Sports drinks – isotonic or hypotonic? What and how much?


An ideal sports drink should provide fluids and carbohydrates as quickly as possible to support the sporting performance. A book on sports nutrition dating back to 1939 already pointed out that such a drink should have a similar composition to blood. Thus, the concept of isotonic sports drinks was born.

Up until the beginning of the modern sports drinks era in the 1970s and 1980s, isotonic drinks were considered ideal for sports. Accordingly, the "isotonic" attribute was well used in marketing. But what exactly is an isotonic drink? And are isotonic sports drinks really ideal for sports?

The term isotonic originally comes from biology. Various substances float around in the cells and blood of all living creatures. In humans, the best-known example is blood sugar. The simple number of these substances is called osmolality. If, for example, the blood that flows straight through the muscles has the same osmolality as the surrounding muscle cells, the blood and muscle cells are isotonic to each other (“iso” means same). If there were less substances swimming around in the cells, the cells would then be hypotonic relative to the blood (“hypo” means under, below). 

The terms isotonic or hypotonic thus always describes the ratio between two neighbouring fluids. In the case of drinks, it always describes the ratio to blood. Therefore, an isotonic drink has the same number of substances floating around as in blood.

Why isotonic?

Isotonic drinks are supposedly absorbed faster in the small intestine than other drinks and therefore get into the bloodstream faster. Accordingly, the muscles benefit earlier from the substances contained in such a drink: primarily water and carbohydrates. The question is, however, are isotonic sports drinks really absorbed faster? 

The absorption speed of drinks has been directly measured in many studies. As early as 1999, these concluded that hypotonic drinks effectuate a faster absorption of water in the small intestine than isotonic drinks. To put it in concrete terms: a drink with an osmolality of 200-250 mmol/kg is absorbed faster in the small intestine than an isotonic drink (isotonic as in blood would have a value of around 280 mmol/kg). Hypertonic drinks starting from around 300 mmol/kg are absorbed even slower.

Thus, hypotonic and not isotonic drinks are ideal for sports. This knowledge has also been incorporated into food legislation since 1 May 2017: drinks with less than 260 mmol/l are allowed to be described as a hypotonic. Even if hypotonic drinks are therefore ideal, the difference between isotonic drinks is not huge. So, those who are not absolute perfectionists and don’t need to achieve maximum performance will also do well on isotonic drinks.

What and how much?

Sport drinks make a lot of sense for intensive sporting performances that last longer than approx. 45 to 60 min and where the focus is on performance. When it comes to shorter exertions, water is sufficient. And whereas “drinking absolutely nothing” was recommended up until the 1960s, in sports today the basic recommendation is about 0.4 to 0.8 l/h. In warm situations where there is a higher sweat loss, you can easily consume the upper limit and maybe even drink a little bit more. But not much more. Otherwise you could end up drinking more than you sweat during the physical exertion. In endurance sports, however, you shouldn’t lose more than 4 % of your body weight in sweat. Otherwise you will get tired faster and the exertion will be perceived as more strenuous. In other types of sports, the border lies at around 2 % of body weight. 

In endurance sports, drinking according to thirst also works really well. Current recommendations thus state that drinking according to thirst is a sensible alternative to drinking a fixed quantity at a fixed time. You can test this during your training to see which method suits you best. 

A sports drink not only needs to contain water but also carbohydrates – plus a pinch of salt after around 2 hours of physical exertion. The ideal drink contains a mixture of a source of glucose (dextrose, maltodextrin), and fructose (fruit sugar, household sugar). While the perfect ratio has not yet been determined, a bit more glucose than fructose seems to make sense. 30 to 60 g of carbohydrates per hour are a good amount, after 3 or 4 hours this can be increased to 90 g. You do, however, need to practice consuming such high quantities, or else you might end up with gastrointestinal problems. Each new combination of water and carbohydrates (also via gel) needs to be tried out during your training. This is because the tolerability can vary from person to person. As such, blindly copying a top athlete doesn’t always make sense. The people at the top didn’t just simply emulate others but also tried it out for themselves.

Summed up for use in practice

  • Sports drinks with carbohydrates make sense for intensive physical exertions of 45-60 minutes and more
  • The target quantity of carbohydrates is then 30 to 60 g/h
  • Hypotonic sport drinks are absorbed somewhat more rapidly than isotonic sports drinks
  • Drinking according to your thirst makes sense in endurance sports, alternatively you can apply the 0.4-0.8 l/h recommendation
  • Losing up to approximately 4 % of your body weight in sweat during endurance sports will have a negligible impact on your performance