How much fluid do you need?

24. June 2020

The fluid balance in sports is a constant source of discussion – the schools of thought are in dispute.

Before the start of marathon races in the 1990s, the final request that repeatedly resounded from the loudspeakers was to make sure that you drink enough. Today, the specialists have more differentiated opinions when it comes to fluid intake recommendations.

Emil Zatopek and Abebe Bikila won the marathon at the 1952 and 1960 Olympic Games. Both are said to have not drunk anything during the race. Only at the beginning of the 1970s was there a radical change in fluid intake recommendations: it changed from «nothing at all» to «drink as much as you sweat» to «drink before you get thirsty» to «drink as much as you can tolerate».

1985 saw one of the many turning points in sports physiology. While the fear of heat exhaustion still prevailed during endurance events, there was another main reason for hospital admissions at the 90-km Comrades Marathon in South Africa: water intoxication! In the same year, 20 percent of the finishers at the Ironman in Hawaii received the same diagnosis.

The term water intoxication primarily serves as a deterrent. This is because thin blood resulting from an excessive fluid intake is in most cases unproblematic in terms of health. Only in rare extreme cases and presumably with the appropriate disposition can it lead to death. On the other hand, there are no performance-related benefits if you drink too much – on the contrary!

Water intoxication due to hyponatraemia

Despite the description of water intoxication and the underlying hyponatraemia (= low level of sodium in the blood; indicator of thin blood), the majority of specialists didn’t change their minds for a long time. The fluid intake recommendations were not moderated and reached their peak in 1996. At that time, the internationally recognised American College of Sports Medicine recommended drinking 0.6 to 1.2 litres per hour or as much as could be tolerated. Only with time did more and more specialists start to question these recommendations.

Two schools of thought emerged. The new school propagates moderate recommendations. The main focus is to avoid hyponatraemia and drink according to thirst. The classical school still focuses on heat exhaustion and doubts that drinking according to thirst is sufficient. And while the classical school insists a maximum sweat loss of 1–2 percent of body mass does not impair performance, the same value for the new school is 4 percent, which is significantly higher.

Practical tips

In fact, the maximum tolerable sweat loss of 1–2 percent seems to be set somewhat low. Top marathon runners with times of around 2:10 hours lose 9 percent of body mass on average and don’t collapse. However, they are extremely well trained and can tolerate a much higher degree of dehydration than most. For good, average endurance athletes, the value at which there will still be no drop in performance is probably about 4 percent.

With the hydration calculator on the website of the Swiss Sports Nutrition Society (www.ssns.ch), it is possible to determine the fluid intakes that lead to different levels of dehydration. You can thus see at a glance the sensible fluid intake required for your individual needs. If you don't like calculating and prefer to use fixed recommendations, you can work with 0.4 to 0.8 litres of fluid per hour. In the cold or with a generally low sweat loss, you will more likely start at 0.4 litres, in the heat or with a high sweat loss, more likely 0.8 litres. Drinking large quantities of more than one litre of fluid per hour should be treated with caution, as the risk of drinking too much increases rapidly.