Magnesium deficiency: risks, diagnosis and supplementation

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Magnesium is essential for life and is necessary for more than 600 enzymatic reactions. It is involved in many functions, such as energy metabolism, nerve and muscle function, bone health, protein and DNA synthesis, blood glucose regulation, maintaining a steady heart rhythm and normal blood pressure, and supporting a healthy immune system.

The kidneys are the main regulator of magnesium balance by urinary excretion or retention, and the nutrient is needed in the diet to combat daily losses via kidneys and sweat.

Hidden deficiency

There is approximately 25g of magnesium in the human body, mostly in tissues, muscles, organs and bone. Less than 1 per cent is in the blood, so serum levels do not accurately reflect total body stores. Deficiency can occur in two ways:

• Acute hypomagnesaemia (low serum magnesium, which can be identified with a blood test)
• Chronic magnesium deficiency (where serum magnesium levels may be in the normal range)

A chronic magnesium deficiency may be masked by a ‘normal’ blood test result and can remain hidden and untreated.

Calcium – and the knowledge that blood levels are not representative of bone calcium levels – has featured in healthcare professional education for decades. Recommending or prescribing a calcium supplement is not reliant upon an abnormal blood test.

Conversely, magnesium has been largely absent from routine healthcare professional training.

Many healthcare professionals are not aware that a chronic deficiency may be present with a normal blood test result. Thus, it is often overlooked.

Who is at risk?

A significant number of people are deficient, including up to 60 per cent of critically ill patients. Inadequate dietary magnesium intake and malnutrition can lead to a deficient status.

Regular high alcohol intake or chronic alcoholism can result in increased renal losses.

“Less than 1 per cent of magnesium is found in the blood, so serum levels do not accurately reflect total body stores.”

Regular and strenuous exercise increases nutritional demands and may also lead to increased magnesium loss through excessive sweating. Hard water contains more dissolved magnesium, but people living in soft water areas have lower levels in their tap water.

Pregnancy and lactation can result in maternal losses if dietary intake is inadequate. This may lead to magnesium deficiency in the mother and baby. Oestrogen deficiency causes increased losses in the urine, which increases the risk of deficiency in post-menopausal women. Chronic magnesium deficiency is more common in the elderly.

Impact of medicines 

Proton pump inhibitors reduce magnesium absorption and there have been case reports of hypomagnesaemia from as soon as three months after starting these medicines. An MHRA alert recommends looking for signs and symptoms of deficiency in those taking PPIs.

Diuretics increase renal losses and cancer chemotherapy, particularly cisplatin, can lead to magnesium deficiency via the kidneys.

Some antimicrobials (e.g. gentamicin, amphotericin B, pentamidine) and immunosuppressants (e.g. ciclosporin, tacrolimus) can also cause deficiency. Long-term IV nutrition or fluid therapy may result in chronic deficiency due to inadequate levels of magnesium in the feed.

Diagnosis

William Osler was a physician who, in his training of doctors, said: ‘Listen to your patient, he is telling you the diagnosis.’ In recent decades, medicine has increasingly become reliant on diagnostic test results. Chronic magnesium deficiency has a plethora of signs and symptoms (see table).

Diet or supplements?

Ideally, all nutrients should come from a healthy diet and supplements should not be needed. Magnesium is found in a wide variety of foods, with rich sources being nuts and green leafy vegetables. However, over the last 60 years, the magnesium content in fruit and vegetables has decreased by 20–30 per cent.

The Western diet now also contains more refined grains and processed food. It is estimated that 80–90 per cent of magnesium is lost during food processing. This means that even with a balanced diet and optimal intake of magnesium-rich foods, dietary magnesium intake may still be inadequate and a supplement needed.

Optimum dosage

The UK magnesium reference nutrient intake for adults is 270mg (10.9mmol) in women and 300mg (12.3mmol) in men per day. However, the data used to form this recommendation is limited and from a time when the magnesium content in foods was higher. The doses in prescribable and OTC supplements vary.

Recommended doses for oral magnesium preparations to correct a deficiency range from 10 to 24mmol a day. If oral intake exceeds daily requirements, gut absorption is reduced; diarrhoea is the main side-effect. Caution is needed with supplementation in those with kidney dysfunction.

Bioavailability varies between magnesium preparations. It is relatively poor with magnesium oxide and greater with magnesium chloride, lactate, citrate, glycinate and aspartate supplements.

A magnesium supplement may still be needed to correct a chronic deficiency, even when there is a normal serum magnesium result. Response to oral supplementation is slow and may take up to 40 weeks to reach a steady state.

Author
• Lisa Jamieson BPharm (Hons), MSc (Clin Pharm), MSc (Nutr Med), MRPharmS is a pharmacist and medical writer with a specialist interest in nutritional medicine. Find her on X: @LoveInYourTummy