Most of the sodium we consume is in the form of sodium chloride, or the common table salt found in food and drinks. We take salt for granted these days as we've developed ways to make it widely available. Everyone loses a different amount of sodium in their sweat.
Up to a certain point, taking in plain water is enough to mitigate sweat losses. But, as those losses start to mount up, you need to replace sodium too to avoid your blood becoming diluted. This is a potentially disastrous condition called hyponatremia, which can certainly ruin your race and, tragically, has even been fatal on occasion.
For optimal sports performance, especially in hot environments, it is important to drink regularly. Beverages that contain sodium may help to rehydrate faster than those without, and added carbohydrates may ward off muscle fatigue, so we have to prefer isotonic drinks near sport.
Dott.ssa Annalisa Faè R&D ProAction