Symptoms of magnesium deficiency can range from subtle fatigue and muscle cramps to serious cardiac arrhythmias and seizures. Magnesium deficiency, or hypomagnesaemia, occurs when serum magnesium falls below approximately 0.7 mmol/L. This essential mineral acts as a cofactor in over 300 enzymatic reactions, supporting nerve function, muscle contraction, and cardiovascular health. Whilst uncommon in healthy individuals, certain medications, gastrointestinal disorders, and chronic conditions significantly increase risk. Early recognition of symptoms enables prompt treatment through dietary modification, oral supplementation, or intravenous replacement in severe cases. Understanding who is at risk and when to seek medical attention is crucial for preventing potentially life-threatening complications.

What Is Magnesium Deficiency?

Magnesium deficiency, clinically termed hypomagnesaemia, occurs when serum magnesium levels fall below the normal reference range of approximately 0.7–1.0 mmol/L (though laboratory reference ranges may vary slightly). Magnesium is the fourth most abundant cation in the body and the second most abundant intracellular cation after potassium. It serves as a cofactor for over 300 enzymatic reactions, playing crucial roles in protein synthesis, muscle and nerve function, blood glucose control, and blood pressure regulation.

True magnesium deficiency is relatively uncommon in healthy individuals consuming a balanced diet, as the kidneys efficiently regulate magnesium excretion. The body maintains tight homeostatic control of magnesium through intestinal absorption and renal excretion, with approximately 99% of total body magnesium stored in bone, muscle, and soft tissues, leaving only 1% in extracellular fluid.

Diagnosis can be challenging because serum magnesium levels do not always accurately reflect total body magnesium stores. A person may have depleted intracellular magnesium whilst maintaining normal serum levels initially. For this reason, clinical assessment must consider both biochemical measurements and presenting symptoms. Standard diagnostic work-up includes repeat serum magnesium measurement, concurrent electrolytes (particularly potassium and calcium), ECG if symptomatic, and sometimes 24-hour urinary magnesium or fractional excretion to assess renal losses.

The condition may develop gradually over time or occur acutely in specific clinical scenarios, such as severe gastrointestinal losses or certain medication use. Severe hypomagnesaemia (typically <0.5 mmol/L) can affect multiple organ systems, leading to a constellation of neuromuscular, cardiovascular, and metabolic disturbances that require prompt recognition and appropriate management.

Common Symptoms of Magnesium Deficiency

The clinical presentation of magnesium deficiency varies considerably depending on the severity and duration of depletion. Early or mild deficiency may produce subtle, non-specific symptoms that are easily overlooked or attributed to other causes. These include:

• Fatigue and weakness – a general sense of tiredness and reduced energy levels

• Loss of appetite – decreased interest in food, which may further compromise nutritional intake

• Nausea – mild gastrointestinal discomfort without specific cause

As deficiency progresses to moderate or severe levels, more distinctive neuromuscular symptoms emerge. Muscle cramps, particularly affecting the legs and feet, are among the most commonly reported complaints. Patients may experience muscle twitching (fasciculations), tremor, or generalised muscle weakness. These symptoms reflect magnesium's essential role in neuromuscular transmission and muscle contraction.

Neurological manifestations can include paraesthesia (tingling or numbness), typically in the extremities, and in severe cases, seizures may occur. Tetany may develop, characterised by carpopedal spasm and sometimes positive Trousseau's or Chvostek's signs. Personality changes, apathy, or increased irritability have also been documented, though these are less specific.

Cardiovascular symptoms represent potentially serious complications of magnesium deficiency. Cardiac arrhythmias, including atrial fibrillation, ventricular tachycardia, and characteristic ECG changes (prolonged QT interval) may develop. Prolonged QT can precipitate torsades de pointes, a potentially life-threatening arrhythmia. Patients might experience palpitations or irregular heartbeat sensations.

It is important to note that magnesium deficiency rarely occurs in isolation. It frequently coexists with other electrolyte disturbances, particularly hypokalaemia (low potassium) and hypocalcaemia (low calcium), which can be refractory to treatment until magnesium levels are corrected. This interrelationship can complicate the clinical picture and necessitates comprehensive electrolyte assessment when magnesium deficiency is suspected.

Who Is at Risk of Low Magnesium Levels?

Several population groups and clinical conditions predispose individuals to magnesium deficiency. Understanding these risk factors enables targeted screening and preventive strategies.

