Physical performance and heat tolerance after chronic water loading and heat acclimation.

Heat acclimation (HA) and forced water intake (FWI) have both been found to improve the endurance of human subjects working in hot environments. Therefore, we studied the interaction between HA and FWI. Prior to any treatment (control, AI and BI) the subjects (n = 9) underwent a heat tolerance (HT) test. Thereafter, they were divided into two groups. The first (n = 5) were heat-acclimated (AII), underwent a second HT test, doubled their normal daily water intake for 1 week (AIII), and underwent a third HT test; the second group (n = 4) were subjected to the same protocol, except that the FWI came before and during HA (BII). It was found that both regimens (phases AII and BII) significantly increased work duration. Although the results of the two methods were similar, their combination somewhat lengthened work tolerance time (phases AIII, BIII). Maximal oxygen uptake did not change after HA (BII) or FWI (AII), but the maximal values were attained at significantly lower heart rates, both after BII alone or combined with HA (BIII). In an additional experiment, the time needed to "ride" 15 km on a bicycle ergometer was reduced by 10% after FWI as compared to control time.

Improved thermoregulation caused by forced water intake in human desert dwellers.

Residents of the Negev desert in Israel sustain a mild state of dehydration. Low, concentrated urine outputs, high incidence of kidney diseases and high hematocrit ratios characterize this population. Educational programs to increase the awareness of the population to the dangers of dehydration have undoubtedly failed. It was our purpose to see whether forced increased drinking will affect the above variables. Ten healthy subjects were asked to double their normal voluntary water intake without (phase II) and with salt supplements (50 mM NaCl, 20 mM KCl) (phase III), for one week. After phases II and III significant increases in body masses, decreased concentrations of serum proteins, hemoglobin, hematocrit ratios and serum osmolalities were found. No significant changes were found in the concentrations of sodium and potassium in the serum. At the end of each phase, the subjects were asked to exercise on a bicycle ergometer for 60 min at 50% VO2max in a heated chamber at 45 degrees C, and 30%-50% relative humidity. Experiments were terminated if and when heart-rates exceeded 180 bpm or the rectal temperature increased to 39 degrees C. After both experimental phases, subjects increased their tolerance to heat, extending the exercise periods by 25% and 30%. Compared with their starting levels, hematocrit ratios, serum proteins and hemoglobin concentrations increased in phases II and III while no changes were recorded in the control period (phase I). It is suggested that spontaneous voluntary water drinking in desert dwellers is not enough to achieve a true state of "euhydration".