A follow up question: Are they eliminated via the kidneys in any significant amount? For years I experienced muscle cramps while engaged in distance running, alpine climbing, and backpacking. I experimented with oral rehydration salts diluted to half strength and that seemed to help, but only a little. More recently I have been using a sports drink that supplies Ca++ at 100 mg, Mg++ at 150 mg, Na+ at 270 mg, and K+ at 160 mg/32 gr serving, mixed in 12 oz of water, along with 24 gr of carbohydrate. Voila, no cramps! I've got to think it was either the Ca++ or Mg++, since I had already tried Na+ and K+ in the rehydration salts. This leads to my question of how I am losing the Ca++ and Mg++ in the first place. If not by sweating, what is the mechanism? I take a daily mineral supplement that provides 100% of the RDV for both Ca++ and Mg++, along with a diet rich in nuts, low fat dairy, and vegetables, so I doubt I would be deficient going into endurance exercise. I'd sure appreciate your, and John's view on this one.