magnesium

see also:

• hypomagnesaemia
• hypermagnesaemia
• eclampsia

• magnesium is the fourth most abundant mineral present in the human body. More than 600 enzymes require it as a co-factor and almost 200 require it to activate critical processes in the body.
• Mg is the 2nd most abundant intracellular cation, playing major roles in:
  • energy transfer, storage, and use including glycolysis, oxidative phosphorylation
    • Mg is bound to ATP inside the cell, thus shifts in intracellular magnesium concentration may help to regulate cellular bioenergetics, such as mitochondrial respiration
  • DNA and protein synthesis
  • molecular stabilizer of RNA, DNA, and ribosomes
  • protein, carbohydrate, and fat metabolism
  • maintenance of normal cell membrane function
  • regulation of parathyroid hormone (PTH) secretion
• almost all enzymatic processes using phosphorus as an energy source require magnesium for activation
• extracellular Mg blocks neurosynaptic transmission by interfering with the release of acetylcholine
• there appears to be a strong link between low magnesium levels (<18mg/L) and high amounts of a genotoxic amino acid called homocysteine (esp. if also have low B12 and folate levels) making people more susceptible to Alzheimer's and Parkinson's disease, gastrointestinal diseases, a range of cancers, and diabetes ¹⁾
• a low intake of magnesium (less than 300mg per day) can increase the risk of many diseases
• there appears to be a protective effect of higher dietary magnesium (550mg/day compared to “normal” intake of 300mg/d) on reducing brain aging, reduction in brain size and dementia ²⁾
• magnesium-rich foods include wholegrains, dark green leafy vegetables, nuts, beans and dark chocolate

• total body magnesium content of an average adult is 25 g, or 1000 mmol
  • 60% in bone, 20% in muscle, 20% in soft tissue and liver
  • only 1% is extracellular
• normal plasma magnesium concentration is 1.7-2.1 mg/dL (0.7-0.9 mmol, or 1.4-1.8 mEq/L)
  • 70% in the plasma is ionised or compexed to filtrable ions and thus available to renal excretion
  • 20% is protein bound
• daily dietary requirement:
  • 0.15-0.2 mmol/kg/d for healthy adults
• control of body magnesium levels:
  • GIT absorption
    • mainly in small intestine
    • absorption rate depends upon dietary load - usually 30% absorbed but ranges 25-80%
    • absorption may be decreased with:
      • GIT phosphates or fat
      • high calcium intake
    • absorption may be increased by:
      • parathyroid hormone (PTH)
      • corticosteroids
      • vitamin D
  • renal excretion
    • limited hormonal control
    • 60-70% is passively reabsorbed in the thick ascending limb of loop of henle
    • 15-25% is reabsorbed in proximal convoluted tubule
    • 5-10% is actively reabsorbed in distal convoluted tubule
  • NB. unlike calcium, there is little exchange between bone and plasma and thus access to bone reservoirs takes weeks to correct low plasma levels

• paediatric asthma:
  • paediatric dose in severe acute asthma:
    • load dose infusion 50mg/kg of magnesium sulphate infused over 20min:
      • 0.1ml/kg of 50% magnesium sulphate diluted to 20ml with 0.9% saline and infuse at 1ml/min for 20min.
    • maintenance infusion not usually given for paediatric asthma unless patient is being transferred to a critical care unit:
      • 0.12mmol/kg/hr (to max. 8mmol/hr) = 0.06ml/kg/hr (max. 4ml/hr) of 50% solution (2mmol/ml)
      • aim for target serum magnesium level of 1.5-2.5mM
• pre-eclampsia and eclampsia
  • NSW NETS - Magnesium sulphate for eclampsia (pdf)
• torsade de pointes VT
• ethanol withdrawal seizures
• constipation
• ethanol-induced atrial fibrillation - 20mmol in 100ml 5% dextrose over 30-60min, can give further 20mmol over next 2hrs

• Aust Presc 2007 - Magnesium: the forgotten electrolyte