Originally Posted by CarolynC

The problem with magnesium oxide is that it has a low solubility in water. (This follows the gen chem solubility rule that "All oxides are insoluble EXCEPT those of calcium, barium and Alkali metal (Group I) cations.") Therefore, because the tablet doesn't dissolve well parts of it can pass though the body unused, which gives it a low bioavailability. The smaller the tablet, the more magnesium oxide will dissolve and be usable by the body. If the magnesium oxide is "in solution" in a coated capsule, then it's already dissolved and should have the same bioavailability as any other form of magnesium; for example, the Liquid Magnesium softgels made by Carlson.

Also, since magnesium oxide is a base, it will be most soluble in acid. A person with low levels of stomach acid won't dissolve magnesium oxide as well as a person with normal levels of stomach acid.