Here's a good explanation on Sodium from Mark Allen.
Sodium
The third and final section to work on right now is whether or not you need to supplement your fuel intake with sodium. The first question to ask about this is whether or not you ever crave something salty during a long ride? If so, you might benefit from having a source of sodium with you during your race. A key note is that if you have any history of hypertension, consult with your primary health care provider before experimenting with this.
Research has shown that after about three hours of maximum sweating, performance begins to drop off if sodium is not taken in. On average, a person loses 350mg of sodium per hour. So after you lose about 1000mg of sodium the performance begins to drop. Keep in mind that this is VERY individual. Some people will sweat out twice this and others only half. Experiment with what you need.
One sign that your sodium intake is not enough to keep up with your sodium sweat rate is that if you are drinking enough and eating enough calories but still feel like you are flat or feel like you are sort of bonking and are having trouble concentrating and are losing motivation, this is a sign that you may be low on sodium. The best source of sodium is buffered sodium tablets that you can get at any pharmacy. There are a lot of electrolyte tablets available, but most of them are in a capsule that makes it impossible for you to taste the salt when you take it. It is very important to be able to have the tablet melt in your mouth. This will enable you to keep it in until your body says “enough” at which point you can spit out any remaining part of the tablet.
Keep track of how much sodium your electrolyte drink is providing. Keep track of how you feel during your workouts, especially after about three hours. Try sucking on a buffered salt tablet and see how your body and energy levels respond. Start conservative and build up until you find the right amount for you, especially during hot and long workouts.