Mild preexercise hyperhydration with electrolyte-containing beverages: effect on thirst, water intake, and physiologic function

The American College of Sports Medicine\u27s (ACSM) position stand on appropriate fluid intake for long duration exercise states that individuals should drink fluids containing sodium (0.5 - 0.7 g/l), potassium (0.08 - 0.2 g/l), and carbohydrate (5 - 10%) at a rate of 400 - 800 ml/h, depending on the individual\u27s sweat rate and body size, in order to keep body water losses \u3c 2%. The ACSM guidelines for preexercise hydration are unclear. Beverages containing significant amounts of sodium increase water intake and retention, subsequently improving fluid balance during and after exercise. However, whether sodium containing beverages ingested before exercise influence water intake and urine production during endurance exercise has not been studied. Two primary studies were conducted in order to test the efficacy of chicken noodle soup (167 mmol Na+/l) to improve fluid balance by increasing water intake and retention during exercise in thermoneutral (WBGT = 16°C) and hot, dry (WBGT = 26°C) environments. In both studies, fluid balance was improved during 90 min of moderate intensity exercise (∼55% VO2peak) 45 min after ingesting 355 ml of chicken noodle soup (CNS) compared with a similar quantity of water (WATER). The improvement in fluid balance was mainly because of an increase in ad libitum water intake that persisted throughout the 90 min of exercise. The increase in water intake was apparent in both the thermoneutral (801 +/- 415 vs. 594 +/- 391; CNS vs. WATER) and hot, dry (1434 +/- 592 vs. 1163 +/- 427; CNS vs. WATER) environments. Water retention was also improved after CNS in the thermoneutral environment, but not the hot, dry environment despite decreased calculated free water clearance in both studies. Although fluid balance was greater in both studies, no cardiovascular, temperature regulatory, or performance benefits were observed. Perceived thirst was also similar in all trials when water intake was allowed ad libitum. When water ingested at a similar rate to the CNS trial during a second water hyperhydration trial (WATER/R) in the hot, dry environment, ratings of perceived thirst decreased. Interestingly, when participants were forced to drink more water than normal in the hot, dry environment (WATER/R) without the additional preexercise sodium load (CNS), plasma sodium concentrations were consistently lower during exercise. From these studies we conclude that preexercise beverage composition can affect both dipsogenic drive and kidney function resulting in improved body water status during exercise. Also, these results indicate that it may not be necessary to replace sodium losses during exercise if anticipated losses are ingested before exercise