There are other electrolytes besides sodium, and those are more likely to be the problem. Generally, you'll only have problems with hyponatremia if you drink very large amounts of plain water after sweating.
So if you've been sweating a huge amount, substitute something else for *part* of the plain water. Fruit juices are generally high in potassium and some other minerals. Chocolate milk is the classic post workout drink because it has small amounts of all sorts of minerals, plus a little sugar. If you don't want the sugar, plain lowfat milk would have most of the same effect.
And I personally have had really good experiences with coconut water to stave off dehydration headache or cramps. The first time I had it, I was on a jungle hike (and I was NOT fit) when a friend noticed I didn't look good, so he grabbed a coconut off a tree and hacked it open for me! But the stuff in a can seems to do the trick just as well without the risk of losing fingers. Just read the label to make sure it's just coconut water (sometimes called green coconut juice) without a bunch of added sugar.
If you do really need to add sodium, then iodized table salt kills two birds with one stone. Iodine is important for thyroid function and some other things I can't remember right now, and most people don't get enough. Adding iodine to table salt was probably the biggest public health victory in history, or maybe second to polio vaccination, but now that people are trying to lower their sodium, we're starting to see a re-emergence of the old problems. It might even be a minor factor in the "obesity epidemic." A tiny sprinkle of regular old table salt a few times a week won't hurt you if you're not worried about excess sodium, and it might help.
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