I was a Personal and Group Fitness coach early in my career – so I can say from professional experience that athlete hydration is complicated…sports nutrition and hydration in general are complicated. However, we can boil it down to one simple thing: the wet bulb.

If you and I go for a run or a run outside, we will sweat. Your body is constantly trying to maintain homeostasis – for example, it wants to stay at the body temperature set point regulated by your brain. When you, me or an Olympic athlete exercise… our muscles contract, our heart rate increases: overall, your cells use more energy, which produces more heat. Sweating is how humans cool down. The heat from your skin warms the sweat, which then evaporates.

KOAA

It takes energy to turn water into vapor, and this heat energy comes from your skin. So – as sweat evaporates, you cool down. The more we exercise, the more heat we produce, and the more we sweat as our bodies try to control our temperature.

But…let’s talk about humidity…something we don’t really care about here. It’s hard for water to evaporate into wet air, and it’s easy to evaporate into dry air. Think of a sponge. A dry sponge absorbs water easily. A wet sponge absorbs water easily! It already has a lot of water in it, and it can only hold so much. The atmosphere works the same way. This is evaporative cooling…in short. Water evaporates into the air, which cools it. Wetter air doesn’t cool it as much as drier air. If you have a swamp cooler, you know this: it works well on a dry day, not so well on a wet day.

The ability of water evaporation 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 is the temperature that the air can reach by evaporating as much water as it can hold (putting as much water as it can hold in a sponge and not a drop too much). Since evaporative cooling is how humans cool down, the wet bulb temperature is a more useful measure of heat stress and water needs than the normal temperature. Meteorologist Alex O’Brien wrote a story about the Wet Bulb Globe Temperature, which combines the wet bulb temperature with other factors to classify heat danger levels.

As the wet bulb temperature increases, we sweat more doing the same exercise at the same intensity. This is because sweating becomes less effective. Because cooling is all about evaporation… all that water soaking your skin and shirt isn’t actually cooling you down. It’s the sweat you don’t see that does the work. When the wet bulb rises, less water enters the air and more stays on you, so your body increases sweat output to compensate.

So – how much should an Olympian drink? Well – it depends on the athlete and the sport. It’s actually quite personal and depends on many individual genetic factors. Also, since we lose salts (called electrolytes) when we sweat, athletes often consume sports drinks to replace these instead of water during activity. Otherwise, electrolyte balance can be disrupted. Let’s look specifically at endurance running.

Large-scale studies with large groups of people have generally calculated hydration needs based on how much weight is lost through sweating during exercise sustained at different wet-bulb temperatures. If the wet-bulb temperature is below about 81 degrees, 3/4 quart per hour is the average replacement requirement of an electrolyte-containing beverage for an elite runner. Above that, a full quart is required. Above 86 degrees, high-intensity endurance exercise can become dangerous and emergency rule changes may be made to the competition, as was the case in Men’s Tennis on Tuesday.

Paris will cool down a bit by the weekend with mostly cloudy conditions dropping into the upper 70s and lower 80s. By next Tuesday, they will warm up and temperatures will be more of a concern again.

____

Have a question or story idea you’d like the First Alert 5 Weather team to consider? Email: [email protected]

Watch KOAA News5 anytime, anywhere with our free streaming app available for your Roku, FireTV, AppleTV and Android TV. Just search for KOAA News5, download it and start watching.