How Magnesium Is Absorbed: Glycinate vs Citrate Bioavailability Explained
Updated 11 April 2026
Not all magnesium is created equal. The compound magnesium is bonded to determines how well your body absorbs it. This page explains the science of magnesium absorption in plain English: chelation, organic vs inorganic salts, bioavailability percentages, and why a 500 mg oxide capsule gives you less usable magnesium than a 500 mg glycinate capsule.
What Is Bioavailability?
Bioavailability is the proportion of a substance that enters your bloodstream when you consume it. For magnesium supplements, it answers the question: "Of the magnesium I swallow, how much does my body actually get to use?"
A supplement with 30% bioavailability means that for every 100 mg of elemental magnesium you take, about 30 mg reaches your bloodstream. The other 70 mg passes through your digestive tract and is excreted. This is why the form of magnesium matters so much: a cheaper form with low bioavailability may deliver less magnesium than a more expensive form with high bioavailability.
Organic vs Inorganic Magnesium
Organic salts (better absorbed)
Magnesium bonded to organic (carbon-containing) compounds. Higher water solubility and better absorption through the intestinal wall.
- - Glycinate (bonded to glycine)
- - Citrate (bonded to citric acid)
- - Malate (bonded to malic acid)
- - Taurate (bonded to taurine)
- - Threonate (bonded to threonic acid)
- - Orotate (bonded to orotic acid)
Inorganic salts (poorly absorbed)
Magnesium bonded to inorganic compounds. Lower water solubility, poor absorption, and more GI side effects.
- - Oxide (bonded to oxygen)
- - Sulfate (Epsom salt)
- - Carbonate (like chalk)
- - Hydroxide (milk of magnesia)
How Glycinate Is Absorbed
Magnesium glycinate is an amino acid chelate. The magnesium atom is bonded to two glycine molecules, creating a stable, protected complex. This chelate structure gives glycinate a unique absorption advantage.
Instead of relying solely on the standard mineral absorption pathway, glycinate can also be absorbed through amino acid transport channels in the small intestine. These dipeptide and amino acid transporters are separate from mineral transporters and are not saturated by other minerals. This means glycinate absorption is less affected by competing minerals (calcium, iron, zinc) than other forms.
The chelate structure also protects the magnesium from binding to phytates and other anti-nutrients in food that would otherwise reduce absorption. Once inside the intestinal cell, the magnesium is released from the glycine and enters the bloodstream.
Estimated bioavailability: 25 to 35%
Based on available comparative studies (Lindberg 1990, Walker 2003)
How Citrate Is Absorbed
Magnesium citrate is a salt of magnesium and citric acid. It has excellent water solubility, which means it dissolves readily in the stomach's acidic environment. This solubility is the primary driver of its good bioavailability.
Once dissolved, the magnesium ions are absorbed through the intestinal wall via two pathways: paracellular transport (between cells, driven by concentration gradient) and transcellular transport (through cells, via TRPM6 and TRPM7 ion channels). Citric acid may also enhance absorption by acidifying the local environment.
However, citrate's osmotic effect means that some of the dissolved magnesium draws water into the intestinal lumen before it can be absorbed. At higher doses, this osmotic effect increases the speed of intestinal transit, reducing the time available for absorption. This is why citrate's effective bioavailability may be slightly lower than glycinate's at higher doses.
Estimated bioavailability: 25 to 35%
May be slightly lower at higher doses due to osmotic laxative effect
Why Oxide Is So Poorly Absorbed
Magnesium oxide has the highest elemental magnesium percentage of any form (about 60% by weight), which is why manufacturers like it: they can put "500 mg magnesium" on the label for a single capsule. But this is misleading because the bioavailability is only 4 to 5%.
Oxide has extremely low water solubility. It does not dissolve well in stomach acid, meaning most of it passes through the digestive tract as an unabsorbed powder. Studies by Firoz and Graber (2001) and Lindberg et al. (1990) both confirmed that oxide is significantly less bioavailable than organic forms.
The math: A 500 mg magnesium oxide capsule contains about 300 mg of elemental magnesium. At 4% bioavailability, only 12 mg actually reaches your bloodstream. A 500 mg glycinate capsule contains about 70 mg elemental, but at 30% bioavailability, about 21 mg is absorbed. The cheaper oxide delivers less usable magnesium.
Factors That Affect Absorption
Food
Taking magnesium with food generally improves absorption. Food slows gastric emptying, increasing contact time with intestinal absorption sites. Protein and fat in the meal may further enhance uptake.
Vitamin D status
Vitamin D enhances intestinal magnesium absorption by upregulating TRPM6 channels. Low vitamin D status can reduce magnesium absorption. Conversely, magnesium is needed to activate vitamin D. Correcting both together is more effective than correcting either alone.
Stomach acid levels
Adequate stomach acid is needed to dissolve magnesium salts. People taking PPIs (omeprazole, etc.) have reduced stomach acid, which can impair absorption. This is especially problematic for oxide, which requires more acid to dissolve.
Dose size
Absorption is inversely related to dose. Your body absorbs a higher percentage from smaller doses (50 to 100 mg) than from large doses (400+ mg at once). Splitting daily intake into 2 or 3 doses maximises total absorption.
Gut health
Conditions that damage the intestinal lining (Crohn's disease, celiac disease, chronic diarrhoea) reduce magnesium absorption surface area. People with GI conditions may need higher supplement doses and should work with their doctor.
Competing minerals
High doses of calcium, iron, or zinc taken at the same time may compete with magnesium for absorption. This competition is less relevant for glycinate (which uses amino acid transport) than for citrate or other salt forms.
Absorption Comparison: All Forms
| Form | Type | Solubility | Bioavailability | Elemental % |
|---|---|---|---|---|
| Glycinate | Organic | High | 25-35% | ~14% |
| Citrate | Organic | High | 25-35% | ~16% |
| Malate | Organic | High | 20-30% | ~12% |
| Taurate | Organic | High | 20-30% | ~9% |
| Threonate | Organic | High | High* | ~8% |
| Chloride | Inorganic | High | 20-30% | ~12% |
| Oxide | Inorganic | Very low | 4-5% | ~60% |
| Sulfate | Inorganic | Moderate | Low | ~10% |
*Threonate crosses the blood-brain barrier; systemic bioavailability differs from brain bioavailability.
Disclaimer: Bioavailability estimates are based on published research and may vary by individual, brand, and testing methodology. This content is for educational purposes only.