I believe that one of the main reasons people who supplement Mg don't get all the benefits of Mg is because no one tells people about the elemental content of the various Mg forms. Even the US RDA is based on elemental Mg. But the forms we buy only contain between 10-60% of the amount on the front. Unfortunately, the form with the highest percentage(oxide=60%) also has one of the worst absorption rates at about 4%.

Selecting the forms with the highest % elemental Mg, and the best absorption rates is difficult enough for those who know the difference, but is near impossible for those who have no knowledge that Mg salts and elemental Mg are different things.

Many people are using a couple of 200mg Mg C tablets and believing that they are getting 400mg of Mg. Actually, they are getting about 64mg if they absorb every drop, and about 20mg if they absorb the usual 30%.

This link shows some of the differences in elemental content between forms.

http://ods.od.nih.gov/factsheets/magnesium.asp