"Isotonic" has been a quality that certain sports drinks have used to advertise themselves for decades. What is it all about?

It's all about the number of dissolved particles in a drink. The concentration of a drink is measured in units of osmolarity - mosmoles per liter - or osmolality - mosmoles per kilogram. The value indicates how many particles are dissolved per kilogram or per liter. It does not matter how large a particle is. Isotonic now means that the quantity of dissolved particles corresponds approximately to that of blood.

Isotonic drinks, i.e. those that have the same concentration as blood, are said to be absorbed more quickly by the body, which is an advantage during sport. Is this their key quality?

Basically, you can say that pure water passes through the stomach the fastest. The more dissolved particles there are in the water, the slower the liquid passes through the stomach and the slower it is generally absorbed into the body. Up to a roughly isotonic concentration, the effects are small and probably not very relevant in practice. However, hypotonic drinks, i.e. drinks with an osmolarity of less than 260 mosmol per liter, are effectively absorbed somewhat faster than isotonic drinks. Hypertonic drinks are then increasingly and relevantly absorbed more slowly and the likelihood of tolerance problems increases, especially if the osmolarity rises above 400.

What concentration is considered isotonic and what range may be advertised as isotonic by law?

According to the law, drinks may be described as isotonic if they have an osmolarity of 260 to 300 mosmol per liter, which is in the range of the blood concentration, whereby this legal tolerance seems reasonable and also irrelevant from a biological point of view.

And this brings us to the initial question, is beer isotonic?

No. Normal beer is far from isotonic. This is mainly due to the alcohol, which drives the osmolarity extremely high. The osmolarity of beer is around 1000 mosmol per liter. However, drinks containing alcohol are not primarily not recommended because of the high osmolarity, but because of the alcohol content itself, which has a negative effect on the performance and recovery of athletes. There's nothing to sugarcoat.

What about non-alcoholic beer?

When the alcohol is removed, beer is naturally roughly in the isotonic range.

Many sports drinks, even those that claim to be isotonic, are highly sweetened. How healthy and useful are they for athletes?

The key question is whether or not there is a high carbohydrate requirement. If I was primarily sweating, for example because it was hot, but I didn't train that intensively, then a high intake of liquid carbohydrates is probably neither necessary nor sensible. However, if the carbohydrate and energy requirement is very high due to the high sporting performance, then sugary drinks, which sports drinks de facto are, can be very valuable and sensible sources of energy.

Which is healthier after sport: a non-alcoholic beer (0.0%) or a sweet sports drink?

Non-alcoholic beer is a drink that provides water and carbohydrates, but less than an average sports drink. Non-alcoholic beer can be considered a natural product with no additives. It can also be a tasty alternative to the sports drink that may already have been consumed during exercise. If someone likes to drink non-alcoholic beer, then it can help to absorb the fluid and carbohydrates needed after sport. It all depends on what the objective is and what your individual preferences are.

What's wrong with treating yourself to a sugary drink or sweetened food after an exhausting endurance event?

If my energy stores are empty and I need a lot of energy for recovery, then liquid carbohydrate sources such as drinks are not only helpful, but in this situation they are also not harmful to your health in any way. Ultimately, sugary drinks are quickly available sources of carbohydrates that can also be consumed in large quantities and are relatively well tolerated. For an inactive person, these properties are probably not really useful. For an athlete who is under a lot of strain and has completely depleted energy stores, however, they certainly do.

Samuel Mettler is a sports scientist and doctor of sports nutrition. He runs courses in sports nutrition at Bern University of Applied Sciences. He has competed in athletics at a competitive level.