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Mechanism of action
Because of the important interaction between phosphate and magnesium ions, magnesium ions are essential to the basic nucleic acid chemistry of life, and thus are essential to all cells of all known living organisms. Over 300 enzymes require the presence of magnesium ions for their catalytic action, including all enzymes utilizing or synthesizing ATP, or those that use other nucleotides to synthesize deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). ATP exists in cells normally as a chelate of ATP and a magnesium ion.

Dietary sources
Spices, nuts, cereals, coffee, cocoa, tea, and vegetables are rich sources of magnesium. Green leafy vegetables such as spinach are also rich in magnesium since they contain chlorophyll.

Deficiency
Magnesium is an essential component of the human diet. Human magnesium deficiency (including conditions that show few overt symptoms) is relatively rare although only 32% of people in the United States meet the RDA-DRI; low levels of magnesium have been associated with the development of a number of human illnesses such as asthma, diabetes, and osteoporosis. Alcoholism can produce a magnesium deficiency, which is reversible by oral or parenteral administration, depending on the degree of deficiency.

Disease prevention
Magnesium plays a role in preventing both stroke and heart attack. The symptoms of people with fibromyalgia, migraines, and premenstrual syndrome are less severe, and magnesium can shorten the length of the migraine symptoms.

Distribution
Adult human bodies contain about 24 grams of magnesium, with 60% in the skeleton, 39% intracellular (20% in skeletal muscle), and 1% extracellular. Serum levels are typically 0.7–1.0 mmol/L or 1.8–2.4 mEq/L. Serum magnesium levels may appear normal even in cases of underlying intracellular deficiency, although no known mechanism maintains a homeostatic level in the blood other than renal excretion of high blood levels.

Intracellular magnesium is correlated with intracellular potassium. Magnesium is absorbed in the gastrointestinal tract, with more absorbed when status is lower. Magnesium competes with calcium in the human body, in this way it actually keeps calcium in check. However, this can cause a calcium deficiency if calcium levels are already low. Low and high protein intake inhibit magnesium absorption, and other factors such as phosphate, phytate, and fat affect absorption. Excess dietary magnesium is excreted in feces, urine, and sweat. Magnesium status may be assessed roughly through serum and erythrocyte Mg concentrations and urinary and fecal excretion, but intravenous magnesium loading tests are likely the most accurate and practical in most people. In these tests, magnesium is injected intravenously; a retention of 20% or more indicates deficiency. Other nutrient deficiencies are identified through biomarkers, but none are established for magnesium.

Recommended intake
The UK recommended daily values for magnesium is 300 mg for men and 270 mg for women. Observations of reduced dietary magnesium intake in modern Western countries compared to earlier generations may be related to food refining and modern fertilizers that contain no magnesium.

Therapy
Magnesium therapy is recommended by the ACC/AHA/ESC 2006 Guidelines for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death for patients with ventricular arrhythmia associated with torsades de pointes who present with long QT syndrome as well as for the treatment of patients with digoxin intoxication-induced arrhythmias. Magnesium is also the drug of choice in the management of pre-eclampsia and eclampsia. Magnesium improves calcification. Magnesium supplementation helps to improve carotid intima media thickness in patients on hemodialysis |journal = Int Urol Nephrol |volume=40 |pages=1075–1082 |year=2008 |doi= 10.1007/s11255-008-9410-3 |pmid= 18568412|issue= 4 }}

Supplementation
Numerous pharmaceutical preparations of magnesium, as well as magnesium dietary supplements are available. Magnesium oxide, one of the most common forms in magnesium dietary supplements because it has high magnesium content per weight, has been reported the least bioavailable. Magnesium citrate has been reported as more bioavailable than oxide or amino-acid chelate (glycinate) forms.

Overdose
Excess magnesium in the blood is freely filtered at the kidneys, and for this reason it is difficult to overdose on magnesium from dietary sources alone. With supplements, overdose is possible, in particular in people with poor renal function; occasionally, with use of high cathartic doses of magnesium salts, severe hypermagnesemia has been reported to occur even without renal dysfunction.

Detection in biological fluids
Magnesium concentrations in plasma or serum may be measured to monitor for efficacy and safety in those receiving the drug therapeutically, to confirm the diagnosis in potential poisoning victims or to assist in the forensic investigation in a case of fatal overdosage. The newborn children of mothers having received parenteral magnesium sulfate during labor may exhibit toxicity at serum magnesium levels that were considered appropriate for the mothers.

Risk of disease
Low serum magnesium levels are associated with metabolic syndrome, diabetes mellitus type 2 and hypertension. Low serum magnesium levels have been associated with a higher risk of developing metabolic syndrome. Several in vitro and animal studies point toward a protective role of magnesium through multiple molecular mechanisms. Magnesium is a natural calcium antagonist, and both human and animal studies have shown that low circulating magnesium levels are associated with vascular calcification. Results from an observational study conducted in the general Japanese population demonstrated that lower serum magnesium levels were significantly and independently associated with a greater average intima-media thickness and the risk of at least two carotid plaques. Magnesium supplementation might be useful in reducing the progression of atherosclerosis in chronic dialysis patients. and patients with mildly elevated serum magnesium levels could have a survival advantage over those with lower magnesium levels. Patients with chronic kidney disease have a high prevalence of vascular calcification, and cardiovascular disease is the leading cause of death in this population.

Overdose
Excess magnesium in the blood is freely filtered at the kidneys, and for this reason it is difficult to overdose on magnesium from dietary sources alone. With supplements, overdose is possible, in particular in people with poor renal function; occasionally, with use of high cathartic doses of magnesium salts, severe hypermagnesemia has been reported to occur even without renal dysfunction. Since the kidneys are responsible for the excretion of magnesium, anyone with a heart or kidney disorder should not take magnesium supplementation except under their doctor's direction. It is very rare to overdose on magnesium from food, however, ingestion of large amounts of milk of magnesia (as a laxative or antacid), Epsom salts (as a laxative or tonic), or magnesium supplements may overdose, especially if they suffer from kidney problems. Too much magnesium can cause several serious health problems, including nausea, vomiting, severely lowered blood pressure, confusion, slowed heart rate, respiratory paralysis, deficiencies of other minerals, coma, cardiac arrhythmia, cardiac arrest, and eventually death. The most common side effects of magnesium toxicity are stomach upset and diarrhea.

Function in plants
Plants have an additional use for magnesium: chlorophyll is a magnesium-centered porphyrin. Magnesium deficiency in plants causes late-season yellowing between leaf veins, especially in older leaves, and can be corrected by applying Epsom salts (which is rapidly leached), or else crushed dolomitic limestone to the soil.