My wife and I learned about electrolyte depletion back in '63 when we started jogging here on the Gulf Coast. In our case, it was mostly the loss of Potassium and Magnesium. There's a lot more to the issue than just salt (Sodium Chloride). From the hyperlink above:

"A deficiency in calcium, for example, can lead to achy joints, heart palpitations, nervousness, and hypertension. A deficiency in potassium will manifest itself in muscular fatigue, diminished reflex function, fluctuations in heartbeat, headaches, and edema. The signs of magnesium deficiency include insomnia, chronic fatigue, poor digestion (to the point where the stomach will shut down), and irritability. A lack of manganese can result in excess perspiration, rapid pulse, and hypertension. During an endurance event and in particular an ultra marathon type of event, these problems become more realistic. Making sure your electrolyte needs are met will help you avoid not only cramping, but a host of other potential disasters."

"There is no sports drink in existence that provides electrolytes in substantial amounts."

We have been mixing Gookinaid Hydralyte in our water for close to 40 years. With a pinch of salt added (to meet the Chris Carmichael criteria), it's a complete electrolyte replacement. I use it at a little more than half the recommended strength based on experience with it and the fact that the gel I use has some electrolytes as well.

http://www.gookinaid.com/

Al