It is not commonly know that all forms of magnesium are equal in the world. There are a variety of different forms of magnesium on the market, and many off the shelf multivitamins are also listed as containing magnesium. The vast majority of these contain magnesium oxide. This in of itself is not a bad thing. It is of course a source of magnesium. What is less well known is that as magnesium sources go, it is a pretty poor one. It is not easily absorbed by the body.
The reason for this is that chemically, in the magnesium oxide molecule, the bonds between magnesium and oxygen are extremely strong. Even though the compound is ionic, like sodium chloride for example, the magnesium and oxygen atoms are tightly bound together in a lattice, and the bonds are much stronger than other ionic compounds. This means that the compound itself is much less soluble than comparable ionic salts. The solubility of magnesium oxide in water, is about 0.01 grams per litre of water. Thats right, not very much at all. It generally requires an acid dissolve. The general reasoning is that stomach acid is enough to dissolve it, and this is technically true. But conditions in the digestive system are not always ideal for this, which is why studies have shown that its bioavailability is much lower than other forms of magnesium.
This is where the strength of other magnesium compounds becomes apparent. Chelated forms of magnesium are far superior in this regard. There a variety of different forms of chelated magnesium, with either acids or amino acids as chelating agents. After comparing the various costs, it is apparent that magnesium citrate is the most affordable and sensible choice for a magnesium supplement. Some other forms can be considered slightly more effective, but then it becomes a question of if the extra cost is really justified. Magnesium citrate, specifically the trimagnesium form (as there are even more than one type of magnesium citrate) is the most soluble, having a solubility of 200 grams per litre! Thats 20,000 times higher than magnesium oxide. It is not hard to see why magnesium citrate could have a much higher bioavailablity, since it easily dissolves in water alone, regardless of the acid in your stomach.