Magnesium Metabolism

Magnesium: Basics & Distribution - Mighty Mineral Intro

• Total body Mg: 25g (1000 mmol).
• Distribution:
  • Bone: ~60%
  • Intracellular (muscle, soft tissues): ~39%
  • Extracellular Fluid (ECF): ~1%
• Dietary sources: Green leafy vegetables, nuts, whole grains.
• Daily requirement: ~300-400 mg.

⭐ Magnesium is the second most abundant intracellular cation after potassium, with normal serum levels around 1.7-2.2 mg/dL (0.7-0.9 mmol/L).

Magnesium: Absorption & Excretion - In, Out, Regulated

• Absorption (Intestine):
  • Jejunum & ileum (main sites).
  • Paracellular (passive, major) & transcellular (active, TRPM6/7).
  • ↑ by Vit D; ↓ by phytates, high Ca/PO4.
• Excretion (Kidneys):
  • Filtered, then reabsorbed.
  • Proximal Convoluted Tubule (PCT): 15-20%.
  • Thick Ascending Limb (TAL): 60-70% (major).
  • Distal Convoluted Tubule (DCT): 5-10% (active, TRPM6).
  • ↑ excretion: Loop/thiazide diuretics, hypercalcemia.

⭐ The thick ascending limb (TAL) of the loop of Henle is the major site for renal magnesium reabsorption (60-70%), followed by the proximal convoluted tubule (15-20%). oka

Magnesium: Key Functions - Cellular Powerhouse

• Vital intracellular cation.
• Cofactor for >300 enzymes:
  • Energy (ATP) metabolism: Kinases, ATPases.
  • DNA/RNA synthesis & stability.
• Nerve conduction, muscle function (Ca²⁺ channel blocker).
• Maintains cardiac rhythm.
• Blood glucose control.

⭐ Magnesium is a crucial cofactor for over 300 enzymes, particularly those involving ATP metabolism (e.g., kinases) and nucleic acid synthesis.

Hypomagnesemia: Causes & Care - Deficiency Dangers

• Definition: Serum Mg < 1.8 mg/dL (or < 0.7 mmol/L).
• Causes:
  • ↓ Intake/Absorption: Malnutrition, alcoholism, PPIs, malabsorption (Crohn's).
  • ↑ Renal Loss: Diuretics (loop/thiazide), Gitelman/Bartter, nephrotoxins.
  • GI Losses: Diarrhea, vomiting.
  • Redistribution: Refeeding syndrome.
• Clinical Features (Dangers):
  • Neuromuscular: Weakness, tremors, tetany (Chvostek's/Trousseau's +ve), seizures.
  • Cardiac: Arrhythmias (Torsades de Pointes), ECG: ↑PR, ↑QT intervals.
  • ⭐ > Hypomagnesemia is a critical cause of refractory hypokalemia (due to increased renal potassium wasting via ROMK channels) and hypocalcemia (due to impaired PTH secretion and end-organ resistance).
• Management (Care):
  • Treat underlying cause.
  • Oral Mg (mild/asymptomatic).
  • IV MgSO₄ (severe/symptomatic): Torsades 1-2g IV; Seizures 1-2g IV.
  • Monitor DTRs, respiratory rate, urine output.

Hypermagnesemia: Causes & Care - Toxicity Troubles

Causes:

• ↓ Renal Excretion: Acute Kidney Injury (AKI), Chronic Kidney Disease (CKD) (most common).
• ↑ Exogenous Intake:
  • Iatrogenic: IV MgSO₄ (eclampsia, asthma).
  • Oral: Mg-antacids/laxatives (esp. with CKD).
• Other: Lithium, hypothyroidism, Addison's, tumor lysis.

Clinical Manifestations:

• Neuromuscular:
  • 4-7 mEq/L: ↓DTRs (earliest sign), muscle weakness, lethargy.

  ⭐ The earliest sign of hypermagnesemia is often the loss of deep tendon reflexes (DTRs), typically occurring at Mg levels of 4-7 mEq/L.

  • 7-10 mEq/L: Areflexia, somnolence.
  • >10-12 mEq/L: Flaccid paralysis, respiratory depression.
• Cardiovascular:
  • Hypotension, bradycardia.
  • ECG: ↑PR interval, ↑QRS duration, ↑QT interval, AV block.
  • Cardiac arrest (>15 mEq/L).
• Other: Nausea, vomiting, flushing.

Management:

• Stop all Mg intake.
• Antagonism: IV Calcium Gluconate (10-20ml of 10% solution).
• Enhance Elimination:
  • IV Saline + Furosemide.
  • Hemodialysis (HD) for severe cases or renal failure.
• Supportive: Mechanical ventilation.

High‑Yield Points - ⚡ Biggest Takeaways

• Normal serum Mg²⁺: 1.7-2.2 mg/dL.
• Crucial cofactor for ATP-dependent enzymes; modulates neuromuscular excitability and cardiac function.
• Absorption: Jejunum & ileum; Excretion: Renal, mainly via TAL & DCT reabsorption.
• Hypomagnesemia: Neuromuscular hyperexcitability (tetany, Torsades), hypokalemia, hypocalcemia. Caused by diuretics, PPIs, alcohol.
• Hypermagnesemia: Due to renal failure or excess intake. Leads to neuromuscular depression (↓DTRs), hypotension, bradycardia.
• Treat severe hypermagnesemia with IV Calcium gluconate for cardiac protection.