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Hi everyone, I am still hearing so much about what is the best form of magnesium to use. There is a lot of misinformation and lack of understanding as to the bioavailability/ elemental components of magnesium. And there are different magnesiums for different purposes. So I do not just have one type to treat my clients with, I use what is the best one for my particular client. And yes there are times(rare) that I will use mag oxide, but not for ever. There is so much info out there now from Dr's and scientists who have done the research and have shown how the body uses these different components/salts the magnesium is attached to. 
I am sorry at the moment I am so bogged down in my studies and exams I don't have time to write this all out for you, Let me get to the end and there will be a lot to write and catch up on. But here is a short snippet from someone else whom I thought has put some together well for you to get an idea and maybe start your own research. Hope this helps.

Bioavilabilty Of Different Magnesium supplements -

All Magnesium supplements are a combination of Magnesium with another substance such as a salt. Every salt provides different amounts of elemental Magnesium. The amount of Magnesium and its bioavailability alter the effectiveness of the supplement. Other factors affecting absorption of Magnesium are the existing Magnesium levels of the individual, as Magnesium will be less rapidly absorbed if body levels are already adequate and excreted through the urine or stools if given in excess. Also all the points mentioned above will have an influence on Magnesium absorption.

Bioavailability refers to the amount of elemental Magnesium actually absorbed by the body.

In short, the amount of Magnesium that your tissues can use readily is based on how soluble the Magnesium product is and the amount of elemental or ionic Magnesium that is released.

A value called the “stability constant” is based on the metal-ligand complex. Stability constants are a measure of the strength of the bonds of the compound molecule and vary from 0 upwards.

The lower the stability constant, the more easily it dissolves or dissociates into its metal ions due to weak ionic bonds). This means the body can easily absorb the metal in ionic form in a pH from 2 (stomach acid) to 7.4 (serum and lymph) ( Thomas E. Furia. 1972).

Metal ions easily pass between the cells. They are under the control of gravity, moving body fluids and the electric charge of the cell membrane. Metal ions may react with the cell membranes or be taken into the cell. Magnesium ions are present in much greater concentration inside cells than in the serum, being actively brought into the cell, as the cell needs them.

So although Magnesium oxide has the highest elemental Magnesium (60%), it also has a high stability constant, meaning that it does not dissociate, or ionize and is therefore poorly bioavailable Gut absorption is believed to be as low as 4%( leaving 288mg of a 500mg capsule unabsorbed in the intestines).

You will find that Magnesium oxide is very common in poor quality supplements simply because it is cheap however, only about 4% of its elemental magnesium is absorbed, equivalent to about 12 mg out of a 500 mg tablet.

Magnesium Chloride Supplements.

Magnesium chloride (12% elemental Magnesium) has a stability constant of 0 and is completely ionized across a large pH range, 2 (found in stomach acid) to 7.4 found in extracellular tissues such as blood and lymph. Magnesium chloride has the chloride part of its compound to produce hydrochloric acid in the stomach and enhance its absorption. This is particularly suitable for anybody with low stomach acid.

Magnesium Malate Supplements

Magnesium malate (6.5% elemental Magnesium) has a stability constant of 1.55 and is nearly completely ionisable. Again the weak ionic bonds of Magnesium and malic acid are easily broken making it readily soluble in the body.

Magnesium Citrate Supplements

Magnesium citrate (16% bioavailability) and stability constant of 2.8. Weak bonds provide a high bioavailability. Magnesium citrate works by attracting water through the tissues by osmosis. When the Magnesium citrate reaches the small intestine it attracts enough water to induce defecation. The extra water helps create more faeces, stimulating bowel motility and may have a mild laxative effect. This form of Magnesium functions best on an empty stomach followed by a full glass of water or juice to aid absorption.

Researchers have demonstrated that Magnesium bioavailability is greater in citrate than oxide taking the pH of stomach acid and alkalinity of pancreas into consideration. (Lindberg, Zobitz et al. 1990)

Magnesium sulphate Supplements

Magnesium sulphate (10% elemental Magnesium) is also known as Epsom salts. It contains Magnesium; Sulphur and Oxygen. It is the main preparation of intravenous Magnesium. Bioavailability is limited and variable with degrees of mild diarrhoea. (Morris, LeRoy et al. 1987).

Magnesium Ascorbate Supplements

Magnesium Ascorbate (6.4% elemental Magnesium) is a source of both vitamin C and Magnesium. It is a neutral salt having a significantly higher gastrointestinal tolerance than some of the other forms.

Magnesium Phosphate Supplements

Magnesium (19% elemental Magnesium) but practically insoluble in water. Magnesium is bound to phosphate in teeth and bone.

Magnesium Carbonate Supplements

Magnesium Carbonate (42% elemental Magnesium). Research sources suggests different bioavailability rates between 5-30%. In large doses this form may have a mild laxative effect. Magnesium carbonate reacts with hydrochloric stomach acid to form Magnesium chloride. This conversion is dependent on adequate stomach acid levels.

Magnesium Hydroxide Supplements

At 41.67%, Magnesium Hydroxide has a relatively high percentage of elemental magnesium but has a low solubility in water, suggesting poor absorption. When in a suspension in water it is often called milk of magnesia, used as an antacid or laxative.

Although a high percentage of elemental Magnesium, the Magnesium ion is very poorly absorbed from the intestinal tract, drawing water from the surrounding tissues by osmosis.

Interestingly research done in Magnesium depleted rats using 10 organic and inorganic Magnesium salts using a stable isotope approach concluded that all salts were equally efficient in restoring blood and plasma Magnesium levels. (Coudray, Rambeau. 2005).

This has not been repeated in humans who are more complex in terms of having other factors that influence absorption.