HUMAN BODY-FLUID DISTRIBUTION DURING EXERCISE IN HOT, TEMPERATE AND COOL ENVIRONMENTS

Using a simultaneous-dilution technique, we investigated body-fluid vo lume changes during exercise in seven males, during 50 min of cycling (50% maximal work rate) in hot (36.2 degrees C), temperate (22.0 degre es C) and cool conditions (14.4 degrees C). Total body water (TBW), ex tracellular fluid (ECFV), plasma (PV) and erythrocyte volumes (RCV) we re measured, while blood volume (BV), interstitial fluid volume (IFV), extracellular water (ECW) and intracellular water volumes (ICW) were derived. During the initial 10 min of cycling, BV decreased in all env ironments (P = 0.01), primarily because of a PV reduction (P = 0.01), while IFV, ECFV and ICW were not significantly changed. By 30 min, BV recovered in the temperate and cool conditions, despite mass losses of 563 and 520 mt (respectively), but remained depleted in the hot condi tion (P = 0.01). The 50-min volume changes revealed that, throughout e xercise, body-fluid losses appeared to be drawn primarily from the ext racellular space, regardless of air temperature. In the hot condition, the PV change represented 63% of the TBW loss, with the ICW contribut ing 23%. it was concluded that, during cycling, progressive dehydratio n mainly affected the extracellular space, with the intravascular and intracellular spaces being defended in less stressful conditions.