Hypotonic, isotonic and hypertonic sports drinks: which to choose and when to use them?

Many athletes ask themselves which type of sports drink to choose – hypotonic, isotonic or hypertonic. These drinks differ in osmolality, that is, the concentration of dissolved particles (carbohydrates and sodium) in the solution, which is compared with the osmolality of blood plasma (roughly 280–300 mOsm/kg). It is precisely this parameter that determines the direction in which water moves across the intestinal wall and the rate at which the stomach empties – and so decides how quickly the body absorbs both fluid and energy.

1. How is water absorbed in the human body and what does hydration mean?

99 % of water absorption takes place in the small intestine, with the largest part (roughly 72 %) absorbed right at its beginning – in the duodenum, and further in the jejunum. This movement is entirely passive, based on the simple physical laws of osmosis:

Hydration of the body expresses a state of dynamic balance between fluid intake and loss. During intense exercise in the heat the body can lose an enormous amount of water through sweat and become dehydrated.

2. How are carbohydrates and sodium absorbed and what does that have to do with the types of drink?

In the small intestine, carbohydrates are not absorbed by free diffusion like water, but require specific transport proteins (transporters) in the membrane of the intestinal cells. Glucose (and galactose) is carried across the intestinal wall by the active transporter SGLT1, whose function is one hundred per cent dependent on sodium binding and being transported at the same time. Fructose, by contrast, is absorbed by a different route – facilitated diffusion through the transporter GLUT5, which does not depend on sodium. Once these simple sugars get inside the cell, they pass together into the bloodstream across the basolateral membrane using the transporter GLUT2.

The absorption of sodium takes place both passively and actively. The active transport of sodium from the lumen of the intestine into the blood is driven by the sodium-potassium pump (Na+/K+ ATPase) on the basolateral membrane of the cell, which constantly pumps sodium out of the cell and so keeps the sodium concentration inside low. Crucially, a substantial part of the sodium is absorbed in close connection with glucose, precisely through the shared transporter SGLT1.

The total number of particles (osmolality) as the key to the movement of water

Every free sodium ion, glucose molecule or fructose molecule behaves in the intestine as one separate, osmotically active particle. Water in the human body has no active transporter and passes through the intestinal wall entirely passively by osmosis – which means that it always automatically follows the direction with the higher number of dissolved particles (higher osmolality), in order to even out the concentration on both sides.

3. What is osmolality and does it matter?

Osmolality expresses the concentration of dissolved particles (solutes) in one kilogram of water (mOsm/kg) and is defined exclusively by the total number of separately dissolved particles (ions and molecules) in the solution, not by their size or weight.. In practice it is often confused with osmolarity, which gives the number of particles per litre of solution, while the principle remains the same.

For athletes the osmolality of a drink can be an important parameter, because it directly determines the rate at which the stomach empties and the direction in which water moves in the intestine.

What happens when the forces are matched — the isotonic state?

When you drink an isotonic drink, the osmolality in the intestine is essentially the same as the osmolality in the blood (around 290 mOsm/L). At the start, therefore, the force on both sides of the rope is matched. Neither side forcibly pulls water towards itself purely on the basis of the initial osmotic pressure.

At this moment, however, the active SGLT1 transporters come into play. These transporters start actively taking glucose and sodium from the intestine and pumping them through the wall into the intestinal cells and then into the blood. This active transport does something crucial: the transporters move particles from the intestine into the blood → this immediately creates a new local excess of particles on the blood side → water responds immediately to this new excess and starts to be absorbed smoothly after them (solvent drag).

An isotonic solution is a perfectly balanced compromise. Because it is osmotically matched with the blood at the start, the digestive tract does not have to fight the water in any way, the stomach does not have to hold things up by diluting, and as soon as the active transport of sugar and sodium starts, the water literally “hitches a ride" with them straight into the bloodstream. You thus get both fast hydration and an optimal dose of energy for the muscles without the risk of water starting to move in the opposite direction.

4. Hypotonic, isotonic and hypertonic drinks

According to the value of their osmolality relative to blood plasma, we divide sports drinks into three basic categories:

5. Why can I drink more carbohydrates with maltodextrin than with glucose?

The osmolality of a solution is determined exclusively by the number of dissolved molecules (particles) in a given volume, not by their physical weight.

6. Why is it good to wash a gel down with water instead of a sports drink?

Energy gels are essentially extremely concentrated carbohydrate solutions; at best they are designed as isotonic (maltodextrin), at worst hypertonic (glucose).

7. How can the findings above be used in practice?

In practice what matters most is the athlete's particular hydration and nutrition strategy. In general it is recommended to stay with isotonic or hypotonic osmolality for the reasons we mentioned in the introduction to the article. Even though hypotonic drinks are the most effective in terms of hydration (Rowlands et al., 2022), in the context of a long race where we rely, for example, on energy gels, everything gets mixed in the stomach anyway. That is why it is a good idea always to wash energy gels or bars down with plain water, so that we bring the tonicity down to the lowest possible level.

If we take, for example, riders in the Tour de France, they mostly use various sources of carbohydrate (isotonic drinks, gels, chews, bars and so on). What matters is that the ratio of carbohydrates is always the same in every product, e.g. 2:1, the so-called “mix and match", and at the same time that in trying to secure better hydration through a hypertonic drink they do not exceed their planned carbohydrate intake per hour.

Practical recommendations according to the length of the effort:

A practical guide: how to prepare a drink from hyve Energy Drink

Each serving of hyve Energy Drink contains 30 g of carbohydrates (20 g maltodextrin + 10 g fructose, a 2:1 ratio) and 200 mg of sodium. Simply diluting it in your water bottle lets you set the osmolality precisely according to the conditions and the length of the effort:

What should a good sports drink look like during exercise?

On the basis of the scientific recommendations for hydration and topping up energy during endurance exercise, the ideal sports drink should meet the following criteria:

Conclusion for athletes

Choosing the right sports drink has a direct impact on performance. The key parameter is osmolality, but even more important is the carbohydrate concentration: it decides how quickly the stomach empties and whether water travels to the muscles or, on the contrary, from the blood into the intestine.

The science is clear on this: the fastest hydration is provided by a concentration of around 3–4 % carbohydrate — this weak solution hydrates the body faster than plain water, because it activates the SGLT1 transporters, which actively draw water in (Jeukendrup, 2009). The standard isotonic 6% concentration is a good compromise for efforts where you also need to top up energy. Once you go above 8 %, water absorption slows down and with a sensitive stomach or in extreme heat you risk digestive problems.

Always wash gels down with plain water, never with an isotonic drink. Use different sources of carbohydrate in a "mix and match" way so that you do not go above the total carbohydrate intake per hour you have trained for (e.g. if you have trained for 60 g of carbohydrates/hour and use two gels, you should not take any further carbohydrates from a drink and should rely on water alone for hydration.

And finally — even the best drink will not help you if your intestines are not used to a higher carbohydrate intake. Gut training is therefore just as important a part of preparing for a race as the training itself.

1. Rowlands DS, Kopetschny BH, Badenhorst CE. (2022). The Hydrating Effects of Hypertonic, Isotonic and Hypotonic Sports Drinks and Waters on Central Hydration During Continuous Exercise: A Systematic Meta-Analysis and Perspective. Sports Med, 52(2):349–375.
2. Jeukendrup A, Gleeson M. (2025). Sport nutrition (4th ed.). Human Kinetics.
3. Jeukendrup, A.E., Currell, K., Clarke, J. et al. Effect of beverage glucose and sodium content on fluid delivery. Nutr Metab (Lond) 6, 9 (2009). https://doi.org/10.1186/1743-7075-6-9