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".