- Isonatremic (isotonic) dehydration occurs when the lost fluid is similar in sodium concentration to the blood. Sodium and water losses are of the same relative magnitude in both the intravascular and extravascular fluid compartments.
- Hyponatremic (hypotonic) dehydration occurs when the lost fluid contains more sodium than the blood (loss of hypertonic fluid). Relatively more sodium than water is lost. Because the serum sodium is low, intravascular water shifts to the extravascular space, exaggerating intravascular volume depletion for a given amount of total body water loss.
- Hypernatremic (hypertonic) dehydration occurs when the lost fluid contains less sodium than the blood (loss of hypotonic fluid). Relatively less sodium than water is lost. Because the serum sodium is high, extravascular water shifts to the intravascular space, minimizing intravascular volume depletion for a given amount of total body water loss.
1) Weigh yourself nude right before a run.
2) Run at race pace for one hour, keeping track of how much you drink (in ounces) during the run.
3) After the run, strip down, towel off any sweat, and weigh yourself nude again.
4) Subtract your weight from your prerun weight and convert to ounces. Then add to that number however many ounces of liquid you consumed on your run. (For example, if you lost a pound and drank 16 ounces of fluid, your total fluid loss is 32 ounces.)
5) To determine how much you should be drinking about every 15 minutes, divide your hourly fluid loss by 4 (in the above example it would be 8 ounces).
6) Because the test only determines your sweat losses for the environmental conditions you run in that day, you should retest on another day when conditions are different to see how your sweat rate is affected. You should also redo the test during different seasons, in different environments (such as higher or lower altitudes), and as you become faster, since pace also affects your sweat rate.