01 Magnesium is necessary for the proper functioning of 700-800 enzyme systems in the body - that's why it is implicated in scores of symptoms and 65 health conditions.
02 Most people are magnesium deficient.
03 Excess calcium in the diet depletes magnesium in the body, and many people take too much calcium, either as supplements, in fortified foods or dairy products.
04 Magnesium is often very deficient in the soil and in the food supply, especially in "modern" processed foods, so it must be supplemented.
05 Many forms of magnesium supplements cause a laxative effect, which prevents them from being taken in a therapeutic dose to relieve magnesium deficiency and its symptoms. Magnesium chloride, though, can be taken in therapeutic dosages.
06 Magnesium deficiency can cause mitochondrial dysfunction. ATP (adenosine triphosphate) energy molecules are made in the mitochondria via the Krebs cycle. Six of the 8 steps in that cycle depend on adequate levels of magnesium.
07 The Serum Magnesium test is inaccurate, but is still the standard test used in hospitals, clinics and in most clinical trials, and it doesn't even appear on a standard electrolyte panel.
08 Magnesium deficiency is a major factor in chronic disease - diabetes, heart disease, high blood pressure, high cholesterol, migraines, IBS, and heartburn. Moreover, the drugs used to treat all these conditions deplete magnesium, often making symptoms worse.
09 Telomeres (the DNA sequence at the ends of the chromosomes) hold the key to aging, as does magnesium, which prevents telomeres from deteriorating.
Getting more technical, here are twelve crucial functions of magnesium that appear in a textbook on magnesium in a chapter called "Divalent Cation Metabolism: Magnesium." [4,5] There will be some overlap with the Top Nine Magnesium Facts that help to explain the amazing features of this mineral.
01 Energy: The most important function of magnesium is assisting in the creation of energy in the trillions of cells making up our body. Magnesium is a cofactor in the production of ATP (adenosine triphosphate) via ATP synthase. ATP, the molecule that transfers energy, is manufactured in the mitochondria and it must be bound to a magnesium ion (MgATP) in order to be biologically active. Each human cell contains 1,000-2,000 mitochondria. ATP is made in each one through a series of 8 steps called the Krebs cycle. What's remarkable about magnesium is that it is necessary for 6 of those 8 steps. In this cycle, magnesium is a modulator of oxidative phosphorylation during which electrons are transferred from electron donors to electron acceptors such as oxygen in redox reactions, using magnesium as a cofactor. These redox reactions, called electron transport chains, form a series of protein complexes within the cell's mitochondria that release energy by generating ATP.
02 Transporters and Pumps: ATP has many other functions besides being a source of energy. ATP is required by many transporters ("transmembrane ATPases") that import molecules necessary for cell metabolism and export toxins and wastes across cell membranes. A hydrogen-potassium ATPase creates the gastric proton pump, which acidifies the contents of the stomach. Many other pumps and transporters are directed by ATPases with magnesium as a necessary cofactor.
03 Membrane Stabilizer: Magnesium is an important membrane-stabilizing agent. Stabilization decreases excessive excitation of nerves and contraction of muscle cell membranes.
04 Protein Production: Magnesium is required for the structural integrity of numerous body proteins. To date, over 3,700 magnesium receptor sites have been found on human proteins!
05 RNA & DNA: Magnesium is required for the structural integrity of nucleic acids. Consequently, magnesium is a requirement for the production of RNA and DNA.
06 GTP: Magnesium is a cofactor for the enzyme guanosine triphosphatase (GTPase). This enzyme has many functions: (a) signal transduction, or "switching on" specific receptor proteins located on cell membranes and transmitting that signal to trigger taste, smell, and perception of light; (b) protein biosynthesis; (c) control and differentiation of cell division; (d) translocation of proteins through cell membranes; and (e) transport of vesicles within the cell and assembly of vesicle coats.
07 Phospholipase C: Magnesium is a cofactor for the enzyme phospholipase C, which is a class of enzymes that split phospholipids at the phosphate group. These enzymes define signal transduction pathways. The most important one allows calcium to enter cells.
