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Magnesium Deficiency
Essentials
- Magnesium is a participant in many metabolic processes. Its concentration in the body is regulated by the kidneys. Normal diet contains sufficient amounts of magnesium, and significant magnesium deficiency in healthy individuals with normal nutritional status is rare.
- Magnesium deficiency is common in hospitalized patients, especially in intensive care. It should be suspected if the patient has compatible symptoms or clinical findings or an underlying factor predisposing to such deficiency.
- The most common symptoms and findings in hypomagnesaemia are neurological or psychiatric.
- Decreased plasma magnesium is a sign of magnesium deficiency.
- Only 1% of total body magnesium is in the extracellular fluid, which impedes the evaluation of magnesium deficiency. Normal plasma magnesium concentration does not rule out deficiency.
Magnesium balance
- 1 mmol = 24 mg elemental magnesium (Mg2+ )
- The body of an adult contains about 1 000 mmol of magnesium, of which
- 1 % is in the extracellular fluid space
- 50-60 % bound in the skeleton
- the rest is inside cells.
- For adults, the recommended daily intake of magnesium is 13-18 mmol (310-420 mg), depending on age and sex. The weight-dependent recommended enteral dose is 0.2 mmol (4.8 mg)/kg/day.
- Diet contains on average about 10-15 mmol (240-360 mg) magnesium, of which 30-40% is absorbed.
- During low-magnesium diet, the absorption from the gastrointestinal channel is increased to two-fold.
- Good sources of magnesium include wholegrains, seeds, nuts, green vegetables, as well as internal organs and meat.
- Magnesium balance is regulated by absorption from the intestinal tract, binding to bone tissue and reabsorption from the kidneys.
- Excretion by kidneys is the most important factor regulating magnesium balance.
- When plasma concentration rises, excretion to urine is increased.
- This so-called renal threshold is close to normal plasma concentration.
- The main causes of hypomagnesaemia are inadequate intake or absorption from the intestinal track, loss through kidneys and transfer into cells or the skeleton.
Causes of magnesium deficiency
- Insufficient absorption in the intestine
- Malabsorption conditions
- Alcoholism (often also associated with sparse intake and impaired absorption)
- Prolonged fluid therapy or parenteral nutrition
- Intestinal wasting
- Vomiting, diarrhoea
- Nasogastric suction
- Intestinal fistula
- Renal wasting
- Excessive alcohol consumption increases the excretion of magnesium into the urine.
- Diabetes with poor control
- Acquired renal diseases (interstitial nephritis, acute tubular necrosis, sequela of renal transplantation)
- Hyperthyroidism
- Hyperaldosteronism
- Congenital conditions and syndromes, such as Bartter or Gitelman syndrome or familial hypomagnesaemia-hypercalciuria-nephrocalcinosis
- Other losses
- Blood transfusion, excessive sweating, burn injuries, pancreatitis, pregnancy and breastfeeding
- Drugs
- Transport to the intracellular space or to bones
- E.g. when severe undernutrition is corrected (refeeding syndrome), after surgery for hyperparathyroidism or during recovery from diabetic ketoacidosis or respiratory acidosis
Main symptoms
- Neurological/psychiatric symptoms
- Muscle weakness
- Tremor
- Muscle cramps
- Tetany
- Increased reflexes
- Seizures
- Depression
- Memory difficulties
- Irritability
- Delirium or psychosis
- Cardiac symptoms
- Therapy-resistant arrhythmias (including atrial fibrillation)
- Disorders of conduction
- Hypomagnesaemia increases the cardiac toxicity of digoxin.
- Symptoms caused by other electrolyte disturbances associated with the deficiency
- Persistent hypokalaemia that is not corrected without the administration of magnesium (magnesium deficiency prevents the function of the sodium-potassium pump; potassium cannot enter the intracellular space but is wasted into the urine)
- Hyponatraemia
- Hypocalcaemia (decreased secretion and end-organ effects of PTH)
- Symptoms usually develop only when the plasma magnesium concentration decreases below 0.5 mmol/l.
Diagnosis
- Suspicion may be raised by the presence of risk factors (longstanding diarrhoea, alcoholism, use of PPI drugs or diuretics) or symptoms (unexplained hypocalcaemia, difficult-to-manage hypokalaemia, neuromuscular symptoms, ventricular arrhythmias).
- Determine plasma magnesium; normal range in adults is 0.71-0.94 mmol/l.
- Low plasma magnesium level is a sign of magnesium deficiency (except in acute stress reaction, when magnesium moves from the extracellular space into cells), but a normal level does not exclude deficiency. However, magnesium deficiency is usually associated with a predisposing factor, such as prolonged diarrhoea or alcoholism.
- Magnesium deficiency may also be assessed through daily (24 hour) excretion in urine (about 5.0 mmol/24 h in normomagnesaemia and about 0.5 mmol/24 h in magnesium deficiency).
- The fractional magnesium excretion in daily urine relative to the simultaneously measured plasma creatinine and urine creatinine concentrations gives a more reliable picture of the mechanism of magnesium deficiency.
- A fractional excretion of more than 3-4% indicates magnesium loss from the kidneys when their function is normal.
- The cause of established hypomagnesaemia is often revealed through patient history.
- If the aetiology of magnesium deficiency is not kidney-related, the magnesium amount in a 24-hour urine sample decreases from 3-5 mmol to less than 1 mmol.
- Magnesium tablets are frequently used without plasma magnesium assay for symptomatic treatment of cardiac arrhythmias and leg cramps. The evidence on the effectiveness of the therapy is, however, deficient . A small, even if unnecessary magnesium supplement in the diet is not harmful if the renal function is normal.
- A diagnosed deficiency is treated.
- Treat the underlying cause if such can be identified. If magnesium deficiency has been caused by diuretic therapy that cannot be discontinued, adding a potassium sparing diuretic in the treatment may help.
- Patients on intravenous nutrition need, as maintenance dose, substitution with 4-8 mmol/day (0.1 mmol/kg/day).
- Correcting the deficiency requires higher dose, in adults usually 20-40 mmol/day.
- Due to the low renal threshold of magnesium (in relation to plasma concentration), it is most efficient to correct the deficiency gradually, over several days.
- Mild deficiency is treated orally. Magnesium (Mg2+ ) administered at a dose of 10-40 mmol/day (approximately 240-1 000 mg/day) is usually an appropriate dose to correct the deficiency in a patient with normal renal function. Several different magnesium salts are available, the Mg2+ concentrations of which vary.
- Severe symptomatic deficiency (plasma magnesium < 0.5 mmol/l) is treated with intravenous infusion, and, as necessary, a rapid corrective bolus can be given if the haemodynamic situation is unstable. For such therapy, see local guidance on acute care.
- The patient should be treated on a high dependency unit (ECG monitoring; conduction disorders, including torsades de pointes ventricular tachycardia, are possible).
- Less urgently, 20-30 mmol of magnesium sulphate mixed with 1 000 ml of 5% glucose can be infused over 12-24 h. The infusion can be continued for 3-5 days until plasma magnesium remains above 0.8 mmol/l.
- In renal failure, magnesium should be administered carefully and plasma magnesium concentration should be closely monitored in order to avoid severe hypermagnesaemia.
- If GFR < 30 ml/min/1.73 m2 , dose reduction by at least 50%.
- When using magnesium products, the following conversion can be used: 1 mmol = 2 meq = 24 mg of elementar magnesium (Mg2+ ) = 240 mg of magnesium sulphate.
References