Effect of an exercise-heat acclimation program on body fluid regulatory responses to dehydration in older men

We examined if an exercise-heat acclimation program improves body fluid reg ulatory function in older subjects, as has been reported in younger subject s. Nine older (Old; 70 +/- 3 yr) and six younger (Young; 25 +/- 3 yr) male subjects participated in the study. Body fluid regulatory responses to an a cute thermal dehydration challenge were examined before and after the B-day acclimation session. Acute dehydration was produced by intermittent light exercise [4 bouts of 20-min exercise at 40% peak rate of oxygen consumption (VO2peak) Separated by 10 min rest] in the heat (36 degrees C; 40% relativ e humidity) followed by 30 min of recovery without fluid intake at 25 degre es C. During the 2-h rehydration period the subjects drank a carbohydrate-e lectrolyte solution ad libitum. In the preacclimation test, the Old lost si milar to 0.8 kg during dehydration and recovered 31 +/- 4% of that loss dur ing rehydration, whereas the Young lost similar to 1.2 kg and recovered 56 +/- 8% (P < 0.05, Young vs. Old). During the B-day heat acclimation period all subjects performed the same exercise-heat exposure as in the dehydratio n period. Exercise-heat acclimation increased plasma volume by similar to 5 % (P < 0.05) in Young subjects but not in Old. The body fluid loss during d ehydration in the postacclimation test was similar to that in the preacclim ation in Young and Old. The fractional recovery of lost fluid volume during rehydration increased in Young (by 80 +/- 9%; P < 0.05) but not in Old (by only 34 +/- 5%; NS). The improved recovery from dehydration in Young was m ainly due to increased fluid intake with a small increase in the fluid rete ntion fraction. The greater involuntary dehydration (greater fluid deficit) in Old was accompanied by reduced plasma vasopressin and aldosterone conce ntrations, renin activity, and subjective thirst rating (P < 0.05,Young vs. Old). Thus older people have reduced ability to facilitate body fluid regu latory function by exercise-heat acclimation, which might be involved in at tenuation of the acclimation-induced increase in body fluid volume.