I am a chronic cramper when it comes to endurance sports. I am currently training for a marathon. I went to a nutrition clinic for the MTT teams the other night in hope of finding some info on the cramping. Here is the link to the presentation. The info should be useful to cyclists as well.

http://www.marathontrainingteam.com/...ition-2014.pdf

In relation to the OP's question is how much sodium is lost during exercise. I never imagined it was that much. I started adding lots of salt(as recommended) to the fluids taken on SAG breaks at Saturday's training run. It definitely made a difference. The person doing the presentation recommended the sea salt packets from Wendys.