Runners can lose significant quantities of sodium through sweat. The higher the sweat rate the higher the sodium concentration. This is especially a problem for runners who restrict their salt intake. While there is little direct evidence about what happens when we start to become sodium deficient, it appears that there are two stages.
- Sodium deficient dehydration. If there is insufficient sodium, the body will compensate by reducing the blood volume (hypovolemia) so that the blood sodium concentration remains at the correct level. Thirst will be high, but drinking would just produce urine output without hydration. Often there will be a craving for salt or salty foods.
- Hyponatremia. Under some conditions sodium deficiency can produce Hyponatremia, which is where the blood sodium concentration is too low. There are other causes of Hyponatremia, the main one being excessive drinking.