Effects of Hypotonic and Isotonic Sports Drinks on Endurance Performance and Physiology

Abstract

Isotonic sports drinks containing carbohydrate and electrolytes are often consumed for endurance performance, but hypotonic drinks may be superior. The purpose of this study was to compare effects on performance and related physiology of a new sports drink, Mizone Rapid (hypotonic; containing carbohydrate and electrolytes) with three other drinks: Mizone (hypotonic; carbohydrate only), Powerade (isotonic; carbohydrate and electrolytes), and a water placebo. Methods: Sixteen well-trained cyclists were randomized in balanced doubleblind crossover fashion to consume each of the four drinks on separate days at a rate of 250 ml every 15 min during a 2-h steady ride at constant power, followed by a continuous incremental test to peak power. Tests were performed on a cycle ergometer at room temperature (18-22°C) and were separated by 37 d. Results and Discussion: Peak power with Mizone Rapid was substantially higher than that with water and Mizone (4.3% and 3.2% respectively) but similar to that with Powerade (0.2%) (90% confidence limits for differences, ±2.6%). Analysis of urine composition and volume after the performance test provided evidence that Mizone Rapid was excreted and therefore probably absorbed more rapidly than the other drinks: it produced both the lowest urine osmolarity and the highest urine volume. There were no clear differences between the effects of the drinks in the steady ride on tympanic temperature, heart rate, blood glucose and blood lactate that were consistent with the effects on subsequent performance. Conclusion: Consumption of a hypotonic drink aimed at optimizing fluid absorption has an effect on endurance performance similar to that of an isotonic drink and may have beneficial effects on endurance performance relative to water and a hypotonic drink lacking sodium.