Isotonic drinks

Many sports drinks are marketed as being “isotonic”. It means that they have approximately the same osmolality as body fluid, which is between about 280 and 300 mOsm/kg. The osmolality describes to the number of particles in a solvent unit, and is of interest because it determines the effectiveness of a sport drink at delivering water for rehydration.

The physiological relevance for water uptake is illustrated in the Figure: In general, water tends to move from areas of lower osmolality to areas of higher osmolality until an equilibrium is reached. Isotonic solutions have essentially no osmotic effect on the body fluid balance, while this is different for hypotonic and hypertonic solutions. Hypotonic solutions provide excess water, as the tonicity of the solution is less than that of body fluids. Hence, water from a hypotonic solution will be absorbed quickly. Hypertonic solutions have a greater tonicity than that of body fluids and thus draw water from the surrounding into the lumen, with some adverse effects like the loss of water and electrolytes.

The osmolality of a drink increases with a higher carbohydrate or electrolyte content, or with the dissociation of carbohydrates or electrolytes. The latter plays a role where carbohydrates hydrolyse in the acidic conditions of a sport drink over time. For those drinks, formulated to have isotonic or slightly hypotonic properties, maintaining these during the shelf-life period can be a challenge.

Stable isotonic properties with isomaltulose
Drinks with isomaltulose can be expected to maintain their isotonic properties, due to the high stability of this carbohydrate. With a molar mass of 342.30 g x mol-1, isomaltulose adds to the osmolality of a drink to the same extent as sucrose, and to less extent than monosaccharide sugars like glucose or fructose. In a drink formulation, isomaltulose does not undergo significant hydrolytic changes during processing or storage. It shows a high stability in acidic beverages over the shelf-life period, and it also shows high stability in acidic conditions relevant to the stomach, making it highly likely that ingested isomaltulose reaches the small intestine as disaccharide carbohydrate. Hence, isotonic properties initially defined for a drink with isomaltulose are still relevant when the drink is finally consumed and reaching the place of absorption.

Figure: Osmolality effects on body fluids.