Magnesium: Benefits, Dosage, and Safety

What magnesium is

Magnesium is an abundant essential mineral in the body, naturally present in many foods, added to some fortified foods, sold as a dietary supplement, and present in some medicines such as antacids and laxatives. An adult body contains about 25 g of magnesium, with 50% to 60% in the bones and most of the rest in soft tissues. Less than 1% is in blood serum, where normal concentrations range between 0.75 and 0.95 mmol/L. It acts as a cofactor in more than 300 enzyme systems.

What it does

• Serves as a cofactor in more than 300 enzyme systems that regulate diverse biochemical reactions, including protein synthesis
• Supports normal muscle and nerve function
• Contributes to blood glucose control and blood pressure regulation
• Required for energy production, oxidative phosphorylation, and glycolysis
• Contributes to the structural development of bone
• Required for the synthesis of DNA, RNA, and the antioxidant glutathione
• Plays a role in the active transport of calcium and potassium ions across cell membranes, important to nerve impulse conduction, muscle contraction, and normal heart rhythm

How much magnesium per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Symptomatic deficiency from low dietary intake is uncommon in otherwise-healthy people because the kidneys limit urinary excretion. Early signs of deficiency include loss of appetite, nausea, vomiting, fatigue, and weakness. As deficiency worsens, numbness, tingling, muscle contractions and cramps, seizures, personality changes, abnormal heart rhythms, and coronary spasms can occur. Severe deficiency can result in hypocalcemia or hypokalemia because mineral homeostasis is disrupted. Hypomagnesemia is defined as a serum magnesium level less than 0.75 mmol/L.

Who may need more

• People with gastrointestinal disorders that cause malabsorption (Crohn's disease, celiac disease, regional enteritis, intestinal resection)
• People with type 2 diabetes, who can have increased urinary magnesium losses
• People with chronic alcohol dependence
• Older adults
• People taking loop or thiazide diuretics or long-term proton pump inhibitors, which can lower magnesium status

Food sources

Widely distributed in plant and animal foods. Good sources include green leafy vegetables such as spinach, legumes, nuts, seeds, and whole grains; foods high in dietary fiber generally provide magnesium. Examples per ODS: pumpkin seeds roasted 1 oz (156 mg), chia seeds 1 oz (111 mg), dry-roasted almonds 1 oz (80 mg), boiled spinach 1/2 cup (78 mg), dry-roasted cashews 1 oz (74 mg), oil-roasted peanuts 1/4 cup (63 mg), shredded wheat cereal 2 biscuits (61 mg), soymilk 1 cup (61 mg), cooked black beans 1/2 cup (60 mg), smooth peanut butter 2 tbsp (49 mg). Some breakfast cereals and other foods are fortified. Water can contribute (1 mg/L to more than 120 mg/L).

Forms and absorption

Supplements are available in many forms including magnesium oxide, citrate, and chloride. The Supplement Facts panel declares the amount of elemental magnesium, not the weight of the whole compound. Absorption varies by form: forms that dissolve well in liquid are more completely absorbed, and small studies find magnesium in the aspartate, citrate, lactate, and chloride forms is absorbed more completely and is more bioavailable than magnesium oxide and magnesium sulfate. Roughly 30% to 40% of dietary magnesium is typically absorbed. Very high supplemental zinc (142 mg per day) can interfere with magnesium absorption.

Safety, excess and interactions

Too much

Too much magnesium from food does not pose a health risk in healthy people because the kidneys eliminate the excess in urine. High doses from supplements or medications often cause diarrhea, sometimes with nausea and abdominal cramping; forms most commonly reported to cause diarrhea are magnesium carbonate, chloride, gluconate, and oxide. Very large doses of magnesium-containing laxatives and antacids, typically more than 5,000 mg per day, have been associated with magnesium toxicity, including fatal hypermagnesemia. Toxicity symptoms usually develop after serum concentrations exceed 1.74 to 2.61 mmol/L and can include hypotension, nausea, vomiting, facial flushing, urine retention, ileus, depression, and lethargy, progressing to muscle weakness, difficulty breathing, extreme hypotension, irregular heartbeat, and cardiac arrest. Risk increases with impaired renal function or kidney failure.

Interactions to know

• Bisphosphonates: magnesium-rich supplements or medications can decrease absorption of oral bisphosphonates such as alendronate (Fosamax); separate dosing by at least 2 hours
• Antibiotics: magnesium can form insoluble complexes with tetracyclines (demeclocycline, doxycycline) and quinolones (ciprofloxacin, levofloxacin); take these antibiotics at least 2 hours before or 4 to 6 hours after a magnesium-containing supplement
• Loop and thiazide diuretics: chronic use of furosemide (Lasix), bumetanide (Bumex), hydrochlorothiazide, and ethacrynic acid (Edecrin) can increase urinary magnesium loss and lead to depletion
• Potassium-sparing diuretics: amiloride (Midamor) and spironolactone (Aldactone) reduce magnesium excretion
• Proton pump inhibitors: prolonged use (typically more than a year) of PPIs such as esomeprazole magnesium (Nexium) and lansoprazole (Prevacid) can cause hypomagnesemia; FDA advises checking serum magnesium before and during long-term PPI treatment
• High-dose supplemental zinc (142 mg per day) can interfere with magnesium absorption and disrupt magnesium balance

How status is measured

Most commonly measured by serum magnesium concentration (normal range 0.75 to 0.95 mmol/L), though serum levels correlate poorly with total body or tissue magnesium. Other methods include erythrocyte, salivary, and urinary magnesium; ionized magnesium in blood/plasma/serum; and a magnesium-loading (tolerance) test, which some experts consider the best method in adults. No single method is considered satisfactory. Magnesium is not part of routine electrolyte testing, and dietary intake is often used as a proxy for status.

Regulated as a dietary supplement ingredient under the Dietary Supplement Health and Education Act (DSHEA); not FDA-approved to diagnose, treat, cure, or prevent any disease. The Daily Value (DV) for labeling is 420 mg for adults and children age 4 and older. In 2022 the FDA approved a qualified health claim that diets with adequate magnesium may reduce the risk of hypertension while stating the evidence is inconsistent and inconclusive; products bearing this claim must provide at least 84 mg per serving and, for supplements, no more than 350 mg.

What the evidence supports

We grade the strength of evidence behind each common reason people take magnesium, mirroring how the NIH describes it.