The primary limiting factor in sports that require great endurance is the time it takes for your heart to pump oxygen in your bloodstream from your lungs into your muscles. A study from the University of Connecticut (Medicine and Science in Sports and Exercise, May, 2006) shows that with dehydration, your heart beats with far less force so it pumps far less blood with each beat, and is unable to bring as much oxygen to your muscles.
You can’t depend on thirst to tell you when you lack fluids. Certain brain cells called osmoreceptors tell you when you are thirty, but only after the salt concentration of your blood has risen considerably. When you exercise, you sweat. Sweat contains far more water than salt in comparison to blood. So you lose far more water than salt during exercise and blood levels of salt rise. By the time that a your blood salt concentration is high enough to trip off the osmoreceptors, you are severely dehydrated and it is too late for you to be able to drink enough during exercise to catch up with your water deficit. On the other hand, if you take salt with fluids, then your blood salt levels rise faster and tell you that you are thirsty earlier.
There are other reasons that you should take salt with fluids during prolonged exercise. First, it helps prevent muscle cramps. Remember, during exercise you lose salt and water. If you are replacing only water, you can eventually take in so much water that your salt levels drop to cause muscle cramps. Second, even though salt is a mild diuretic at rest, during exercise it helps your body to retain water. So when you are going to exercise for more than a couple hours, particularly in hot weather, drink small amounts frequently and eat salted foods such as peanuts. Always stop if you feel sick, have chills, headache, severe muscle burning or aching, dizziness, or blurred vision. Seek help if your symptoms do not subside in a few minutes; you could be headed for heat stroke that can kill you.