Glad you stressed bioavailability. For anybody who thinks they're getting magnesium in their daily vitamin pill: look at the bottle. If it's magnesium oxide, it's probably just passing on through.

I make my own magnesium 'oil' (high concentration of magnesium chloride in water - feels oily, hence the name). I started out using it topically for muscle cramps (sort of an Epsom salts on steroids), tried the 'oil' orally, but got some stomach upset. I ended up taking 2g magnesium citrate per day - the cramping stopped. For anyone else looking to do this - beware. Magnesium citrate is a very effective laxative - you have to build up gradually. And, yeah, I probably should have done the same with the chloride.

BTW/FWIW, cramps really, really suck for a distance swimmer, particularly if you don't float and hate towing a buoy.