Earlier in my career, I was a Personal and Group Fitness coach - so I can say from professional experience: athlete hydration is complicated...sports nutrition and hydration in general is fairly complicated. But, we can boil it down to something simple: the wet bulb.
If you and I go for an outdoor run or jog, we're going to sweat. Your body is constantly trying to maintain homeostasis - e.g., it wants to remain at your body temperature set point regulated by your brain. When you, me, or an Olympic athlete works out...our muscles contract, our heart rate increases: overall, your cells are using more energy, which produces more heat. Sweating is the way humans cool down. The heat from your skin heats up the sweat, which then evaporates.
It takes energy to turn water into steam, that heat energy comes from your skin. So - as sweat evaporates, you cool down. The harder we workout, the more heat we produce, and the more we sweat as our body tries to keep our temperature in check.
But...let's talk about humidity...something we don't deal with a lot here. It's hard to get water to evaporate into wet air, and easy to get it to evaporate in dry air. Think of a sponge. It's easy for a dry sponge to pull in water. It's hard for a wet sponge to pull in water! It's already got lots of water in it and it can only hold so much. The atmosphere works the same way. This is the process of evaporational cooling...in a nutshell. Water evaporates into the air, which cools it. Wetter air can't cool as much as dry air. If you have a swamp cooler you know this: it works well on a dry day, not as well on a wet day.
The ability of evaporating water to cool the air, is measured by a number called the wet bulb temperature. It combines humidity and temperature into a single value. Specifically, it's the temperature air would reach, by evaporating as much water as it could possibly hold into it (putting as much water into the sponge as it could hold and not a drop more). Since evaporative cooling is how humans cool down - the wet bulb temperature is a more useful measure of heat stress and hydration need, than normal temperature. Meteorologist Alex O' Brien has a story on the Wet Bulb Globe Temperature, which combines wet-bulb temperature with other factors to classify heat danger levels.
As the wet-bulb temperature goes up, we sweat more doing the same exercise at the same intensity. That's because sweating becomes less efficient. Because cooling is all about evaporation...all that water drenching your skin and shirt isn't actually cooling you down. It's the sweat you don't see that does that job. When the wet bulb goes up, less water is going into the air and more of it stays on you so your body increases sweat output to try and fix it.
So - how much does an Olympic athlete need to drink? Well - it depends on the athlete and the sport. It's actually quite individualized and based on many individual genetic factors. Also, because we lose salts (called electrolytes) when we sweat, athletes will often consume sports drinks to replenish those rather than water, during activity. Otherwise, electrolyte balance can be thrown off. Let's look specifically at endurance running.
General large-scale studies that have been done with large groups of people have calculated hydration needs based on how much weight is lost through sweating during sustained exercise at different wet bulb temperatures. If the wet bulb is below around 81 degrees, 3/4ths of a quart per hour is the average replacement need of an electrolyte containing drink for an elite runner. If it's above that, a full quart is needed. Above 86 degrees, high intensity endurance exercise can become dangerous and emergency rule changes can be made to competition - as occurred on Tuesday for Men's Tennis.
Heading into the weekend, Paris will cool down a bit with mainly cloudy conditions dropping them to the upper 70s to low 80s. By next Tuesday,, they'll warm up and heat will become more of a concern again.
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