Magnesium

Magnesium is the fourth most abundant cation in the human body and an essential cofactor for over 300 enzymatic reactions, including all ATP-dependent processes (biologically active ATP exists as Mg-ATP complex), DNA and RNA synthesis, ribosomal protein synthesis, and voltage-gated ion channel regulation. Approximately 50-60% of body magnesium resides in bone, 40% in soft tissues, and less than 1% in serum.

Requirements and intake gap

The 1997 NAM DRI set RDAs of 400 mg/day for men 19-30 years and 420 mg/day for men 31+, 310 mg/day for women 19-30 and 320 mg/day for women 31+. Pregnancy RDA is 350-400 mg. Analysis of NHANES data indicates approximately 50% of US adults consume below the Estimated Average Requirement. The Tolerable Upper Intake Level of 350 mg/day applies only to supplemental magnesium (food magnesium is unregulated because gastrointestinal tolerance limits excessive absorption from food).

Function

Magnesium's biochemical roles span: (1) ATP handling — every enzymatic ATP hydrolysis requires Mg2+ coordination to the phosphate groups; (2) nucleic acid metabolism — DNA polymerase, RNA polymerase, and ribosomal assembly all require Mg2+; (3) neuromuscular excitability — Mg2+ blocks NMDA glutamate receptors in a voltage-dependent manner and stabilizes cardiac and neuronal membranes; (4) bone mineralization — Mg2+ influences hydroxyapatite crystal structure and bone strength; (5) parathyroid hormone regulation — hypomagnesemia impairs PTH secretion and action.

Deficiency manifestations

Frank hypomagnesemia (serum Mg <1.7 mg/dL) is uncommon but occurs in contexts of: gastrointestinal losses (chronic diarrhea, malabsorption, alcoholism), renal losses (diabetes, loop and thiazide diuretics, proton pump inhibitors), and inadequate intake in the elderly or hospitalized. Clinical manifestations include neuromuscular hyperexcitability (tremor, tetany, seizures), cardiac arrhythmia (torsades de pointes, atrial fibrillation), and concurrent hypokalemia and hypocalcemia that resist replacement until magnesium is corrected.

Subclinical magnesium inadequacy — normal serum magnesium but insufficient intake — is more common and has been associated with increased risk of type 2 diabetes, hypertension, cardiovascular events, migraine, and all-cause mortality in observational studies. Interventional evidence for magnesium supplementation in these conditions is mixed but generally favorable for blood pressure (modest reduction) and migraine frequency.

Status assessment

Serum magnesium reflects less than 1% of total body magnesium and can remain normal despite substantial intracellular depletion. Alternative markers — erythrocyte magnesium, ionized magnesium, urinary magnesium excretion after intravenous loading — are available in research and specialty clinical contexts but not routinely used. The practical consequence is that magnesium status cannot be reliably assessed from routine laboratory panels, and subclinical inadequacy is underdiagnosed.

Dietary sources

Per USDA FoodData Central (mg Mg per 100 g): pumpkin seeds 550, chia seeds 335, almonds 270, cashews 290, spinach (cooked) 80, black beans (cooked) 70, quinoa (cooked) 65, dark chocolate (70-85%) 228, whole wheat bread 75, white rice (cooked) 12, refined white bread 25. The refinement-driven magnesium loss — whole grains contain ~3x the magnesium of refined grains — is one reason whole-grain emphasis supports magnesium adequacy.

Supplementation forms

Magnesium salts differ in bioavailability and gastrointestinal tolerance. Magnesium citrate and glycinate are well absorbed with modest gastrointestinal effects. Magnesium oxide has poor absorption (~4%) and more osmotic laxative effect, though is common in supplements. Magnesium sulfate (Epsom salt) is used topically and in medical settings. Typical supplemental doses range 200-400 mg elemental magnesium per day.