1. Introduction
dehydration, mass loss, weight loss, heat illness
DEHYDRATION, HYPOTONIC, RETENTION OF FLUID, FLUID RETENTION
The effects of varying sodium (Na) and carbohydrate (CHO) in oral rehydration solutions (ORS) and sports drinks (SD) for rehydration following exercise are unclear. We compared an ORS and SD for the percent of fluid retained (%FR) following exercise-induced dehydration and hypothesized a more complete rehydration for the ORS (45 mmol Na/L and 2.5% CHO) and that the %FR for the ORS and SD (18 mmol Na/L and 6% CHO) would exceed the water placebo (W). A placebo-controlled, randomized, double-blind clinical trial was conducted. To induce 2.6% body mass loss (BML, Inadequate fluid intake during sports participation and training can lead to dehydration. Persistent dehydration at or beyond 2% of body mass negatively impacts performance during subsequent efforts and increases the risks of heat illness [Rehydration is a function of gastric emptying, intestinal fluid absorption, and retention of fluid to restore body fluid compartments. A comprehensive measure of these processes is the percentage of fluid retained after a defined recovery period following ingestion of a specified volume of fluid [The ingredients in a sports drink that promote rehydration include sodium (Na) and carbohydrate (CHO) with sodium being the primary factor [Carbohydrate content might also promote fluid retention during rehydration by prolonging gastric emptying and intestinal absorption [The effect of varying CHO content in rehydration beverages having 6% or less CHO such as in sports drinks and oral rehydration solutions (ORS) is equivocal. Osterberg et al. found no difference in post-exercise rehydration fluid retention (~75% of the ingested volume) for a 3% vs. 6% CHO [The question that remains is whether variations in CHO concentrations of between 2 and 6% enhance rehydration when Na content also varies in the beverage. A low-CHO, hypotonic beverage, i.e., ~2%, with a Na concentration above 40 mmol/L might promote equal or greater fluid retention compared to a sports drink with 6% CHO and only 10–20 mmol/L of Na. Dumke’s lab examined this and reported no difference in fluid retention for an ORS (3.4% CHO and 60.9 mmol Na/L) or sports drink (6% CHO, 18 mmol Na/L) [The purpose of the present study was to compare beverages that varied in both Na and CHO content within the range found in sports drinks for rehydration properties following exercise-induced dehydration in male athletes. The main outcome variable used to define completeness of rehydration was the percentage of fluid retained during a 3.5 h period following beverage ingestion. To explore the effect of inversely varying Na and CHO, two commercially available and commonly used rehydration beverages were administered in volumes that replaced 100% of the acute body weight loss. A water placebo was compared to an ORS containing 2.5% CHO and 45 mmol/L Na and a standard sports drink containing 6% CHO and 18 mmol/L Na. We hypothesized that the higher Na, lower CHO beverage would promote the greatest rehydration.