08 Adenylate and Guanylate cyclase: Magnesium is a cofactor for the enzyme adenylate cyclase. This enzyme converts ATP to cyclic AMP (cAMP) and pyrophosphate. Cyclic AMP is used for intracellular signal transduction of the effects of hormones like glucagon and adrenaline into cells because the hormones can't pass through cell membranes. Cyclic AMP is involved in the activation of protein kinases and regulates the effects of adrenaline and glucagon. It also binds to and regulates the function of ion channels or gateways into the cell.
Magnesium is also a cofactor for the enzyme guanylate cyclase. This enzyme synthesizes cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP) keeping cGMP-gated ion channels open, allowing calcium to enter the cell. Cyclic GMP is an important second messenger that transmits the message across cell membranes from peptide hormones and nitric oxide, and it can also function in hormone signaling. It can trigger changes requiring protein synthesis. In smooth muscle, cGMP is the signal for relaxation, which can regulate vascular and airway tone, insulin secretion, and peristalsis.
09 700-800 Enzyme Processes: Magnesium is a required cofactor for the activity of hundreds of enzyme processes. The authors of "Magnesium in Man: Implications for Health and Disease" assure us that the number of magnesium enzymatic reactions is more than 600. Andrea Rosanoff, PhD, says, "While it was estimated in 1968 that magnesium was a required cofactor for over 300 enzyme processes, that number is now more reliably estimated at 700 to 800." 
10 Regulates Ion Channels: Magnesium is a direct regulator of ion channels, most notably via the other key electrolytes potassium, calcium and sodium. Magnesium is intimately involved in potassium transport. Magnesium and potassium depletion cause similar damaging effects on the heart. Furthermore, it is impossible to overcome potassium deficiency without replacing magnesium. That's why hospitals often seem to have such a difficult time finding the right electrolyte balance of sodium, potassium, calcium, and chloride: they ignore magnesium and do not routinely measure it in their electrolyte panels and when they do test for it, they use the inaccurate serum magnesium test.
11 Intracellular Signaling: Magnesium is an important intracellular signaling molecule itself. While I've mentioned signaling several times; the role of cell signaling cannot be underestimated. Without intracellular communication the cells of the body would not be able to function at all.
12 Nerve and Muscle Function: Magnesium is intimately involved in efficient nerve conduction. Although calcium is vital for proper nervous system function, too much calcium is dangerous. Excess calcium is pro-inflammatory and can excite nerves to the point of cell death. Magnesium helps cells to regulate calcium levels. Magnesium is intimately involved in efficient muscle function. The mechanisms are varied and include oxygen uptake, electrolyte balance, and energy production. Magnesium is important for properly functioning muscles, allowing calcium to cause muscle contraction and then pushing calcium out of the muscle cells to allow the relaxation phase. In the same way that nerve cells can be "excited to death," muscle cells stimulated by too much calcium can go into uncontrollable spasms or cramps, resulting in tissue damage such as occurs in a heart attack.
4. McCarthy JT, Kumar R, "Divalent cation metabolism: magnesium," in Schrier R (series ed.), The Atlas of Diseases of the Kidney, Blackwell, Oxfordshire, 1999.
5. Heaton FW, "Role of magnesium in enzyme systems," in Siegel H (ed.), Metal Ions in Biologic Systems, Marcel Dekker, New York, 1990.
6. de Baaij, JHF. et. al. Magnesium in man: Implications for health and disease. Physiological Reviews. Jan 1, 2015 Vol. 95 no. 1, 1-46. http://physrev.physiology.org/content/95/1/1.long
7. Rosanoff A. The Essential Nutrient Magnesium - Key to Mitochondrial ATP Production and Much More (2009). https://www.prohealth.com/library/print.cfm?libid=14606.
8. Abraham GE, Flechas JD, "Management of fibromyalgia: rationale for the use of magnesium and malic acid." J Nutr Med, vol. 3, pp. 49-59, 1992. https://www.ncbi.nlm.nih.gov/pubmed/8587088