Magnesium: The Underestimated Mineral

Magnesium is sometimes called the forgotten electrolyte. It is not included in many routine blood panels, its blood level reflects only a tiny fraction of body stores, and its deficiency produces symptoms easily attributed to other causes. At the same time, magnesium has become one of the most popular supplements, recommended for sleep, anxiety, cramps, energy and much else. Both the neglect and the enthusiasm deserve scrutiny.

Where magnesium is and what it does

An adult body contains roughly 25 grams of magnesium. About half to two thirds is in bone, most of the rest is inside cells, particularly muscle, and less than one percent is in the blood. This distribution has important consequences for testing.

Magnesium participates in several hundred enzymatic reactions. Among its most important roles:

• Energy metabolism. ATP, the cell’s energy currency, functions largely as a complex with magnesium.
• Nucleic acid and protein synthesis.
• Nerve and muscle function. Magnesium regulates ion channels and acts as a natural calcium antagonist, influencing muscle contraction and nerve excitability.
• Cardiac rhythm. Low magnesium predisposes to arrhythmias.
• Calcium and potassium balance. Magnesium is needed for parathyroid hormone secretion and action, and for renal potassium handling. Severe magnesium deficiency causes low calcium and low potassium that do not correct until magnesium is replaced.
• Glucose metabolism. Magnesium is involved in insulin signalling, and low magnesium is associated with insulin resistance and type 2 diabetes.

Who becomes deficient

The link with proton pump inhibitors is worth emphasising. Regulators have warned that long term use, generally more than a year, can cause low magnesium, sometimes severe, and that it may not correct with supplements until the medicine is stopped.

Signs of deficiency

Mild deficiency may cause no clear symptoms, or nonspecific ones such as fatigue, weakness and reduced appetite. More significant deficiency can cause muscle cramps, tremor, twitching, tetany, palpitations and arrhythmias, and in severe cases seizures. Laboratory clues include low potassium or low calcium that are difficult to correct.

Why testing is difficult

Serum magnesium is the standard test, but because so little magnesium is in the blood, and because the body tightly regulates serum levels, a normal result does not exclude depletion of body stores. A low serum magnesium, however, reliably indicates deficiency.

Other tests have been proposed. Red blood cell magnesium is offered by some laboratories but is not well standardised and its clinical value is uncertain. A 24 hour urine magnesium can help distinguish renal loss from other causes. Specialised loading tests are used mainly in research. In practice, assessment combines the serum level with the clinical picture, dietary intake and risk factors.

Where magnesium therapy has evidence

The pattern is familiar from other nutrients: clear benefit where there is deficiency or a specific pharmacological use, and weaker evidence for broad supplementation in people who are not deficient.

Food first

Good dietary sources include pumpkin and other seeds, nuts such as almonds and cashews, legumes, whole grains, dark leafy greens, dark chocolate and some mineral waters. Diets based on refined grains and processed foods tend to be low in magnesium. For many people, improving diet quality addresses marginal intake more comprehensively than a supplement.

Supplements: forms and cautions

• Forms differ in absorption and tolerability. Magnesium oxide contains a high proportion of magnesium but is less well absorbed and more laxative. Citrate, glycinate and some other organic forms are often better absorbed. Evidence comparing forms is limited, and differences may be modest.
• Diarrhoea is the most common side effect, particularly with higher doses and oxide or citrate forms.
• Kidney impairment is the key safety concern. The kidneys excrete excess magnesium, so people with reduced kidney function can accumulate it and develop toxicity, causing weakness, low blood pressure and, in severe cases, cardiac effects. Magnesium containing laxatives and antacids need particular caution in older adults and people with kidney disease.
• Interactions: magnesium can reduce absorption of some antibiotics, bisphosphonates and levothyroxine, so separating doses is recommended.

An illustrative case

A 69 year old woman with hypertension, treated with a thiazide diuretic, and reflux, treated with a proton pump inhibitor for 6 years, reports muscle cramps, fatigue and palpitations. Her potassium is persistently low despite supplements. Her doctor checks magnesium, which is low. Magnesium replacement is started, after which her potassium corrects. Her doctor reviews whether the proton pump inhibitor is still needed and considers alternatives for her blood pressure. Her cramps and palpitations settle. 2 routine medicines had combined to produce a deficiency that explained all 3 problems.

Neither forgotten nor a cure all

Magnesium deserves more attention than it usually receives: its deficiency is common in people taking certain medicines, misses easily on routine tests, and has real consequences. It deserves less attention as a general remedy for sleep, stress and wellbeing, where evidence is thin. The practitioner’s task is to find the patients who are genuinely deficient and to use magnesium where it is known to help.