Perspectives on enhancing human performance in the heat: Is the solution to simply "just add water"?

Human exercise performance is influenced by factors related to inherent individual characteristics along with other modifiable factors. During exercise in the heat, sweating provides the major avenue for cooling. When body water losses exceed 2% body mass, changes in physiological responses are observed in a dose-response manner. Human sweat varies in electrolyte content due to differences in ion channel re-absorption in the sweat duct. Moderate hypohydration (> 2% body mass) is associated with physical and mental performance impairments, although this depends on the environment (e.g., warm to hot) and type of exercise (e.g., endurance) or cognitive task (e.g., sustained attention, executive function, motor coordination) involved. This begs the question: Is simply adding water the optimal "solution" to improving human performance during events eliciting dehydration? This review focuses on literature applicable to athletes and military personnel during exertion in the heat. Historically, optimally formulating a sports drink to ingest during exercise has focused on appropriate levels of carbohydrate, with more recent interest spanning from higher electrolyte concentrations to amino acid formulations. Evidence to support recommendations regarding beverage bioavailability during exercise comes from studies comparing the appearance of heavy water (tagged within a beverage) in blood. Fluid delivery appears enhanced with moderately concentrated carbohydrate while electrolyte composition plays a lesser role. Despite the robust historical scientific literature related to fluid replacement, the quest for the optimal sports drink during exercise in the heat continues to generate interest considering global warming trends and the increasing numbers of new hydration-related products for exercising individuals.