Vitamin D & Magnesium
Low magnesium levels makes vitamin D ineffective. Screening for chronic magnesium deficiency is difficult because a normal serum level may still be associated with moderate to severe deficiency.
To date, there is no simple and accurate laboratory test to determine the total body magnesium status in humans.
Magnesium is essential in the metabolism of vitamin D, and taking large doses of vitamin D can induce severe depletion of Mg. Adequate Mg supplementation should be considered as an important aspect of vitamin D therapy.