Just take the electrolytes supplements you have been using in training. More Mg on race day won't help and might hurt. Sodium is by FAR the most important exercise related electrolyte.
As for muscle relaxation and bowel movements Mg does play a role in both of these things but Machka's explanation of these effects is wrong. Serum Mg levels are tightly regulated as you would die if they went signicantly out of range.
It takes about a month straight of daily Mg supplementation to correct a Mg deficiency as whole body stores of Mg are huge. You are not going to "run out" of Mg during your ride. If you actually are Mg deficient you should take a Mg supplement everyday from now till then, but not race day.
As for Mg relaxing the bowels leading to the runs, that is crap
. The reason high dose Mg acts as a laxative is because it causes more water to be retained in the gut - the reason for this is, as above, the body won't allow serum Mg levels to go out of range. What occurs is that the body dilutes the Mg in the gut both from fluids you drink and from your body's water stores in order to get the Mg level into the safe range before absorption of the solution. If the Mg dose is too large for dilution and absorption you will take a runny dump and be done with it.
Last edited by Enthalpic; 01-24-08 at 10:31 PM.