Also, don't take calcium supplements without magnesium. It's important for bone health that your intake of calcium and magnesium be in balance.

Personally, if my leg cramps are any indication, I sweat out more magnesium than calcium. I find it hard to believe that the electrolytes in your sweat depend on the sport you're doing, though. I think it's just that cyclists tend to spend a lot more time at their sport than other athletes, and thus sweat out more electrolytes total. (Imagine if it was commonplace for running clubs to not even bother with less than a 2.5-hour run, e.g.) Potassium is plentiful in food and as for sodium, you can just grab the salt shaker. Not so easy to grab the calcium-magnesium shaker...