What it does in the body
Magnesium is the fourth most abundant mineral in the body and is essential to good health. Approximately half of the body's magnesium is found in bone. The other half is found predominantly inside cells of body tissues and organs. While only one per cent of magnesium is found in blood, the body works very hard to keep blood levels of magnesium constant.¹

Magnesium is critical for energy production, enzyme activation and helps to build and strengthen bone².  Deficiency signs include fatigue, mental confusion, irritability, weakness, heart disturbances, muscle cramps, loss of appetite, insomnia and predisposition to stress. 

Based on analysis of dietary intakes it is estimated that we are getting less than half the amount of magnesium we did 100 years ago.  We can blame this on modern-day foods that are highly processed and refined.  To make matters worse, researchers suggest that the amount of magnesium required for optimum health has been underestimated in the past.³   Calcium supplementation generally requires supplementation of magnesium as well.

Food Sources

• Magnesium is found abundantly in whole foods.  Best food sources for magnesium include tofu, legumes, seeds, nuts, whole grains and green leafy vegetables.⁴   Few Canadians eat enough of these to ensure an adequate magnesium intake.

Recommended Dietary Allowance

• The RDA for magnesium ranges between 240 mg to 420 mg.

Orthomolecular Dosage Range:

• Somewhat higher than the RDA.

"Magnesium supplements are commonly available as inexpensive magnesium oxide in 100 or 250 mg tablets. For better absorption, physicians often prefer amino acid chelated magnesium tablets or magnesium citrate. Magnesium is available without prescription at discount and health food stores everywhere. People typically start supplementation with 200mg per day and may slowly increase to 600mg per day, taken in divided doses, some with each meal. Persons with kidney failure should not take supplemental magnesium unless directed to by their physician. Otherwise, magnesium toxicity is extremely rare. There have been no deaths from dietary supplementation with magnesium." (Orthomolecular Medicine News Service, October 23, 2007. More Magnesium Means Better Health.  http://orthomolecular.org/resources/omns/v03n08.shtml )
 
"Hypomagnesia is due to dietary deficiency, defective absorption, or excessive loss. Some of the most common causes are chronic alcoholism, chronic liver disease, uncontrollable diabetes mellitus, excessive use of diuretics or cardiac glycosides, and malabsorption syndromes.⁵ . . . Magnesium deficiency should be suspected in any situations associated with potassium deficiency, even though serum levels are normal.⁶ A few cases have been diagnosed as multiple sclerosis. A clinical background of diuretics, steroid treatment, hypercalcemia, diarrhea, alcoholism, hypokalemia, and liquid protein diets should lead one to suspect magnesium deficiency.

"Magnesium deficiency may be another factor that predisposes people to cancer because magnesium plays a major role in controlling growth of cells.⁷ Alternative treatments for cancer generally emphasize green vegetables, perhaps due to the fact that chlorophyll is a magnesium-containing molecule. In Poland, it was found that there were fewer cases of leukemia when magnesium was plentiful in soil and water. . . .⁸

"Magnesium is decreased in hypertension, and the incidence of hypertension is high in areas where drinking water is soft or where there is little magnesium in the soil. It has been known since 1925 that magnesium salts lower blood pressure. Rats made moderately magnesium deficient suffered an increase in blood pressure (from 111 to 131). With more severe deficiency, it went up to 143, an increase of 29 percent."⁹

1   Office of dietary supplements, US national institute of health http://dietary-supplements.info.nih.gov/factsheets/magnesium.asp#h1
2  Walsh, Stephen.  Plant Based Nutrition and Health. The Vegan Society: 2003. Pg. 56
3   http://www.eatrightontario.ca/en/ViewDocument.aspx?id=67&Topic=7&Cat=379
4   http://www.eatrightontario.ca/en/ViewDocument.aspx?id=67&Topic=7&Cat=379

(From: Hoffer A and Saul AW. Orthomolecular Medicine for Everyone, Basic Health Publications, 2008.)

References:
⁵  Graber, T.W., A.S. Yee, and F.J. Baker. "Magnesium: Physiology, Clinical Disorders and Therapy." Ann Emerg Med (1981): 49-57.
6   Fouty, R.A. "Liquid Protein Diet, Magnesium Deficiency and Cardiac Arrest." JAMA 240 (1978): 2632-2633.
⁷  Rubin, H. "Growth Regulation, Reverse Transformation and Adaptability of 3T3 Cells in Decreased Mi + Concentration." Proc Natl Acad Sci 78 (1981): 328-332.
⁸  Seelig, M.S. "Magnesium in Oncogenesis and in Anti-cancer Treatment Interaction with Minerals and Vitamins." In Quillan, P., and R.M. Williams (eds.). Adjuvant Nutrition in Cancer Treatment. Arlington Heights, IL: Cancer Treatment Research Foundation, 1994, pp. 238-318. Available online at: http://www.mgwater.com/cancer.shtml.
⁹  Altura, B.M., B.J. Altura, A. Gebrewold, et al. "Magnesium Deficiency and Hypertension: Correlation between Magnesium Deficiency Diets and Micro-circulatory Changes in Situ." Science 223 (1984): 1315-1317.

Also see:
Orthomolecular Medicine News Service, October 23, 2007. More Magnesium Means Better Health.  http://orthomolecular.org/resources/omns/v03n08.shtml