I know it is because they are trying to create equilibrium with salt (they sweat and release salt and by drinking sports beverages they gain back some of the sodium that they lost), but what sorts of molecules do these ingredients supply?

You pretty much know what I do… hah.
But I know it is something about their electrolites.
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They are trying to rehydrate them, and I believe if they are low in sodium they die off.. and you need them.

But, that is all that I know! I just saw tha tnobody had answered this, so I thought I would share what little knowledge I have. :]