If the casual observer were to look at me after a workout they would wonder if I had been doused with a fire hose. Doesn't matter what time of the year (save for the coldest weeks), what distance, or what intensity. I still am soaking wet. My college cross-country coach, my wife, my athletes and my friends all marvel at what has politely been described as "a very efficient cooling system."
I used to joke, "I sweat when I think." Who would have considered that to be true? I didn't, until I started looking closer at thermoregulation. Our skeletal muscles (including those muscles used for running), liver, heart and our brain all generate body heat. On those days when we're hotter than our surroundings we can radiate heat (think about those infrared scopes used by military and police to observe in dark conditions) to maintain a proper temperature. When the air temperature is hotter than us we absorb heat. Add to the outside heat the heat we generate from our running, and thinking about running...you are thinking about running, right...?
External heat and internal heating can raise our body's core temperature to a point where our brains have to make a command decision; how long can the body go before vital organs are injured? Our brain decides to start shutting less important functions down in order to protect itself, the heart and other vital organs. That usually means skeletal muscles take the hit. Entire muscles don't get shut down entirely, mind you, but a few fibers here and there at first...which we usually recognize as breaks in running form or decreased range of motion (shortened, choppy strides). As the temperature continues to increase the brain becomes more concerned and shuts down more muscle fibers ("is that my calf cramping?") until we decide to either cool things down by slowing down, drinking cool fluid, getting into a cooler environment or stop exercising altogether.
So it's important to keep the core temperature as low as possible, either by deflecting radiated heat from the sun and other surfaces, or by drinking so you can continue to sweat.
Runners who sweat heavily, naturally, will need to take in more fluid per hour. You can determine your sweat ratio by stepping on a scale (naked) before and after a run and looking at the weight difference, adding the volume of fluid you drank during the run, then doing the mathematics to figure out the ratio per hour. Hotter, more humid days might demand more and colder days less, but you'll have a ballpark figure on which to base your hydration plan.
After the run, it's important to begin the recovery process by replacing the fluids lost. Again, the easiest way to know how much is by stepping on the scale to find the difference in pounds between when you started and when you got done. Multiply by 16 to get the number of ounces, then start replacing fluid on a one-to-one or a one-and-a-half-to one ratio.
Naturally, the next question is "what's the best drink?" Water has been the ideal beverage for many years, and works great for most athletes, but an entire industry has developed in the past four decades because of this question. Sweat not only contains water but minerals, lactate, urea and some trace elements. The most common elements found in sweat are the electrolytes; sodium (average 0.9 grams per liter), potassium (average 0.2 grams per liter), calcium (average 0.015 grams per liter) and magnesium (average 0.0013 grams per liter). These values naturally will vary from person to person.
One of the most popular sports drinks on the market provides .4-.5 grams per liter of sodium, .125 grams per liter of potassium and 200-to-220 calories of energy per liter from carbohydrates. There are sports beverages on the market which have protein, others have electrolytes but no carbohydrates, and some include other trace minerals like chromium. This ideally allows the athlete to make up for some of the lost minerals and nutrients before the workout or race is over. The best sports drink, however, is the one which the athlete will actually drink; all the beneficial elements and chemicals a laboratory can offer (whether or not they truly help!) are of little use if the flavor of drink is terrible.
So don't forget to keep that bottle of water or sports drink handy during your runs, or make certain your run routes have places where you can get (or stash) fluids. Drink early and drink often; if you wait until you feel thirsty the damage has already been done.