Isotonic, hypotonic, or hypertonic drinks might sound like a dull topic, but it's essential knowledge when planning hydration and energy during training and races. Here, we clarify these concepts, along with which type of drink truly proves most beneficial in practice. The conclusion is straightforward: the critical factor isn't what the drink is called, but making sure you consume enough fluids and the correct amount of carbohydrates for the situation.

An isotonic sports drink means that the drink's carbohydrate and/or salt content matches the body's own levels, ensuring the volume of cells remains unchanged upon intake. The drink's osmolarity (amount of soluble particles) aligns with what is naturally found in our body. If the drink is hypertonic, it contains more carbohydrates or salt than the body, while a hypotonic drink contains less than the body's cells.

Quite a dry fact area—but still useful to know in certain situations.

There's some confusion about what an isotonic drink really is, so let's clarify it here. A drink can be isotonic in terms of the amount of carbohydrates. In practice, this means it contains just the right amount of dissolved carbohydrates to match the body's osmolarity. For example, a solution with the starch maltodextrin is isotonic at 15–20 percent. This means you can mix 15–20 g of maltodextrin per deciliter of water to get an isotonic solution. If you add a little extra water, the drink becomes hypotonic and leaves the stomach faster.

When it comes to carbohydrates, a hypotonic drink—meaning a drink with a low carbohydrate content—passes through the stomach faster than an isotonic one. A hypertonic drink passes through more slowly. If we add salt to an isotonic drink, it becomes hypertonic because it then has more dissolved particles than the body's cells. You would then need to add more water to make it isotonic again. So, a 6 percent solution (in this case, 60 g of sports drink powder per liter) can also be hypertonic if it contains a lot of salt.

Isotonic sports drinks are quite a broad category; many are isotonic around 6–8 percent (60–80 g per liter). But as you see in the example with maltodextrin, that number can be significantly higher, and the amount of salt also impacts this. And honestly, we don't always know if the sports drinks on the market claiming to be isotonic are so in terms of salts, carbohydrates, or overall. So we'll leave that detail there.

When talking about isotonic solutions in healthcare, it's usually the salt content that's referred to. An isotonic saline solution contains as much salt as the body's cells and doesn't pull extra water into the cells or dehydrate them—assuming the body's fluid status is balanced. Logically, an isotonic drink makes sense when you need to rehydrate: it replenishes without overfilling more than necessary.

Which one should you choose?

Here, researchers examined whether a mixture with 6 percent carbohydrates (60 g per liter) containing varying amounts of salt—making the drink hypo-, iso-, or hypertonic—affects fluid balance. The researchers found that the amount of salt in the drink did not affect the fluid balance at all in the test subjects who ran for 80 minutes on a treadmill. What influenced the fluid transport most were the carbohydrate molecules.

The same research team conducted another study the following year with similar results. They had a 6 percent carbohydrate solution with little, moderate, or a lot of salt in it—therefore hypo-, iso-, or hypertonic. The same results appeared here: the amount of salt in the drink did not have a major effect on fluid uptake. The primary factor for maintaining fluid balance during activity is the carbohydrate molecules' ability to transport fluid.

“Glucose in the infusion solution is the more important factor determining intestinal water absorption than Na+. This study suggests that adding Na+ to fluid replacement beverages may not be a factor in fluid absorption.”

There should be salt in the drink to match losses through sweat, but the amount itself is not the most important factor for the body's fluid balance. There is a very good reason why the oral rehydration solution you mix at home contains a substantial amount of sugar. It's not primarily because the dehydrated person needs energy, but because carbohydrates (the sugar in this case) transport water very effectively.

For every glucose molecule you ingest, about 2.7 water molecules are transported along with the glucose molecule through the intestinal membrane and into the cells. If you’ve ever wondered why you feel a bit bloated after eating too much candy, or why you get very thirsty from it, here’s your answer.

• Water is king when it comes to rehydration.
• Water plus salt is more effective.
• Water plus salt plus carbohydrates is the most effective when we solely focus on fluid absorption.