What causes a low magnesium level, and why should you be concerned? Low magnesium, or hypomagnesaemia, occurs when serum magnesium falls below 0.7 mmol/L. This essential mineral supports over 300 enzymatic reactions, including muscle and nerve function, heart rhythm regulation, and bone health. Despite its importance, magnesium deficiency often goes undiagnosed because routine NHS blood tests don't always include it. Common causes include inadequate dietary intake, certain medications (particularly proton pump inhibitors and diuretics), gastrointestinal disorders, diabetes, and excessive alcohol consumption. Severe deficiency can lead to cardiac arrhythmias and seizures. Understanding the causes helps with prevention and appropriate management, improving overall health outcomes.

What Is Low Magnesium and Why Does It Matter?

Low magnesium, medically termed hypomagnesaemia, occurs when serum magnesium levels fall below 0.7 mmol/L (with the normal UK reference range typically being 0.7-1.0 mmol/L). Magnesium is the fourth most abundant mineral in the human body and plays a crucial role in over 300 enzymatic reactions. It is essential for maintaining normal muscle and nerve function, supporting a healthy immune system, keeping the heart rhythm steady, and contributing to bone strength. Additionally, magnesium helps regulate blood glucose levels and is involved in energy production and protein synthesis.

Despite its importance, magnesium deficiency is relatively common and often goes undiagnosed because routine blood tests do not always include magnesium levels. Furthermore, serum magnesium (the standard NHS test) may not perfectly reflect total body stores, as most magnesium resides within cells and bone tissue rather than in the bloodstream.

Low magnesium levels have been associated with several health conditions, including cardiovascular disease, type 2 diabetes, osteoporosis, and migraine headaches, though causality is not established for all these relationships. In severe cases, hypomagnesaemia can lead to potentially serious complications including cardiac arrhythmias and seizures. Understanding what causes low magnesium is therefore essential for both prevention and appropriate management. Early identification and correction of magnesium deficiency can help prevent these complications and improve overall health outcomes.

Common Causes of Low Magnesium Levels

Magnesium deficiency typically arises from three main mechanisms: inadequate dietary intake, increased losses from the body, or impaired absorption from the gastrointestinal tract. Understanding these pathways helps identify individuals at risk and guides appropriate intervention.

Dietary insufficiency is a common contributor, particularly in Western diets that are high in processed foods and low in magnesium-rich whole foods. Good dietary sources of magnesium include:

• Green leafy vegetables (spinach, kale)

• Nuts and seeds (almonds, pumpkin seeds)

• Whole grains (brown rice, wholemeal bread)

• Legumes (black beans, chickpeas)

• Dark chocolate and avocados

However, low dietary intake alone is rarely the sole cause of clinically significant deficiency in otherwise healthy individuals.

Certain lifestyle factors can also deplete magnesium stores. Excessive alcohol consumption is a significant cause, as alcohol increases urinary magnesium excretion and impairs intestinal absorption. Chronic alcohol use can lead to both acute and chronic magnesium deficiency.

Increased renal losses represent another important pathway. The kidneys normally reabsorb approximately 95% of filtered magnesium, but this process can be disrupted by various factors. Poorly controlled diabetes mellitus causes osmotic diuresis, leading to excessive magnesium excretion in urine. Hypercalcaemia can reduce magnesium reabsorption in the renal tubules, while high sodium intake increases urinary magnesium losses through separate mechanisms. Age-related changes in kidney function may also affect the body's ability to conserve magnesium, particularly in those with other risk factors or medical conditions.

Medical Conditions That Lead to Magnesium Deficiency

Several medical conditions significantly increase the risk of developing low magnesium levels through various pathophysiological mechanisms. Gastrointestinal disorders are among the most common culprits, as they directly impair magnesium absorption. Chronic diarrhoea from any cause results in substantial magnesium losses, as the mineral is primarily absorbed in the small intestine and colon. Conditions such as Crohn's disease, ulcerative colitis, and coeliac disease damage the intestinal mucosa, reducing the absorptive surface area and impairing magnesium uptake. Following bowel resection surgery, particularly of the small intestine, patients often develop malabsorption syndromes that include magnesium deficiency.

Endocrine disorders also play a significant role. Type 2 diabetes mellitus is commonly associated with hypomagnesaemia. The mechanism involves increased urinary magnesium losses due to glycosuria-induced osmotic diuresis. Hyperthyroidism may affect magnesium status through increased metabolic demands and altered renal handling. Primary hyperparathyroidism and hypercalcaemia can interfere with magnesium reabsorption in the kidneys.

Kidney disorders present a complex picture. Advanced chronic kidney disease (CKD) may actually cause magnesium retention as glomerular filtration rate falls. In contrast, specific renal tubular disorders such as Gitelman syndrome and Bartter syndrome result in excessive urinary magnesium wasting due to inherited defects in tubular transport mechanisms.

Other relevant conditions include refeeding syndrome, where rapid reintroduction of nutrition depletes circulating electrolytes including magnesium; acute pancreatitis, where magnesium can be sequestered in areas of fat necrosis; and hungry bone syndrome following parathyroid surgery, where rapid bone remineralisation depletes circulating magnesium. Post-transplant patients on certain immunosuppressants are also at risk. Importantly, hypomagnesaemia often causes refractory hypokalaemia and hypocalcaemia that will not correct until magnesium levels are restored.

