Fluid osmolarity of sports drinks

Fluid osmolarity is one of the most critical factors determining the rate of absorption of fluid. In endurance event, especially in hot conditions, the balance between rehydration and carbohydrate supplementation influences the performance and the risks of life (dehydration, hyperthermia, heat stroke and so on). Previous studies suggested that the concentration of carbohydrate influenced the absorption of fluid and that 3-7% carbohydrate was better than lower and higher concentrations of carbohydrate drink in the absorption of fluid. However, it was not yet determined how osmolarity or the concentration of carbohydrate affects the drinkability for individuals during prolonged endurance exercise. We studied the drinkability based on fluid osmolarity in sports drinks during prolonged submaximal exercise in a hot environment and found that subjects drunk hypotonic fluid 167% as much as pure water during a 90 min cycling exercise at 60% V_{O2 max} in the heat. Also, we examined the effects of more acceptable hypotonic fluid on physiological responses during 90 min cycling exercise. As a result, hypotonic fluid prevented the decrease in maximal voluntary contraction of isometric knee extension after prolonged exercise as well as the rate of carbohydrate oxidation, known as a major factor of fatigue during prolonged exercise. Based on these findings, it might be able to recommend hypotonic fluid as the sports drink used in endurance event in hot environments, taking into account of rehydration and carbohydrate supplementation. In this manuscript, we introduce the importance of osmolarity in sports drink used during endurance exercise.