"The well-studied interaction of magnesium with vitamin D and calcium provides an illustrative example.13 Magnesium is required for both steps in the activation of vitamin D to calcitriol, the form of vitamin D that regulates gene expression and stimulates calcium absorption. Even fully activated vitamin D (calcitriol), however, is useless in the absence of magnesium. Humans who are deficient in magnesium have low blood levels of both calcitriol and calcium, but treating them with calcitriol does nothing to restore calcium levels to normal. The only way to normalize calcium levels in these subjects is to provide them with sufficient magnesium. Magnesium also supports the cellular pumps that keep most calcium out of our soft tissue cells and make it available for the extracellular matrix of bones and teeth."
