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, to be 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.

Plasma osmolarity, which is about 300 mOsm/L, is used as a reference for the osmolarity of beverages. Thus, three types of beverages are distinguished:

• The isotonic drink whose osmolarity will be between 270 and 330 mOsm/L, approaching that of plasma.
• Hypotonic drink , which has a lower osmolarity than plasma, between 150-200 mosm/L
• The hypertonic drink, on the other hand, 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 composition very rich 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) to the plasma, and on the other hand because the electrolytes contained in the most concentrated medium (plasma) will go to the less concentrated medium, until the two environments are balanced.

For a beverage to have the same osmolarity as plasma, it must have enough molecules exerting an osmotic pressure on the membranes equivalent to that of plasma. Plasma osmolarity is calculated by taking into account the natremia, the kalemia, the glycemia and the uremia, that is to say the rate of sodium, potassium, glucose and urea because these are the majority osmotically active molecules 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 of cells. Thanks to their composition of sodium, glucose, potassium and chloride, you simply dilute 2 tablets in 500mL of water for an isotonic drink or 1 tablet in 250ml for a hypotonic drink.
[ plasma osmolarity ]

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

To understand it, we must imagine a membrane with, on one hand, a hypertonic liquid and on the other hand the plasma. The hypertonic medium is more concentrated than the plasma, this will result in a movement of electrolytes to restore a state of equilibrium. 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 least concentrated medium (plasma) to the most concentrated medium (hypertonic liquid) until equilibrium is reached, thus 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 interest 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 at once. But the benefit of an isotonic drink goes beyond simply compensating 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 hydration level!

Indeed, osmolarity has an impact on the movement of water and osmotically active molecules on either side of a membrane. Let's imagine a membrane with plasma on one side and an isotonic liquid on the other. The osmotic pressure exerted on both sides 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 precisely, moreover, if dehydration occurs during exercise, the plasma will become hypertonic rather than hypotonic because water losses will be greater than electrolyte losses.

In your body, the liquid you drink ends up in your intestine where absorption and exchanges with the plasma take 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 efficient as with an isotonic drink where the two environments are balanced.
[ osmotic movement ]

The cotransport mechanism of isotonic drinks

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

• A so-called passive diffusion, that is to say that the molecule passes by itself through the membrane, according to a concentration gradient
• The second mode of absorption is active, this means that the molecule requires a carrier 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 into 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: this 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 to the inside of the cell. Then the sodium exits the other side of my cell by active transport and passively diffuses 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 combination of glucose and electrolytes, also ensuring a nutritional intake necessary for your body.
You can, however, choose a hypotonic drink if you are doing long-term physical activity in very hot weather when the priority is to compensate for water loss rather than sugar loss.

Hydratis allows you to concoct your sports drinks simply wherever you are, thanks to hermetic tubes that can be transported everywhere. Rediscover the pleasure of drinking a flavored isotonic or hypotonic drink to stay in shape and improve your performance while staying well hydrated.

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