You need a certain amount of regular salt; there's a reason it was such a valuable commodity historically.

You also probably need potassium, and the easiest way to get that is with one of the "lite" salts that replaces sodium with potassium. You may also take potassium pills, but they don't provide very much potassium per pill.

Reread what Atkins says about vitamins and make sure you're getting everything else you need--magnesium plays a role in cramps, too.