Magnesium forms and bioavailability: oxide, citrate, glycinate, malate

An evidence-grade comparison of the four most common over-the-counter magnesium salts on absorption, tolerability, and the clinical situations in which the choice of form actually matters.

Background

Magnesium supplements are sold as a salt, not as the elemental ion. A 500 mg “magnesium oxide” tablet contains roughly 300 mg of elemental magnesium because oxide has a high elemental percentage by mass; a 500 mg “magnesium glycinate” tablet contains roughly 70 mg of elemental magnesium because the glycine ligand contributes most of the mass. The label number and the elemental number are different numbers, and the bioavailability question — what fraction of the elemental magnesium is actually absorbed — is a third number that depends on the salt form.

Three measurements matter for clinical and consumer-grade supplementation decisions: elemental content per tablet (a chemistry constant), fractional absorption (varies several-fold across salts), and gastrointestinal tolerability (clinically significant, particularly for cathartic forms). This article summarizes what the human-data literature actually supports across the four most common over-the-counter forms.

Fractional absorption: what the data show

The most cited human comparison is Firoz and Graber (2001), who measured 24-hour urinary magnesium excretion as a proxy for absorption after a single 288 mg elemental dose of magnesium oxide, chloride, lactate, or aspartate in healthy adults. Mean fractional absorption was approximately 4 percent for oxide and approximately 28 to 33 percent for the other three forms — a roughly seven-fold difference. The trial was small (n = 16), the absorption proxy was urinary excretion rather than direct intestinal measurement, and the form panel did not include glycinate or malate. Subsequent work has filled in some of the missing forms but the qualitative conclusion has held: magnesium oxide is poorly absorbed in healthy adults relative to organic salts.

Walker et al. (2003), in a randomized double-blind trial in 46 healthy adults, found that magnesium citrate produced higher urinary and salivary magnesium responses than oxide or amino-acid chelate after acute and 60-day supplementation, with citrate emerging as the most bioavailable in their comparison. Coudray et al. (2005), in magnesium-depleted rats using a stable-isotope absorption method, ranked organic salts (gluconate, lactate, aspartate, citrate) above inorganic salts (oxide, sulfate) by approximately 5 to 10 percentage points in absolute fractional absorption. Schuette et al. (1994), in patients with ileal resection where intestinal absorption is compromised, found magnesium diglycinate (bisglycinate) was better absorbed than oxide, plausibly because the glycinate chelate is taken up via dipeptide transport pathways that bypass some of the impaired free-ion absorption.

The aggregate human-data picture across these studies:

• Magnesium oxide: low fractional absorption (4 to 10 percent in human studies), high elemental content per tablet, frequently cathartic at doses above approximately 250 mg elemental.
• Magnesium citrate: substantially higher fractional absorption than oxide, moderate elemental content, mildly laxative at higher doses (the mechanism that makes it useful as a bowel preparation).
• Magnesium glycinate (bisglycinate): absorbed via at least two pathways (free ion and dipeptide), well-tolerated, lower elemental content per tablet, the form most commonly preferred when GI tolerability is the binding constraint.
• Magnesium malate: limited human bioavailability data, plausible absorption similar to other organic salts, marketed for fatigue and fibromyalgia indications where the malate moiety is hypothesized to support cellular energy metabolism but where clinical evidence is thin.

Tolerability and the cathartic dose

Magnesium salts that reach the colon unabsorbed produce an osmotic effect that draws water into the lumen and accelerates transit. This is the mechanism behind magnesium hydroxide (milk of magnesia) and magnesium citrate as bowel preparations. For supplementation purposes, it is the dose-limiting toxicity for most adults: above approximately 350 mg elemental per day from supplements, loose stool becomes common with oxide and citrate. Glycinate appears better tolerated at equivalent doses, although direct head-to-head comparison data are limited and the clinical observation rests substantially on case series and clinical practice rather than randomized trial evidence.

When the choice of form actually matters

For an adult supplementing within the RDA range (roughly 320 mg elemental for women, 420 mg for men) and not symptomatic of magnesium deficiency, the daily total elemental magnesium delivered is the dominant variable. A 200 mg elemental dose of glycinate and a 200 mg elemental dose of citrate will both substantially raise daily intake regardless of which is chosen.

The choice of form becomes consequential in three situations:

1. Documented hypomagnesemia or chronically low intake, where fractional absorption matters because total daily delivery to the body is the endpoint. Oxide is rarely the right choice here despite its high elemental percentage per tablet.
2. Therapeutic indications such as migraine prophylaxis (where the trial evidence is largely on doses of 400 to 600 mg elemental per day), nocturnal leg cramps, or PMS, where higher absorbed doses are needed and tolerability constrains the form selection.
3. Patients with malabsorption or ileal resection, where chelated forms appear measurably better absorbed than inorganic salts (Schuette et al., 1994).

For everyone else, the practical recommendation supported by the data is: prefer an organic salt (citrate, glycinate, or malate) over oxide, choose by tolerability and price within that group, and read the label for elemental content rather than the salt-mass number.