Isotonic

Isotonic solutions have a similar concentration of salts and sugars to body fluids, resulting in no net movement of water across cell membranes.

Balanced Concentration

An isotonic beverage contains a concentration of non‑permeating solutes close to that of human blood plasma, typically around 280–300 mOsm/kg. In practice this means about 6–8% carbohydrate combined with sodium, potassium and other minerals. Because its effective concentration mirrors that of body fluids, an isotonic drink is absorbed steadily without causing water to move into or out of cells. This balance helps maintain blood volume and supports endurance performance. Most commercial sports drinks fall into this category; they provide 13–19 grams of sugar per 250‑millilitre serving along with electrolytes to replace what is lost through sweat and to fuel working muscles.

Steady‑Energy Hydration

Isotonic drinks are designed for sustained exercise lasting an hour or more. Examples include pre‑mixed sports beverages formulated for team sports, carbohydrate‑electrolyte powders mixed at the recommended strength, and ready‑to‑drink pouches aimed at runners or cyclists. Homemade options can be made by combining fruit juice, water and a small pinch of salt to achieve a moderate carbohydrate concentration. These drinks supply both fluid and readily available energy, making them suitable for training, races, or rehydrating after workouts. Athletes should avoid diluting isotonic drinks too much, which would make them hypotonic, or concentrating them, which would make them hypertonic.

By matching the osmotic strength of body fluids, isotonic beverages strike a balance between rapid hydration and energy delivery.

Related Terms: Hypotonic, Hypertonic, Tonicity, Osmolality, Osmolarity