Gastrointestinal disorders significantly impair magnesium absorption. Patients with chronic diarrhoea, Crohn's disease, coeliac disease, or those who have undergone bowel resection surgery face increased risk due to reduced intestinal absorption. Acute gastroenteritis with prolonged vomiting and diarrhoea can also precipitate deficiency.

Medication use represents a major iatrogenic cause. Proton pump inhibitors (PPIs), widely prescribed for acid reflux and peptic ulcer disease, can reduce magnesium absorption when used long-term. The MHRA has issued a Drug Safety Update recommending baseline and periodic magnesium monitoring in patients on prolonged PPI therapy, particularly when combined with digoxin or diuretics. Loop and thiazide diuretics increase renal magnesium losses and are commonly implicated in deficiency, particularly in elderly patients with heart failure or hypertension. Other medications including certain antibiotics (aminoglycosides), chemotherapy agents (cisplatin), immunosuppressants (ciclosporin, tacrolimus), and antifungals (amphotericin B) can also deplete magnesium.

Type 2 diabetes mellitus is associated with increased urinary magnesium excretion, particularly in poorly controlled diabetes. This may contribute to worsening glycaemic control, as magnesium plays a role in insulin function.

Alcohol use disorder causes magnesium depletion through multiple mechanisms: poor dietary intake, gastrointestinal losses, and increased renal excretion. Elderly individuals are at heightened risk due to reduced dietary intake, decreased intestinal absorption, increased medication use, and age-related changes in renal function.

Refeeding syndrome can cause acute magnesium depletion alongside other electrolyte abnormalities. While advanced chronic kidney disease typically causes hypermagnesaemia (elevated magnesium), certain kidney conditions, tubulopathies, or dialysis may lead to magnesium losses. Individuals following restrictive diets or with eating disorders may also have insufficient magnesium intake. Certain genetic disorders affecting renal magnesium handling represent additional risk factors requiring specialist management.

Treatment Options for Magnesium Deficiency

Management of magnesium deficiency depends on severity, underlying cause, and the presence of symptoms. The primary goals are to restore normal magnesium levels, address the underlying cause, and prevent recurrence.

Dietary modification represents the first-line approach for mild deficiency or prevention in at-risk individuals. Magnesium-rich foods include:

• Green leafy vegetables (spinach, kale)

• Nuts and seeds (almonds, cashews, pumpkin seeds)

• Whole grains (brown rice, wholemeal bread)

• Legumes (black beans, chickpeas)

• Fish (mackerel, salmon)

• Dark chocolate and avocados

The Reference Nutrient Intake (RNI) for magnesium in the UK is 300 mg daily for men and 270 mg daily for women, with no specific increase during pregnancy or lactation.

Oral magnesium supplementation is appropriate for symptomatic mild-to-moderate deficiency. Various formulations are available, including magnesium oxide, citrate, and glycinate. Magnesium citrate and glycinate generally offer better bioavailability than oxide. Typical supplementation doses range from 200–400 mg of elemental magnesium daily, though this should be individualised. The NHS advises not exceeding 400 mg/day of magnesium from supplements unless prescribed. Gastrointestinal side effects, particularly diarrhoea, are common and dose-dependent; dividing doses or taking with food may improve tolerance.

Importantly, magnesium supplements can interact with several medications. Take magnesium supplements 2-4 hours apart from tetracycline antibiotics, quinolone antibiotics, bisphosphonates, and levothyroxine, as magnesium can reduce their absorption.

Intravenous magnesium replacement is reserved for severe deficiency, particularly when accompanied by serious symptoms such as seizures or cardiac arrhythmias, or when oral absorption is compromised. This requires hospital admission with continuous monitoring of vital signs, reflexes, ECG, and renal function, as rapid infusion can cause adverse effects including hypotension and respiratory depression. Dosing should follow BNF guidance.

Addressing underlying causes is essential for long-term management. This may involve reviewing and potentially adjusting medications (e.g., considering alternatives to PPIs where appropriate, but only under medical supervision), optimising management of gastrointestinal disorders, or improving diabetic control. Patients on long-term diuretics may require regular monitoring and consideration of supplementation based on magnesium levels.

When to seek medical attention: Contact your GP if experiencing persistent muscle cramps, tremor, palpitations, or unexplained fatigue, particularly if you have risk factors for deficiency. Call 999 immediately for chest pain, collapse, severe breathlessness, or seizures. Regular monitoring of magnesium levels is advisable for high-risk patients, and any supplementation should be undertaken under medical supervision, particularly in those with renal impairment, as magnesium is renally excreted and accumulation can occur. Report any suspected adverse effects from supplements to the MHRA via the Yellow Card scheme.

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