The purpose of this series of studies was to (1) elucidate the role of fluid intake behavior, Na⁺ consumption, and sweat Na ⁺ loss on fluid and Na⁺ balance in older (Study 1) and young (Study 2 and 3) adults during prolonged exercise and (2) determine the effects dehydration on prolonged running performance in endurance-trained athletes (Study 2) and basketball performance (Study 4) and attention (Study 5) in basketball players.

The purpose of Study 1 was to compare the voluntary fluid intake behavior of older men and women (54-70 yr) when allowed to drink ad libitum between repeated bouts of exercise in the heat. We concluded that women drank more water per kg body mass (BM) than men, which may put smaller women at an increased risk for developing hyponatremia.

In Study 2 we compared the measured serum [Na⁺] (S _[Na⁺]) with that predicted by the Kurtz-Nguyen equation by manipulating Na⁺ intake and ΔBM during prolonged running in the heat. We concluded that when athletes maintained or lost BM, changes in S_[Na⁺] could be accurately predicted by changes in the mass balance of fluid, Na⁺, and K ⁺.

Study 3 tested the hypothesis that ΔBM accurately and reliably reflects the change in total body water (ΔTBW) after prolonged exercise in the heat. The difference between methods was 0.09 ± 1.09 (P=0.58) and the intraclass correlation coefficient was 0.76. Thus, we concluded that measuring pre- to post-exercise ΔBM was an accurate and reliable method to assess the ΔTBW.

The purpose of Study 4 was to determine the effect of -1, -2, -3, and -4% ΔBM vs. 0% ΔBM on basketball skill performance in adult male players. We concluded that basketball players experienced a progressive deterioration in performance as the BM deficit progressed from -1 to -4%. The % BM deficit at which the performance decrement reached statistical significance was -2%.

Study 5 employed the Test of Variables of Attention to investigate the effect of a BM deficit on attention in male basketball players at the end of a simulated basketball game. We concluded that vigilance-related attention was impaired by -1 through -4% BM deficits.