Medications That Can Lower Magnesium

Numerous commonly prescribed medications can cause or contribute to magnesium deficiency, making drug-induced hypomagnesaemia an important consideration in clinical practice. Healthcare professionals should be aware of these interactions, particularly in patients taking multiple medications.

Proton pump inhibitors (PPIs), such as omeprazole, lansoprazole, and esomeprazole, are among the most frequently implicated drugs. These medications reduce gastric acid production, which appears to impair magnesium absorption in the intestine. According to MHRA safety guidance, hypomagnesaemia can develop after at least 3 months of treatment, though most cases occur after a year of continuous therapy. The MHRA recommends that doctors consider measuring magnesium levels before starting long-term PPI treatment and periodically during treatment in at-risk patients, particularly those taking digoxin or other drugs that may cause hypomagnesaemia.

Diuretics represent another major drug class associated with magnesium depletion. Loop diuretics (furosemide, bumetanide) and thiazide diuretics (bendroflumethiazide, indapamide) increase urinary magnesium excretion by interfering with renal tubular reabsorption. This effect is dose-dependent and can be clinically significant, particularly in elderly patients or those on high doses.

Other medications that may lower magnesium include:

• Aminoglycoside antibiotics (gentamicin) – cause renal magnesium wasting

• Chemotherapy agents, particularly cisplatin and cetuximab – damage renal tubules

• Calcineurin inhibitors (tacrolimus, ciclosporin) – used in transplant patients

• Amphotericin B – an antifungal that increases urinary losses

Patients at high risk of deficiency on these medications should have their magnesium levels monitored periodically. If deficiency develops, dose adjustment, medication review, or magnesium supplementation may be necessary. If you suspect you've experienced a side effect from any medication, you can report it through the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk). Never stop prescribed medications without consulting your GP or specialist.

Recognising the Symptoms of Low Magnesium

The clinical presentation of magnesium deficiency varies considerably depending on the severity and duration of depletion. Mild deficiency may be asymptomatic or cause only subtle, non-specific symptoms that are easily overlooked. As deficiency progresses, more characteristic features emerge.

Neuromuscular symptoms are often the most noticeable manifestations. These include:

• Muscle cramps, particularly in the legs and feet

• Muscle twitches or fasciculations

• Tremor

• Generalised weakness and fatigue

• Paraesthesia (tingling or numbness), especially around the mouth and in the extremities

These symptoms occur because magnesium is essential for proper nerve signal transmission and muscle contraction. In severe cases, patients may develop tetany (involuntary muscle contractions), positive Chvostek's sign (facial twitching when the facial nerve is tapped), or Trousseau's sign (carpopedal spasm induced by blood pressure cuff inflation).

Cardiovascular manifestations can be serious and include palpitations and cardiac arrhythmias. Particularly concerning are ventricular arrhythmias such as torsades de pointes, which can occur with QT interval prolongation on the electrocardiogram. Low magnesium has been associated with hypertension and cardiovascular risk in observational studies, though a direct causal relationship remains uncertain.

Neuropsychiatric symptoms may include mood changes, anxiety, irritability, and in severe cases, confusion or seizures. Some patients report difficulty concentrating or memory problems. The evidence linking mild magnesium deficiency to all these neuropsychiatric symptoms is not conclusive, and other causes should always be considered.

Other symptoms may include nausea, loss of appetite, and headaches. It is important to note that magnesium deficiency rarely occurs in isolation and is often accompanied by other electrolyte disturbances, particularly hypokalaemia (low potassium) and hypocalcaemia (low calcium), which can complicate the clinical picture and make symptoms more severe.

When to See Your GP About Low Magnesium

Knowing when to seek medical advice is crucial for timely diagnosis and management of magnesium deficiency. You should contact your GP if you experience persistent symptoms suggestive of low magnesium, particularly if you have risk factors such as chronic gastrointestinal conditions, diabetes, or are taking medications known to affect magnesium levels.

Specific situations warranting GP consultation include:

• Persistent or frequent muscle cramps that interfere with daily activities or sleep

• Unexplained fatigue or weakness that does not improve with rest

• Recurrent palpitations or irregular heartbeat

• Tingling, numbness, or other unusual sensations in your extremities

• New or worsening anxiety, mood changes, or difficulty concentrating

• If you are taking PPIs long-term (more than three months) or high-dose diuretics

Your GP can arrange appropriate investigations, typically including serum magnesium measurement alongside other electrolytes (potassium, calcium), and renal function tests. Specialised tests such as urinary magnesium excretion studies are usually arranged in secondary care if needed.

Seek urgent medical attention (call 999 or go to A&E) if you experience:

• Severe muscle spasms or tetany

• Seizures

• Severe chest pain or significant cardiac arrhythmias

• Confusion or altered consciousness

For urgent but non-life-threatening concerns, contact NHS 111 for advice.

Treatment for confirmed magnesium deficiency typically involves addressing the underlying cause, dietary modification to include magnesium-rich foods, and oral magnesium supplementation (commonly magnesium oxide, citrate, or glycerophosphate). Magnesium supplements should be used with caution in patients with kidney impairment due to the risk of magnesium accumulation. Severe deficiency may require intravenous magnesium replacement in hospital. Regular monitoring ensures that levels return to normal and remain stable, preventing recurrence and associated complications.

Frequently Asked Questions