Magnesium Forms Compared: Glycinate, Citrate, Oxide, Malate, and Threonate — Bioavailability and Outcomes

Magnesium is one of the most-used minerals in supplement form and one of the most confusing. Five common salts dominate the shelf: oxide, citrate, glycinate (bisglycinate), malate, and L-threonate. The science of which form to pick is less dramatic than marketing suggests, but the differences are real and clinically actionable in specific situations.

Why form matters — at all

The cation magnesium is what raises serum and intracellular concentrations. The anion (oxide, citrate, glycinate, etc.) determines elemental magnesium content per gram, solubility, gastrointestinal absorption fraction, and the local osmotic effect in the gut. Two factors drive most clinical differences: percentage of elemental magnesium delivered and the laxative effect of poorly-absorbed salts [1].

Magnesium oxide

The cheapest and most concentrated form (about 60% elemental Mg by weight). Its absorption fraction is the lowest of the common salts — roughly 4% in classic balance studies — because of low aqueous solubility [1]. The unabsorbed fraction acts osmotically in the colon, which is why oxide is the form used as a laxative (milk of magnesia, prep solutions). For replacing a deficit, much of the dose passes through.

Magnesium citrate

About 11% elemental magnesium; high solubility; absorption fraction around 25–30% in human bioavailability comparisons. Citrate is the workhorse "bioavailable" form, well-tolerated up to a few hundred mg of elemental Mg per day, and a reasonable default for replacing dietary inadequacy [2]. Higher doses produce loose stools.

Magnesium glycinate (bisglycinate)

Magnesium chelated to two glycine molecules. Elemental content around 14%. Absorption is comparable to citrate, but the chelated form is generally associated with less gastrointestinal upset and is the form most often used in sleep and anxiety trials. The reduced GI burden is the practical advantage rather than dramatically higher absorption [3].

Magnesium malate

Magnesium bound to malic acid (a Krebs cycle intermediate). About 6% elemental Mg. Bioavailability is comparable to citrate. Malate is sometimes used in fibromyalgia protocols on the theory that malic acid supports muscle energetics; controlled trial evidence is weak and inconsistent. The practical case is well-tolerated GI profile, similar to glycinate.

Magnesium L-threonate

The salt with the smallest market share but the largest marketing claims, based on a single preclinical paper showing increased rat brain magnesium with this form. Elemental content is about 8%. The one randomised human cognition trial (MMFS-01) reported modest improvements in cognitive composite scores in older adults with subjective memory complaints — but the trial was small, industry-funded, and the effect size was inconsistent across subscales [4]. It costs roughly 5–10× more per elemental milligram than citrate.

What this means in practice

For correcting dietary inadequacy or the common deficiency of older adults on diuretics: citrate or glycinate is fine. For sleep or anxiety trials, glycinate is the literature-default. For people who develop loose stools on citrate, glycinate or malate are better-tolerated alternatives. For constipation, oxide is the cheap and effective laxative. For cognitive use, the threonate hype outruns the evidence; if budget is unlimited, a trial is reasonable, but expectations should be modest and the same outcome may be reachable with citrate or glycinate.

Dose and safety

The NIH ODS Tolerable Upper Intake Level for supplemental magnesium in adults is 350 mg of elemental magnesium per day [5]. Higher doses are not toxic in healthy people but produce diarrhea, and risk is real in chronic kidney disease (renal magnesium clearance is reduced). Magnesium also reduces absorption of fluoroquinolone and tetracycline antibiotics, bisphosphonates, and levothyroxine — separate dosing by at least 4 hours.

How to read a magnesium label

Two label-reading habits make magnesium shopping less confusing. First: distinguish "milligrams of magnesium" from "milligrams of magnesium salt." A 500 mg capsule of magnesium oxide is about 300 mg of elemental magnesium; the same weight of magnesium citrate is about 55 mg. The serving panel on US-labelled products lists elemental magnesium; on EU products, the salt weight is sometimes given separately. Second: chelated forms (glycinate, bisglycinate, malate) are often more tolerable but no more strikingly absorbed than citrate in head-to-head trials. The choice is driven by GI tolerability, cost, and trial precedent for the specific indication (sleep/anxiety → glycinate; constipation → oxide or citrate; replacement → citrate; cognition trial precedent → threonate, expensive). For most adults supplementing a marginal diet, 100–200 mg of elemental magnesium per day from any well-tolerated form is sufficient.