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.

Dr. Gabe Mirkin has been a radio talk show host for 25 years and practicing physician for more than 40 years; he is board certified in four specialties, including sports medicine. Read or listen to hundreds of his fitness and health reports at http://www.DrMirkin.com