Why is magnesium oxide so commonly used if absorption is poor?

Magnesium oxide is significantly cheaper than chelated forms and has high elemental magnesium by weight. Many low-cost supplements use it to display high magnesium numbers on the label while delivering little to the body.

What is the absorbed dose difference between glycinate and oxide?

At 400 mg elemental magnesium, glycinate delivers approximately 320 mg absorbed, while oxide delivers approximately 16 mg. This 20x gap makes oxide effectively non-functional for serum magnesium support at standard doses.

Can magnesium glycinate cause diarrhea?

At normal doses (200–400 mg elemental/day), magnesium glycinate is well-tolerated. Unlike oxide, it does not have significant osmotic laxative activity. Very high doses may cause loose stools in some individuals.

Moderate Evidence

Magnesium Glycinate vs Oxide: An 80% vs 4% Absorption Gap

Magnesium glycinate absorbs at over 80% vs approximately 4% for magnesium oxide — a 20x difference that makes oxide largely ineffective for systemic magnesium support.

Updated 2026 · Reviewed by Dr. Brennan Commerford, D.C.

Our Recommendation

Magnesium Glycinate

Magnesium glycinate delivers over 80% absorption with excellent tolerability, versus approximately 4% for magnesium oxide — the most common cheap filler form.

Magnesium Glycinate vs Magnesium Oxide

FormulaForge Choice

Magnesium Glycinate

Pros

+Over 80% bioavailability — among the highest of any magnesium form
+Gentle on the GI tract — does not cause the laxative effect of oxide
+Glycine amino acid co-factor may support relaxation and sleep quality

Cons

-Higher cost per milligram of elemental magnesium than oxide
-Lower elemental magnesium percentage by weight than oxide

Best For

Anyone seeking to effectively support serum magnesium levels for muscle function, sleep, and stress resilience.

S1 bioavailability tier — over 80% absorption

Avoid

Magnesium Oxide

Pros

+Very low cost per gram
+High elemental magnesium percentage by weight (60%)
+Effective as an osmotic laxative

Cons

-Only approximately 4% absorption — most is excreted
-Causes significant GI distress and osmotic diarrhea at magnesium-relevant doses
-Poorly suited for systemic magnesium repletion

Best For

Not recommended for systemic magnesium support. Appropriate only as a laxative, not a magnesium supplement.

S4 bioavailability tier — approximately 4% absorption

Frequently Asked Questions

Why is magnesium oxide so commonly used if absorption is poor?: Magnesium oxide is significantly cheaper than chelated forms and has high elemental magnesium by weight. Many low-cost supplements use it to display high magnesium numbers on the label while delivering little to the body.
What is the absorbed dose difference between glycinate and oxide?: At 400 mg elemental magnesium, glycinate delivers approximately 320 mg absorbed, while oxide delivers approximately 16 mg. This 20x gap makes oxide effectively non-functional for serum magnesium support at standard doses.
Can magnesium glycinate cause diarrhea?: At normal doses (200–400 mg elemental/day), magnesium glycinate is well-tolerated. Unlike oxide, it does not have significant osmotic laxative activity. Very high doses may cause loose stools in some individuals.

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96% of Magnesium Supplements Barely Absorb — Here's Why

Most people buy magnesium oxide. Only 4% reaches your bloodstream. The rest becomes an osmotic laxative. Our magnesium guide ranks all 8 forms by actual bioavailability and which goal each serves best.

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FormulaForge formulates and sells supplements containing the ingredients discussed on this page. Our formulary recommendations are based on peer-reviewed bioavailability research. All cited studies are independently verifiable.