Magnesium is an important nutrient involved in a variety of processes in the body. Acting as a cofactor in more than 300 enzyme systems, magnesium assists in the regulation of protein synthesis, bone development, blood pressure, blood glucose levels, and energy production. Magnesium also plays a role in the active transport of calcium and potassium ions across cell membranes; a process that is important for nerve impulse conduction, muscle contraction, and normal heart rhythm.
Magnesium is an abundant mineral, with an adult body containing approximately 25g. Almost half of the body’s magnesium can be found in the bones and half in soft tissues. Less than 1% of total body magnesium is found in blood serum and these serum levels are kept within the range of 0.75 to 0.95mmol/L. This is largely controlled by the kidneys, which typically excrete around 120mg of magnesium into the urine each day.
Hypomagnesaemia, defined as a serum magnesium level of less than 0.70mmol/L, may occur as a result of renal or gastrointestinal loss. Low intakes do not immediately produce symptoms as renal excretion quickly reduces in order to maintain constant blood levels. However, deficiency may develop if low dietary intake is maintained over extended periods. Some medications and medical conditions can also interfere with the magnesium balance by reducing absorption or increasing excretion.
Due to the wide range of systems that depend upon magnesium, symptoms of hypomagnesaemia vary considerably. Table 1 describes some of the possible signs and symptoms.
Table 1. Symptoms of hypomagnesaemia
|Nucleoside reverse transcriptase inhibitors||Metabolites of active phosphorylated NRTI inhibit viral reverse transcriptase and viral DNA synthesis, preventing HIV replication|
|Neuromuscular||Weakness, muscle fasciculation, tremor, tetany|
|Central nervous system||Personality changes, disorientation, psychosis, convulsions, stupor, coma|
|Cardiovascular||Prolongation of QT interval|
Chronic low magnesium levels are more likely to affect the following patient groups due to increased excretion or impaired absorption:
- People with gastrointestinal diseases, such as Crohn’s disease and coeliac disease
- People with type 2 diabetes
- People with long-term alcoholism; and
- Older people.
Hypermagnesaemia is uncommon as excess magnesium is efficiently excreted by the kidneys of healthy individuals. This condition is generally only seen in patients with renal failure who have an excessive dietary intake of magnesium. Signs and symptoms vary depending on the degree of magnesium excess:
- 1.1-2.5mmol/l: bradycardia, flushing, sweating, sensation of warmth, nausea and vomiting
- 3 mmol/l: drowsiness and decreased deep-tendon reflexes
- 5-7.5 mmol/l: flaccid paralysis and decreased pulse rate and QRS intervals
- >7.5 mmol/l: respiratory distress and asystole.
Sources of Magnesium:
Magnesium is present in many foods (both naturally and fortified). Good sources of dietary magnesium include legumes, nuts, seeds, whole grains, green leafy vegetables, fortified cereals and dairy products. Magnesium is also available in many dietary supplements. Salt forms of magnesium that are more easily absorbed by the body are the aspartate (Magmin®), citrate, lactate, and chloride salts.
Less readily absorbed salts of magnesium are utilised in some laxatives (milk of magnesia) and bowel preparation products (PicoPrep®), owing to their osmotic laxative effect. Magnesium hydroxide is also included in some heartburn and indigestion products (Mylanta®) to neutralise gastric acid.
Daily magnesium requirements depend upon the age and sex of an individual; average daily recommended amounts and the recommended upper limits of supplemental magnesium are listed below.
|Life Stage||Daily Magnesium Requirement||Maximum Supplemental Dose|
|Birth to 6 months||30mg||Not established|
|Infants 7–12 months||75mg||Not established|
|Children 1–3 years||80mg||65mg|
|Children 4–8 years||130mg||110mg|
|Children 9–13 years||240mg||350mg|
|Teen boys 14–18 years||410mg||350mg|
|Teen girls 14–18 years||360mg||350mg|
High intakes of magnesium from dietary supplements and medications can cause diarrhoea, nausea, and abdominal cramping; extremely high intakes can result in irregular heartbeat and cardiac arrest. Magnesium that is naturally present in food is not harmful however, the dose of supplemental magnesium should not exceed the recommended maximum unless recommended by a healthcare provider.
Magnesium supplements can interact with some medicines and should be avoided in some patient groups. Below are some examples:
- Bisphosphonates, used to treat osteoporosis, are not well absorbed when administered with high amounts of magnesium. It is recommended to take magnesium rich medications at least two hours before or after a bisphosphonate.
- Antibiotics, such as tetracyclines (doxycycline, minocycline) and quinolones (ciprofloxacin, norfloxacin), can form insoluble complexes with magnesium and lead to poor absorption of the antibiotic. It is recommended that these antibiotics be taken at least two hours before or after a magnesium containing supplement.
- Diuretics can either increase or decrease the loss of magnesium through urine. Loop diuretics (frusemide, bumetanide) and hydrochlorothiazide increase urinary excretion of magnesium; potassium sparing diuretics (amiloride, spironolactone) reduce magnesium excretion.
- Proton pump inhibitors, used to ease symptoms of acid reflux or peptic ulcer disease (esomeprazole, omeprazole, pantoprazole) can cause low blood levels of magnesium when taken over a long period of time.
- Very high doses of zinc supplements can interfere with the body’s ability to absorb and regulate magnesium.
- Calcium-channel blockers (nifedipine, felodipine, diltiazem, verapamil), often used to treat hypertension, may be potentiated by magnesium. Patients should be monitored as they may be more likely to suffer from side effects such as dizziness, nausea and fluid retention.
Research is continuing in an effort to understand how magnesium affects health. Here are some examples of what this research has shown.
High blood pressure and heart disease
Magnesium supplements have been shown to achieve small, but statistically significant, reductions in blood pressure. Some studies show that people who have more magnesium in their diets have a lower risk of some types of heart disease and stroke. Further studies are required in order to determine appropriate doses for this effect.
Type 2 diabetes
People with higher amounts of magnesium in their diets tend to have a lower risk of developing type 2 diabetes. Magnesium helps the body break down sugars and might help reduce the risk of insulin resistance. More research is required to determine whether magnesium supplements might help people who already have type 2 diabetes control their disease.
Magnesium is important for maintaining healthy bone mineral density, which is imperative in reducing the risk of bone fractures and osteoporosis. Increasing magnesium intake may help older women improve their bone mineral density. More research is needed to better understand whether magnesium supplements can help reduce the risk of osteoporosis or treat this condition.
People who have migraine headaches sometimes have low levels of magnesium in their blood and other tissues. Several small studies found that magnesium supplements can modestly reduce the frequency of migraines. However, as the doses recommended for this purpose exceeds the upper limits of the recommended daily intakes, this should only be initiated under the care of a healthcare provider.
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- Memorial Sloan Kettering Cancer Center. Magnesium. New York: Memorial Sloan Kettering Cancer Center; 2015.
- Mims Online [Internet]. St Leonards: UBM Medica; 2015.
- Office of Dietary Supplements. Magnesium fact sheet for health professionals. National Institute of Health: Bethesda. 2013.
- Pharmaceutical Society of Australia. Australian pharmaceutical formulary and handbook: the everyday guide to pharmacy practice. 20th ed. Deakin West: Pharmaceutical Society of Australia; 2006.