What is the recommended dose of magnesium chloride (MgCl2) to replace a 250 mg magnesium oxide (MgO) regimen, given options of 64 mg and 70 mg delayed release?

Magnesium Chloride Dosing for Replacement of Magnesium Oxide

The 64 mg delayed release magnesium chloride is the appropriate dose to replace 250 mg of magnesium oxide, as it provides comparable elemental magnesium content while potentially offering better bioavailability and tolerability.

Understanding Magnesium Content and Bioavailability

Elemental Magnesium Comparison

• Magnesium oxide (MgO) contains approximately 60% elemental magnesium, meaning 250 mg MgO provides about 150 mg of elemental magnesium 1
• Magnesium chloride (MgCl2) contains approximately 12% elemental magnesium
• Therefore:
  • 64 mg MgCl2 provides approximately 7.7 mg elemental magnesium
  • 70 mg MgCl2 provides approximately 8.4 mg elemental magnesium

Bioavailability Factors

• MgO has poor bioavailability (estimated 4-5% absorption) despite high elemental content 2
• MgCl2 has significantly better bioavailability (estimated 15-20% absorption) 3
• Delayed-release formulations improve absorption by:
  • Providing continuous release throughout the gastrointestinal tract
  • Reducing gastrointestinal side effects
  • Allowing for better absorption in different segments of the intestine 3

Clinical Decision-Making Algorithm

1. Calculate effective magnesium delivery:

   • 250 mg MgO (60% elemental) × 4% bioavailability = ~6 mg absorbed magnesium
   • 64 mg MgCl2 (12% elemental) × 15% bioavailability = ~7.7 mg absorbed magnesium

2. Consider formulation advantages:

   • Delayed-release MgCl2 provides continuous absorption over 6 hours vs. MgO which releases 100% within 1 hour 3
   • Lower risk of gastrointestinal side effects with delayed-release formulation

3. Evaluate patient factors:

   • If patient has normal renal function, either dose (64 mg or 70 mg) is acceptable
   • If patient has impaired renal function (eGFR <30 mL/min), avoid magnesium supplements altogether 4

Important Clinical Considerations

Advantages of Magnesium Chloride

• Better absorbed in acidic environment of stomach compared to MgO 2
• Delayed-release formulation improves tolerability for long-term use 3
• Lower risk of diarrhea and other gastrointestinal side effects compared to higher doses of MgO 1

Monitoring Recommendations

• Target plasma magnesium level should be >0.6 mmol/L (>1.5 mg/dL) 4
• Monitor for signs of magnesium toxicity in patients with impaired renal function:
  • Loss of deep tendon reflexes
  • Muscle weakness
  • Respiratory depression
  • ECG changes 4

Special Considerations

• Patients taking proton pump inhibitors or H2 receptor antagonists may require higher doses of magnesium due to reduced gastric acid production affecting magnesium solubility 2
• Patients with short bowel syndrome may require higher doses or parenteral magnesium 1

Conclusion

The 64 mg delayed-release magnesium chloride formulation provides comparable effective magnesium replacement to 250 mg magnesium oxide with potentially better absorption and tolerability. The slightly higher 70 mg dose would provide marginally more elemental magnesium but is not necessary for equivalent replacement.