About half the small amount of magnesium (Mg) in the body is found in bones. It is important in the functioning of over 300 enzymes, and acts in a similar way to calcium in the nervous system. Alterations in the level of magnesium in the blood also cause a change in the level of calcium. Excess is removed from the body through the kidneys.
Blood tests can be performed to determine the amount of magnesium present. The normal range is between 0.7 and 1.0 mmol/L (1.7-2.3 mg/100 mL).
High levels occur in long-term kidney failure, malabsorption syndromes, diarrhoea, diabetic ketoacidosis, malnutrition, vomiting, and prolonged intravenous therapy. Medications such as diuretics, amphotericin, gentamicin, laxatives, cisplatin and cytotoxics may also be responsible. Low levels may cause an irregular heartbeat.
Urinary levels of magnesium may be important in some diseases. The normal level in urine is 2.5 to 8.0 µmol/day (20 to 180 mg/day) and low levels may occur in malabsorption syndromes (inability to absorb magnesium from gut), severe body fluid loss, alcoholism, diabetic ketoacidosis (severe form of uncontrolled diabetes), liver cirrhosis, primary aldosteronism (Conn syndrome) and long-term kidney failure.
The level of magnesium in faeces may also be measured in the investigation of unexplained diarrhoea. A high level may indicate an abuse of laxatives containing magnesium, or hypomagnesaemia (lack of magnesium in blood) due to loss of magnesium into gut. The normal level in faeces is 45 mmol/L or less than 15 mmol/day.
The salts of magnesium (eg. magnesium hydroxide) are widely used in antacids.