What is the difference in bioavailability between magnesium oxide and magnesium glycinate?

Magnesium Glycinate Has Significantly Higher Bioavailability Than Magnesium Oxide

Magnesium glycinate has substantially higher bioavailability compared to magnesium oxide, with magnesium oxide having only about 4% absorption rate despite its higher elemental magnesium content. 1

Bioavailability Comparison

Magnesium Oxide

• Extremely poor bioavailability at approximately 4% absorption 1, 2
• Despite having higher elemental magnesium content, very little is actually absorbed 1
• Shows no significant difference from placebo in bioavailability studies 3
• Failed to significantly increase 24-hour urinary magnesium excretion in controlled studies 4

Magnesium Glycinate

• As an amino acid-bound (organic) magnesium compound, it has significantly better absorption 1
• Organic magnesium salts consistently demonstrate superior bioavailability compared to inorganic forms like magnesium oxide 1, 5
• Amino acid chelates like magnesium glycinate facilitate enhanced absorption through different transport mechanisms compared to inorganic salts 5

Clinical Implications

When selecting between these two supplements:

1. For Therapeutic Supplementation:

   • Choose magnesium glycinate when the goal is to effectively increase magnesium levels 1
   • Magnesium oxide may be more appropriate when a laxative effect is desired, as its poor absorption creates an osmotic gradient in the intestines 1

2. Dosing Considerations:

   • Smaller, divided doses throughout the day may improve relative absorption compared to a single large dose 1
   • Taking magnesium at night when intestinal transit is slower may improve absorption 1

3. Side Effect Profile:

   • Magnesium oxide is more likely to cause diarrhea due to its poor absorption and osmotic effects 1
   • Magnesium glycinate is better tolerated by most patients due to reduced gastrointestinal side effects 1

Important Caveats

• Renal function must be considered when selecting any magnesium supplement, as patients with significant renal impairment are at risk of hypermagnesemia 1
• Bioavailability can be affected by various factors including:
  • Dosage amount
  • Administration timing
  • Individual gastrointestinal health
  • Concurrent medications
  • Overall magnesium status 5

While magnesium citrate has shown the highest bioavailability in some studies 3, 4, when specifically comparing magnesium oxide to magnesium glycinate, the glycinate form is clearly superior in terms of absorption and bioavailability.