When I was a kid, I heard the saying "Pigs sweat, men perspire and women glisten." This went right along with "Sweat like a pig" which always seemed to prompt someone to mention that pigs don't sweat. Turns out that the pigs in question aren't the animal, but pig iron. Or maybe not, the source is a bit muddy. Maybe the phrase should be updated to say, "Athletes sweat, others glisten."
Sweating/perspiring is natural and our bodies way of regulating our temperature. When we exercise, our core temperature increases. This increase triggers our brain to remove the excess heat to keep our organs from getting too hot. To do this, our blood vessels expand near the skin so we can radiate heat. We also begin to sweat to start the process of the sweat evaporating and cooling us off in a secondary manner. All of this is how our body thermoregulates.
Sweat is about 99% water with the remainder made up of the electrolytes sodium, calcium, potassium, magnesium and tiny amounts of other trace minerals. This is why Gatorade contains sodium, potassium and carbohydrates to give you energy and replace the minerals lost during exercise. It's also why there are bananas available at the end of races. The percentage of sweat that isn't water varies based on the availability of these minerals in your body and your fitness level.
I was noticing this year during my running that I seemed to be sweating more than I did last summer. I thought it might be faded memory or a difference in hair length, but then I read that as fitness increases, our bodies will sweat more quickly during activity. The percentage of sodium in the sweat decreases as our fitness increases, too. This helps athletes to have more endurance because they're not going to overheat, or run out of electrolytes.
I think many people think that out of shape people will start sweating sooner during activities than someone who is in shape. Turns out that may be the opposite of the reality. It also means that even really athletic people need to be aware of sweating, dehydration and replacing electrolytes. We should all make sure to hydrate properly every day and especially during exercise.