Losing just 2% of your body weight in fluids can have a negative impact on performance. Realizing the importance of hydration for competitive athletes, a study published in the International Journal of Sport Nutrition and Exercise Metabolism put plain water up against coconut water to see which worked best.
Ten male volunteers in their 20s cycled on a stationary bike for an hour drinking 250 ml of water or coconut water every 15 minutes. Then they competed in a 10 km time trial. They could drink as much as they wanted during the last 5 minutes of the sub-maximal ride and the time trial.
There weren’t any significant differences between rate of perceived exertion, feeling of fullness, rating of thirst or time trial performance. The only real difference was subjects assigned coconut water drank less compared to the plain water group.