![]() These conditions include inflammatory bowel disease, chronic pancreatitis and alcoholism. 1 Extra-renal causesĬonditions that cause malabsorption may lead to decreased gastrointestinal absorption of magnesium. In the presence of hypomagnesaemia, FEMg less than 2% indicates appropriate response to hypomagnesaemia while FEMg greater than 2% indicates renal wasting. Alternatively, the fractional excretion of magnesium (FEMg) on a random urine specimen can be used. A value greater than 1.0 mmol indicates abnormal renal wasting. In the presence of hypomagnesaemia, a 24-hour urine total magnesium less than 0.5 mmol is evidence of an intact renal response to hypomagnesaemia. A 24-hour urine collection can be used to determine the presence or absence of renal magnesium wasting. The causes of hypomagnesaemia are extra-renal or renal ( Table 1). 2 Occasionally the magnesium-loading test is required to confirm magnesium deficiency. Changes in magnesium concentrations for an individual might be significant, even if they remain within the normal range. Magnesium is often not included in routine electrolyte testing, so it is important that clinicians remember to order and monitor it. 1, 2, 3, 7, 8Īs total blood magnesium concentrations do not always reflect total body stores, a high index of suspicion is needed particularly in patients at high risk of magnesium deficiency (see Table 1). 2 This can cause a dilemma particularly in determining the presence or absence of hypomagnesaemia.ĭue to magnesium's wide-ranging functions, magnesium deficiency may be the cause of numerous serious pathologies. 5 One reason for this is that in acidosis magnesium shifts from the intracellular to the extracellular space, whereas in alkalosis the reverse occurs. Blood levels may not reflect total body stores. Caution should be taken in interpreting results from patients who have low total magnesium and low albumin, as they may have normal concentrations of ionised magnesium. The normal range for total magnesium is 0.7-1.0 mmol/L. Most laboratories measure total magnesium measurement of ionised magnesium is not standard practice. 2 Magnesium's relationship with calcium means that patients with hypomagnesaemia may have a low plasma calcium that remains refractory to calcium supplementation until the magnesium deficiency is corrected. 2 Secondly, magnesium is needed for the release and action of parathyroid hormone. In magnesium deficiency the inverse occurs and consequently the intracellular concentration of calcium rises. When the intracellular magnesium concentration is high, calcium transport into the cell and from the sarcoplasmic reticulum is inhibited. Firstly, many calcium channels are dependent on magnesium. Magnesium also affects calcium homeostasis through two mechanisms. 1This can lead to a hypokalaemia which only responds to magnesium replacement. 2 Loss of intracellular potassium also occurs in the renal tubules. Magnesium is needed for the proper functioning of the Na +/K +-ATPase pump, so a deficiency causes an increase in intracellular sodium and allows potassium to leak out of cells. It is needed in energy metabolism, glucose utilisation, protein synthesis, fatty acid synthesis and breakdown, muscle contraction, all ATPase functions, for almost all hormonal reactions, and in the maintenance of cellular ionic balance. Magnesium is involved in over 300 enzymatic reactions. The reabsorption of magnesium and calcium parallel each other in this segment, but the hormonal regulation of magnesium homeostasis is incompletely understood. Situations that abolish the positive luminal charge (for example loop diuretics, hypercalcaemia) will reduce the reabsorption of magnesium. 1, 4 The amount reabsorbed is inversely related to tubular flow. Most reabsorption occurs in the thick ascending limb of the loop of Henle where 65-75% of filtered magnesium is reabsorbed passively down an electrochemical gradient which is actively maintained. 1, 4 The amount reabsorbed can vary, however, from nearly zero to 99.5% depending on the individual's magnesium balance. To maintain homeostasis, the nephrons normally reabsorb more than 96% of the filtered magnesium. 3, 6 Ionised and complexed magnesium are freely filtered at the glomerulus (70% of circulating magnesium). There is a circadian excretory rhythm with more magnesium excretion occurring at night. The kidney's handling of magnesium is more complicated. Approximately 1 mmol is lost in gastrointestinal secretions daily. 4Absorption can range from 24% of ingested magnesium in magnesium replete states to 76% in deficient states. The main site of absorption is the small intestine with smaller amounts absorbed in the colon. Homeostasis depends on the balance between intestinal absorption and renal excretion, with kidney tubules having primary control. Under normal conditions the body maintains constant circulating concentrations of magnesium in the blood. ![]()
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