What is an isotonic drink?

Perhaps you have already heard of so-called exercise, sports or energy drinks, all of these names refer to the “ isotonic ” drink.

A drink is isotonic when its osmolarity is close to that of blood plasma.

Osmolarity is expressed in mOsm/L, read milliosmole per liter of solute, corresponds to the number of osmotically active particles ( electrolytes ), which exert pressure on a membrane, in a solution. There is also osmolality, which corresponds to the same thing expressed in mOsm/kg, only the units change.

We use the osmolarity of plasma, which is approximately 300 mOsm/L, as a reference for the osmolarity of drinks. Thus, we distinguish three types of drinks:

• The isotonic drink whose osmolarity will be between 270 and 330 mOsm/L, approaching that of plasma.
• The hypotonic drink , which has an osmolarity lower than plasma, between 150-200 mosm/L
• The hypertonic drink, for its part, has an osmolarity higher than plasma

 

What do hypotonic, isotonic and hypertonic drinks contain?

Hypotonic drinks are low in solutes, these are drinks such as tea or mineral water for example. They contain electrolytes (sodium, bicarbonates, potassium, etc.) in a lower proportion than blood plasma. This type of drink is particularly suitable for maintaining hydration for a long time. Indeed, thanks to its very rich composition in water compared to plasma, the absorption of water will be favored over that of electrolytes, on the one hand because the water will go from the most diluted medium (hypotonic drink) towards the plasma , on the other hand because the electrolytes contained in the most concentrated medium (plasma), will go towards the less concentrated medium until the equilibrium between the two environments is reached.

For a drink to have the same osmolarity as plasma, it must have enough molecules exerting osmotic pressure on membranes equivalent to that of plasma. We calculate the osmolarity of plasma by taking into account serum sodium, serum potassium, blood sugar and uremia, that is to say the level of sodium, potassium, glucose and urea because these are the molecules osmotically majority active substances in plasma although electrolytes such as chloride or bicarbonates also participate.

The isotonic drink therefore preferably contains sodium, potassium and glucose, diluted in a sufficient quantity of water. Hydratis is a solution specifically dedicated to body hydration. It allows you to reconstitute an isotonic or hypotonic drink easily and designed to not only hydrate effectively but also facilitate recovery and fight against oxidative stress in cells. Thanks to their composition of sodium, glucose, potassium and chloride, you just need to dilute 2 tablets in 500mL of water for an isotonic drink or 1 tablet in 250ml for a hypotonic drink.
[ plasma osmolarity ]

Hypertonic drinks, for their part, are particularly rich in electrolytes and sugars since they contain higher quantities of sugar and mineral salts than plasma like sodas. These drinks should be consumed when you are looking to provide nutrients and energy to your body as a priority and when you are not at risk of dehydration. Indeed, hypertonic drinks will compensate very well for sugar losses but can worsen water losses.

To understand it, we must imagine a membrane with, on the one hand, a hypertonic liquid and on the other hand plasma. The hypertonic medium is more concentrated than plasma, this will result in a movement of electrolytes to restore a state of balance. Here, the electrolytes will go from the most concentrated medium (hypertonic liquid) to the plasma. Associated with this movement, there will also be a leak of water from the less concentrated medium (plasma) towards the more concentrated medium (hypertonic liquid) until equilibrium is reached, therefore leading to a loss of water and occasionally digestive discomfort. This is why you should not consume a hypertonic drink if you want to hydrate yourself since the effect will be the opposite!

Why drink an isotonic drink?

The benefit of isotonic drinks during exercise is multiple. During an intensive sports session such as a long race (marathon, trail, etc.) your body loses water and electrolytes.

An isotonic drink allows you to compensate for these losses since you provide your body with water, electrolytes and carbohydrates in one go. But the benefit of an isotonic drink goes beyond simple compensation for losses since the combination of electrolytes, carbohydrates and water with an osmolarity similar to blood plasma maximizes their absorption by the body and therefore your level. hydration!

Indeed, osmolarity impacts the movement of water and osmotically active molecules across a membrane. Let's imagine a membrane with plasma on one side and an isotonic liquid on the other. The osmotic pressure exerted on either side of this membrane is equivalent, the electrolyte concentrations are the same, the exchanges around this membrane will therefore be balanced. The isotonic drink restores electrolytes and water losses continuously to avoid dehydration, moreover, if dehydration occurs during exercise, the plasma will become hypertonic rather than hypotonic because the water losses will be greater than the electrolyte losses. .

In your body, the liquid you drink ends up in your intestine where absorption and exchange with plasma takes place. If you drink a hypertonic liquid, the exchanges between the external environment (intestine) and the blood plasma will be unbalanced, the absorption of water will not be as effective as with an isotonic drink where the two environments are balanced.
[ osmotic movement ]

The cotransport mechanism of isotonic drinks

To fully understand the mechanism of isotonic drinks, it is necessary to understand the mechanism of liquid cotransport. There are several modes of movement of molecules classified into two main categories:

• A so-called passive diffusion, that is to say that the molecule passes on its own through the membrane, according to a concentration gradient
• The second mode of absorption is active, this means that the molecule requires a transporter and energy to pass the membrane, of which the cotransport mechanism is a part.

Among the components of the isotonic drink, water will pass the intestinal membrane towards the blood plasma passively, by diffusion. Electrolytes can pass the intestinal membrane passively, actively or both. This is particularly the case for sodium which, initially, needs glucose to be absorbed by the intestinal cell: it is a cotransport mechanism.

Sodium and glucose pass through a transporter that requires the presence of both molecules to transfer them from the intestinal lumen into the cell. Then the sodium exits the other side of my cell by active transport and diffuses passively into the plasma. Sodium helps maintain osmolarity and therefore helps facilitate the absorption of water and electrolytes.

[ glucose sodium cotransport ]

When to drink isotonic drinks?

Due to its properties, the isotonic drink is the ideal ally for hydration during exercise, allowing you to stay hydrated throughout your sports session, by compensating for your water and electrolyte losses quickly and optimally thanks to the The combination of glucose and electrolytes also ensures the nutritional supply your body needs.
You can nevertheless choose a hypotonic drink if you do long-term physical activity and in high heat when the priority is to compensate for water losses rather than sugar losses.

Hydratis allows you to easily prepare your exercise drinks wherever you are, thanks to airtight tubes that can be transported anywhere. Rediscover the pleasure of drinking an isotonic or hypotonic flavored drink to keep you energized and improve your performance by staying well hydrated.

Find more details during our interview on Europe 1 as well as our products on the Hydratis Shop .