When a beverage is isotonic, it is absorbed straight into the bloodstream without the need for digestion. For that to happen, it must have the right osmolality, which is a measure of density expressed in units of measurement known as osmoles—or milliosmoles per 100g of solvent (mosm/kg). The more particles a sports drink contains (such as carbohydrate, electrolytes, amino acids, antioxidants, protein or flavoring), the higher its osmolality.
Controlling the osmolality of a beverage is key to its effectiveness. Optimal absorption occurs within the body when a drink’s solution is equal in osmolality to one’s natural osmolality level.
For example, your gut and plasma have an osmolality level of 250-300mosm/kg. When you exceed this osmolality level, you divert water, energy and blood to the digestive system and away from the working muscles. This diversion limits the body’s ability to absorb water, thus effecting hydration and performance output.
For optimal performance, you want working muscles to be hydrated and the gut to be free of excess water. In addition, properly hydrated muscles will help prevent gastric distress when the body is under stress from high performance output.