Tonicity

Tonicity refers to the effective concentration of a solution in relation to a semipermeable membrane, reflecting how water will move across the membrane.

What Tonicity Means

Tonicity is often described as the effective osmolality of a solution because it takes into account both the concentration of dissolved particles and whether those particles can cross cell membranes. Unlike osmolality or osmolarity, which measure all solute particles, tonicity considers only solutes that do not freely cross the membrane. A solution isotonic to blood has a similar effective concentration of non-permeating solutes and therefore does not cause net water movement into or out of cells. Hypotonic solutions contain a lower concentration of non-permeating solutes, causing water to enter cells and potentially leading to swelling, while hypertonic solutions have a higher concentration and draw water out of cells, causing them to shrink. Because cell membranes are selectively permeable, two iso-osmolal solutions can differ in tonicity; an iso-osmolal urea solution is hypotonic because urea crosses membranes, whereas an iso-osmolal sodium chloride solution is isotonic because sodium and chloride do not.

Implications for Sports Hydration

Tonicity influences how a sports drink interacts with the gastrointestinal tract and bloodstream. Drinks formulated to be isotonic—usually containing around 6–8% carbohydrate and a salt concentration similar to blood—do not cause significant fluid shifts across the intestinal lining, supporting steady absorption and preventing gastrointestinal upset. Hypotonic drinks have a lower concentration of non-permeating solutes and are absorbed even more quickly; they are useful for rapid rehydration in hot conditions or for athletes with high sweat losses. Hypertonic beverages like concentrated juices or gels have a high effective solute concentration; they draw water into the gut and slow absorption, making them more suitable for delivering calories than for hydration unless consumed with additional water. Understanding tonicity helps athletes choose drinks that match the demands of their activity.

Selecting beverages with appropriate tonicity ensures that fluids and nutrients are absorbed efficiently without causing cell swelling or dehydration.

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