Hyponatremia is from excessive water intake..not from lack of supplemental sodium..

this is interesting

http://bjsm.bmj.com/cgi/content/abstract/40/3/255

Great. How much is 'excessive,' then? Is what's 'excessive' for me the same as what's 'excessive' for you?


That is interesting. There is another paper linked at the bottom that cites it - I can only see the abstract, but that looks interesting too:

http://bjsm.bmj.com/cgi/content/extract/41/2/111

Looks like it's turning out to be a complicated issue, not as simple as 'too much water.' The story that Dodger cited says the same. Maybe dietary sodium has to be considered.
A bit of salt in one's electrolyte drink is hardly a high-risk proposition, and until we know more, it might help prevent at least headaches after a long hot run. So it's worth considering.

I'll stick to replacing some salt, rather than just plain water. Given how gritty and covered in crusty salt I am after a long run in the heat I have no qualms about replacing salt and potassium. Right now I am using an Amphipod Full-Tilt belt pack for my long runs. That has a 20oz. bottle that lies horizontally in my lumbar area. I fill that with Gatorade, usually diluted to half strength. On the front of the belt I have two smaller Amphipod bottles (10.5oz ea.) with plain water. Ran for 2 hours yesterday in 80º temps and fairly high humidity and about drained all 3 bottles. Felt pretty well for the duration of the run, though near the end I was getting a little chilled from overheating. That's pretty par for the course for me.