Mineral supplements are mostly salts - consisting of the cation (usually what you're interested in) and an anion partner. Cations are positively charged; anions are negatively charged. You only care about the cation, and not the anion, so the anion essentially becomes a filler. Consider Magnesium Orotate. It has a molecular weight of 334.482. Magnesium has a molecular weight of 24.31. So the % Mg in Magnesium orotate is just 7%. So every 100 mg of magnesium orotate has just 7 mg of useful Mg!

Consider Magnesium oxide now. Molecular weight of 40.30, so 60% Mg by mass. 100 mg of magnesium oxide has 60 mg magnesium.

I would guess that if you took Mg oxide with a meal, you'd get closer to 10% absorption (30 mg out of a 500 mg tablet). Essentially the same you get from 500 mg of Mg orotate. Orotate helps you absorb nearly all the Mg prsent, but it's takes a lot of bulk to do so. I'd venture a guess that the other forms of magnesium are in line with these two. Essentially out of 500 mg of Mg salt, you get about 30 mg of Mg taking into account counterions and pharmacokinetic absorption profiles.

Dang, Matt beat me to it. He even included the Wikipedia link I would have. But just to reiterate. Look up the chemical formula of the complex you care about: http://en.wikipedia.org/wiki/Magnesium_orotate

It's weight is about 334 daltons. Mg weighs 24 daltons, so Mg is 24/334ths or 7% Mg by weight. 200g of MgOrotate delivers 200*7% or 14g of Mg.

Matt covered the other important point, you actually need to absorb the Mg for it to be useful. It's possible that MgOrotate is more readily absorbed than other complexes. The only way to know is to take the supplement then measure your serum levels. But that's too complicated and expensive. So as always, go by how you look, feel, and perform.

I do a tablespoon of Natural Calm, I don't even know how much I'm taking as far as how much Mg I get, but I do know that it helps me sleep, it helps me perform because of the vasodialation, so I don't worry about it beyond that.

I have a followup question for someone with chemistry knowledge. Can anyone write out the formula for the reaction that occurs when I mix my magnesium citrate powder with hot water? A precipitate forms and sinks to the bottom of the cup. Does this percipitates contain magnesium, or is it a useless byproduct?

Chelates are the mineral bound to amino acids. The specific amino acid does matter. The ones I know about and have used are:

Mg Taurate is bound to Taurine, which has beneficial effects independent of the Mg.

Mg Aspartate is bound to Aspartic acid which is recommended for some conditions or panned because of its association with glutamate as an excitotoxin (and in Aspartame) though certainly milder and perhaps the net effect with the calming of Mg is mild stimulation which could be desirable. Depends on your baseline. Aspartate has been shown to have a mild anabolic androgenic effect. It is found in some sports/mineral supplements or capsules.

BTW, I have often pointed out to people that glutamate and glutamine are different aminos. That is true, but recently I ran across an explanation that glutamine too could contribute to excitotoxin effect and glutamate toxicity, along with NAC and a few other what could be called glutamate agonists. That may explain why glutamine, so often touted as a safe and useful supplement does not sit well with me.

Citrate and Malate are two simple organic acids, not amino acids, the first named for lemon, the second for apples. They play roles in the energy cycle. Malic acid or malate is often recommended (with Mg or separately) for chronic fatigue or similar conditions.

Mg bound to organic acids like citrate and malate or to inorganic acid/anion is called a salt. In solution the anions and cations are floating around mixed/distributed among other electrolyte species. Mg reacts to fatty acids to form soaps. See comment below.

Although very high in the Mg per unit weight category I personally think that slow Mag type Mg Cl is not a good source compared to those which depend partly on your body's own regulation of absorption. It is an unnatural way to take large quantities of Mg which you may not need. Mineral waters and even tap water in some places can have significant Mg in a mix of other minerals like Na and K. Studies have shown that in places with Mg rich water the rate of CHD is lower. Taurine is thought to be good for heart so the combination is frequently promoted for heart health.

Much more could be said but leaving it to what is most essential in choosing the supplement form. There are good Mg food sources too.

Too much Mg in the wrong form will be laxative... think Milk of Magnesia.

About the precipitate, I recall an explanation in fizzy mineral supplements that it's harmless, perhaps depends partly on your water hardness.