Since magnesium is required for the conversion of Vitamin D into its active form, it’s also true that taking Vitamin D may not raise Vitamin D blood levels in those who are magnesium deficient.
Magnesium is ‘used up’ when Vitamin D is converted into its active form in the blood. Magnesium is ‘required’ to convert Vitamin D into its active form in the blood. It works BOTH ways. Magnesium is not JUST depleted, but you will not convert Vitamin D unless you have enough magnesium in order to allow Vitamin D to be converted. In many cases where large doses of Vitamin D are taken but the Vitamin D level does not come up, both the person deficient and their doctor believe that they are having Vitamin D absorption problems.