Actually, there don't appear to be significant differences in bioavailability between the various forms of supplemental magnesium. If taken on an empty stomach

Actually, there don't appear to be significant differences in bioavailability between the various forms of supplemental magnesium. If taken on an empty stomach, the oxide and carbonate forms are less available because the Mg cation won't dissociate. However if they're taken with food, these chemicals dissolve in your gastric acid, and the magnesium will be absorbed the same as any other magnesium salt. Physical packaging can greatly affect absorption; for example, extended release formulations often show very poor availability. But minimally packaged magnesium salts are all pretty much the same in terms of total uptake. Here's a good summary: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5652077/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5652077/ - "Intestinal Absorption and Factors Influencing Bioavailability of Magnesium-An Update-"