As everyone knows it’s hot outside and you need to stay hydrated, especially while running, biking and even swimming. The body cools itself by sweating and if you don’t know how this works, here you go...The maximum amount of sweat a person can produce is about 1 litre per hour. Sweat glands release water and salt in an attempt to cool the body by removing excess heat through heat vaporization. This heat energy increases the speed of the water molecules so that they can escape into the air. Typically, all of the sweat does not evaporate, but rather runs off your skin. In addition, not all heat energy produced by the body is lost through sweat. A major factor that influences the rate of evaporation is the relative humidity of the air around you. If the air is humid, then it already has water vapor in it, probably near saturation, and cannot take any more. Therefore, sweat does not evaporate and cool your body as efficiently as when the air is dry.
Another factor effecting the body during exercise is your heart rate. The body produces sweat when it starts to heat up from exercise. Blood is sent to the skin which increases your heart rate since your heart must work to pump the blood to the skin, muscles and brain. When it is hot out more blood will be sent to your skin than normal for sweating. Your heart will work harder to do this and will lead to a higher heart rate than normal. This means less blood is going to your muscles and to your brain, which is why you'll feel weak and dizzy or disoriented if you get too hot.
The whole thing gets worse if you don't drink enough water. Your body loses a lot of moisture through sweating, and if you don't have enough water coming in to replace that lost water, you'll get dehydrated. Once dehydrated, your body has less fluid available to produce more sweat, your body will get hotter still, and the whole thing escalates.
When you become deydrated your body will stop sweating and may lead to heat exhaustion. You skin might feel cool to the touch and you could become nauseated, have a headache or feel dizzy. If this happens, stop running, get into the shade and find some water.
It’s difficult to say how much water one needs while running in the heat since everyone is different. It’s a good idea to drink more than normal and try to exercise early in the morning or late at night. Don’t drink too much water or you’ll run into Hyponatremia which is the body being oversaturated and not having enough salt based on a water/salt ratio.
I have a little experience with dehydration while training. I weigh about 250 lbs and was running 18 miles last summer. I started at 6am and during the last 5 miles the heat index must have been over 100 degrees before 9am. I felt relatively fine but noticed my arms were completly dry and sweat had stopped dripping of the brim of my hat. I still had 3 miles to go. As soon as I noticed I stopped running and found a water fountain and filled up my camelbak. (I normally run with a camelbak on any run over over 6 or 8 miles depending on the temperature.) That day I had filled the 50-60oz reservior (still can’t figure out how much it actually holds) 2 times and had almost finished it when I stopped sweating. This was on top of the 25-30oz of gatorade I drank at the club’s water stop. I had been running for a little over 2.5 hours at this point, drank 120 ounces of water/gatorade and still stopped sweating. I managed to make it back to the start without any problems but, I think it took me until later that night to become properly hydrated again.
The best way to judge how much water your body loses is to weigh yourself before and after running. If you can keep track of how much water you drink you’ll be able to figure out how much water you’re losing during running. I don’t think it’s necessary to try drinking enough water during the run so that you weigh the same since you can recover the water in the hours following the run.
So, if you weighed 200 pounds before the run and weigh 197 pounds after your run and drank 32 ounces of water along the run, your body probably lost 5 pounds (80 ounces) of water during the run.
It’s a little more difficult to tell how much you’re actually sweating while biking since you generally have a nice breeze.
Sweating while swimming is probably overlooked by most people since it’s difficult to tell you’re sweating. Yeah, the water is generally cooler than your body temperature but, most pools (the ones I’ve been to) are kept at 82-84 degrees which can feel hot after 30-45 minutes of swimming. One of my friends swam at an outdoor pool for a master’s swim here in DC last week and the water temperature was around 90 degrees. He said people were getting out of the water, between sets, to cool down and even hugging metal poles since the poles were cool.