cycled in a neutral (22°C) environment under euhydration (EU), moderate hypohydration (MH), and severe hypohydration (SH). To achieve the desired level of hypohydration, subjects cycled at 50% VO_2max for 1.5 h in a 38°C environment on two separate occasions, 36 h prior to testing. Mean (±SE) percent losses in body weight from baseline during EU, MH, and SH were 0.6 ± 0.3%, 3.3 ± 0.1%, and 5.6 ± 0.4%, respectively. Ventilation, O₂uptake, respiratory exchange ratio, heart rate, plasma free fatty acids, plasma glycerol, blood lactate, and hematocrit were not significantly altered by hypohydration. During EU, hemoglobin concentration was significantly lower than during both MH and SH, but no significant difference was observed for plasma volume loss. Plasma glucose was significantly higher during SH compared with EU and MH. These results suggest that hypohydration of up to 5.6% caused by exercise and fluid manipulation over 36 h does not alter cardiorespiratory or blood lactate responses during progressive incremental submaximal cycling in a neutral environment. However, hepatic metabolism may be altered during hypohydration as indicated by higher plasma glucose levels.