The benefits of proper hydration for human performance and recovery has long been established.  Replacing fluid loss from metabolism, evaporation, and sweating can have a dramatic impact on performance in both team and endurance sports.  As our workouts progress in duration and intensity our requirements for nutrition beyond water alone increase.  Unfortunately, trying to train at a high intensity and drink sufficient fluids can be difficult in many endurance sports.  In fact, prior research shows when athletes are allowed to drink at self selected pace they often become dehydrated up to 3% of their body weight.  

A recent study in the journal Medicine and Science in Sports and Exercise compared a self selected hydration group against a prescribed hydration group as participants performed a cycling trial in the heat (Bardis et al. 2017).  10 elite male cyclists accustomed to riding in the heat performed 3 trials of criterium simulated conditions up to 30km.  The cyclists completed these rides under two conditions, one self selected drinking condition and one requiring them to match fluid losses every km.  The authors found improved performance over the 30 km criterium trial during the prescribed drinking condition secondary to lower dehydration and heat strain, as well as, improved sweating responses.  Specifically, the self selected drinkers had 3 times the amount of dehydration as the prescribed condition.