I don't know how good any scientific evidence is or isn't but anecdotal evidence I certainly have. When I first moved to equitorial Africa 15 years ago I had frequent problems with cramp, even when doing only moderately strenuous activities - working outside on the truck for a few hours was enough to bring it on. Someone gave me salt tablets to take and I stopped having problems. I soon gave up on the tablets and just added more salt to my food. I've never had cramp since - until 18 miles into the Wadsworth Trog last month. Not sure why it suddenly occured then except that since coming back to the UK last year I've reduced my salt intake (on the basis I don't sweat as much when running here as I do simply sitting at my desk in Gabon, but maybe I've overdone it).