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"when exercise is of high-intensity, longer duration (≥ 1 h), or performed in a hot environment, fluid replacement with carbohydrate and electrolytes at a rate sufficient to prevent > 2% hypohydration can be advantageous (discussed in detail in competitive exercise section above)."

The recommended amount of sodium and potassium to be included in a fluid-replacement beverage during exercise (as communicated by The American College of Sports Medicine and the Institute of Medicine) is ∼20 to 30 and ∼2 to 5 mmol/L, respectively (238, 414). This is primarily because the presence of ∼20 to 30 mmol/L sodium in a fluid-replacement beverage has been shown to stimulate physiological thirst and improve beverage palatability and voluntary fluid intake (351, 414, 478, 512). In addition, ingestion of ∼20 to 30 mmol/L of sodium and ∼2 to 5 mmol/L of potassium from a fluid-replacement beverage helps replace sweat electrolyte losses incurred during exercise (414). Sodium intake may be especially important when sweat sodium losses are large. Some types of exercise-associated muscle cramping may be associated with large sweat sodium losses; and cramp-prone athletes can benefit from sodium intake to prevent or offset muscle cramping (45-47, 149, 470). Furthermore, a sodium-containing beverage helps slow the decline in blood sodium concentration during prolonged exercise, with higher concentrations being more effective at offsetting losses compared to plain water (14, 33, 324, 490, 500). Chloride is typically the primary anion included in sports drinks, since it is the anion lost in the greatest amount in sweat. In addition, a small amount of potassium (∼3 mmol/L) is usually included to replace sweat potassium losses. Despite the common perception that magnesium and/or potassium are implicated in the etiology of exercise-associated muscle cramping, there are little to no experimental data to support this notion

http://onlinelibrary.wiley.com/doi/10.1002/cphy.c130014/full#c130014-sec-0007