Calcium and magnesium share some pathways for absorption and compete with each other. Not only is there a limit to how much calcium or magnesium you can absorb,

Calcium and magnesium share some pathways for absorption and compete with each other. Not only is there a limit to how much calcium or magnesium you can absorb, but if you have a high-calcium diet, your body will absorb less magnesium. There are other minerals which have this kind of competition, so supplementing with one mineral can cause problems with absorption of another.