A video created by Andrea Rosanoff, Ph.D. 

Calcium and magnesium share equal importance in our bodies and must be kept in balance for proper cellular function to occur.

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Appropriating Newton’s law of physics that says for every action there is an equal and opposite reaction, neither calcium or magnesium can act without eliciting a reaction from the other. At the biochemical level, magnesium and calcium are known to act antagonistically toward each other. Many enzymes whose activities critically depend on a sufficient amount of intracellular magnesium (10,000 times more than calcium) will be detrimentally affected by small increases in levels of cellular calcium. Growth of cells, cell division, and intermediary metabolism are also absolutely dependent on the availability of magnesium, which can be compromised if excess
calcium is present.1

Reference:

1. Walker GM, “Biotechnological implications of the interactions between magnesium and calcium.” Magnes Res, vol. 12, no. 4, pp. 303–309, 1999.