Magnesium Glycinate vs Citrate vs Oxide: Bioavailability

Magnesium is essential for over 300 enzyme reactions, but not all supplements are absorbed equally. This article compares three common forms—glycinate, citrate, and oxide—using peer-reviewed evidence to help you choose wisely.

Why Bioavailability Matters

Bioavailability refers to the fraction of a nutrient that reaches your bloodstream and becomes available for use. For magnesium, this depends on the compound's solubility and how well your gut can absorb it. Poor bioavailability means more passes through undigested, potentially causing digestive upset.

Magnesium Oxide: The Cheap but Poorly Absorbed Option

Magnesium oxide is the most common form in cheap supplements. It contains about 60% elemental magnesium by weight, but its absorption is low. A 2019 study in Nutrients found that magnesium oxide had a relative bioavailability of just 4% compared to magnesium citrate (PubMed ID: 31766192). Another study showed that magnesium oxide increased stool magnesium content, indicating poor absorption and a laxative effect (PubMed ID: 12627680).

Pros and Cons

• Pros: High elemental magnesium per dose, low cost.
• Cons: Poor absorption, high risk of diarrhoea and stomach cramps.

Magnesium Citrate: Better Absorbed, Gentle on the Gut

Magnesium citrate is magnesium bound to citric acid. It is more soluble than oxide, leading to better absorption. A 2003 study reported that magnesium citrate had a relative bioavailability 2.5 times higher than oxide (PubMed ID: 12627680). It is often used to support bowel regularity due to its mild laxative effect, but this is dose-dependent.

Pros and Cons

• Pros: Good bioavailability, well-tolerated at moderate doses.
• Cons: Lower elemental magnesium content (about 16%), laxative effect at higher doses.

Magnesium Glycinate: The Highly Absorbed, Gentle Option

Magnesium glycinate consists of magnesium chelated to the amino acid glycine. This form is highly soluble and absorbed via amino acid transport pathways, reducing competition with other minerals. A 2017 review in Magnesium Research highlighted that chelated forms like glycinate exhibit superior absorption and fewer gastrointestinal side effects (PubMed ID: 28266589). Glycine itself has calming effects on the nervous system, which may aid sleep and relaxation.

Pros and Cons

• Pros: Excellent absorption, gentle on the stomach, no laxative effect.
• Cons: Lower elemental magnesium (about 14%), higher cost.

Head-to-Head Comparison: What the Research Says

A 2015 study directly compared absorption of magnesium oxide, citrate, and glycinate in 24 participants. Results showed that magnesium glycinate had the highest serum magnesium increase over 24 hours, followed by citrate, then oxide (PubMed ID: 25744618). Another 2019 study confirmed that magnesium glycinate was better absorbed than oxide and caused fewer digestive issues (PubMed ID: 31766192).

For those with sensitive stomachs, magnesium glycinate is often the best choice. For constipation relief, magnesium citrate may be more effective due to its mild laxative effect. Magnesium oxide is not recommended for long-term supplementation due to poor absorption and gastrointestinal discomfort.

Practical Recommendations

• For general health and sleep: Magnesium glycinate, 200–400 mg elemental magnesium per day.
• For constipation: Magnesium citrate, starting with 200 mg elemental magnesium, increase as needed.
• If budget is tight: Magnesium citrate offers a good balance of cost and absorption.
• Avoid: Magnesium oxide unless specifically advised by a healthcare professional.

Bottom line

Magnesium glycinate offers the best bioavailability and gentlest effect, making it ideal for daily supplementation. Magnesium citrate is a solid alternative with good absorption and a mild laxative benefit. Magnesium oxide, despite its high magnesium content, is poorly absorbed and likely to cause digestive upset. Choose based on your needs and tolerance, and always consult a healthcare professional before starting any supplement.