Some claim a dehydration of 1-2% already lowers sport performance by tens of percent. But is this a scientific fact or a marketing campaign of sport drinks? In this article I will summarize some interesting quotes of Professor Ross Tucker and Mike Finch, who recently had a chat about the simple truth about exercise and hydration in The Real Science of Sport Podcast.
An exercising body can not be compared with a body at rest. If you find someone with a heart rate above 160, a breathing frequency above 40 breaths per minute and a core temperature above 38 degrees, you better call an ambulance. Unless… that person is exercising.
90% of endurance athletes finish their events dehydrated without a problem
Professor Ross Tucker and Mike Finch state that the same is true when looking at dehydration. In the real science of sport podcast, Ross says even a 10% dehydration does not have to be a problem when exercising. 90% of endurance athletes finish their events dehydrated without a problem.
In his book Waterlogged: The serious problem of overhydration in endurance sports, author Tim Noakes states drinking before thirsty can even be seriously dangerous. That has everything to do with the osmolality (concentration) of electrolytes (salts) in the blood. Let’s take an example.
Suppose every liter of water in your body contains 10 units of electrolytes. During exercise your body temperature raises, and you start sweating to keep it cool. Sweat itself contains less (say 5) units of electrolytes per liter water. This means sweat contains relatively more water and less electrolytes than the fluid in your body. If you sweat, and therefore take out a lot of water and only a little amount of electrolytes, you’ll increase the electrolyte concentration (units per liter water) in your body. As a result, you’ll become thirsty and drink. No problem.
But what happens if you drink before you are thirsty? For instance because you heard that if your body weight is lower after exercising, you did not drink enough. Let’s find out by again looking at the example. Your body contains 10 units of electrolytes per liter of water. If you drink a sports drink that contains 2 units of electrolytes per liter water, you’ll decrease the electrolyte concentration in your body, even though the sports drink contains electrolytes. It is this concentration that is very important. Especially a decrease in electrolyte concentration in your body can result in serious problems that might get dangerous.
Since there is no sports drink out there that has the same or higher electrolyte units per liter water then your body, every sports drink will lower the electrolyte concentration in your body. Just like every kind of sweat (even the salty kind) will increase the electrolyte concentration in your body.
if you don’t need to remind your body to breath, you don’t need to remind it to drink
Professor Ross Tucker
Drinking way before you are thirsty can become a problem. Less fit people doing endurance events are at most risk. There are two reasons for this. First, less fit people don’t produce a lot of speed or power and therefore don’t increase their body temperature that much. As a result, they sweat less and need to drink less. The second reason is because less fit people are also the once who need more time to get to the finish. If you overdrink every hour, and you exercise more hours, you’ll overdrink more.
In conclusion: from a fluid standpoint, there is no reason to drink before you are thirsty. It can even cause serious damage. Every year, more athletes are (seriously) injured by drinking too much than by dehydration. Or as Ross says: if you don’t need to remind your body to breath, you don’t need to remind it to drink. Do keep in mind though that sport drinks have another purpose, which is energy supply. More about that later. 
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Founder of Molab, Human Movement Scientist and Freelance Content Marketer.