I tried Tums (calcium carbonate) and it works for me. I tried it when the muscles got tight and then I sensed that a cramp is starting. Its the Ultra Strength 1,000mg version per tablet. I chewed 2 of those and drank some of my drink. Of course, I stopped and waited for a couple of minutes just to make sure some of it gets absorbed.

I was able to complete the 53 miler with no muscle tightness. Sport Legs has calcium but also lactate.

http://www.sportlegs.com/what/what.asp