Magnesium oxide

A US Supplement Facts panel declares the element, not the salt. Magnesium oxide has a molar mass of 40.3 against magnesium's 24.305, so 60.32% of its mass is magnesium.

A size 00 capsule holds about 730 mg of powder at powder density 0.6–0.8 g/mL; a size 000 about 1,100 mg.

Counted only for capsule labels naming this salt alone. Full method and the corpus-wide figure: label mass-budget audit.

Absorption is a separate question from mass, and a contested one. Below is what each study did, on whom, and what it measured — with the original linked. See the full literature page for why results conflict.

These entries report what each study measured and on whom. They are not advice, not a ranking, and not a claim about any product on this site. Every reference links to its PubMed record so the original can be read in full.

1. Randomized, open-label study of the short-term pharmacokinetics of oral magnesium oxide in healthy volunteers.

   Scientific reports · 2026 · PMID 42091979

   Design

   Randomized, open-label pharmacokinetic study in 24 healthy male volunteers aged 18-45 years, equally assigned to a single oral dose of either 750 mg or 2000 mg magnesium oxide.

   What it measured

   Serum magnesium concentrations at baseline and multiple time points up to 12 hours post-dose (colorimetric assay); pharmacokinetic parameters Cmax, Tmax, and AUC0-12 via non-compartmental analysis; safety via vital signs, laboratory parameters, ECG, and adverse events.

   Limits

   Male volunteers only, no female data; only two dose levels tested (750 mg, 2000 mg) so dose-proportionality across a wider range is not established; terminal elimination parameters (t1/2, AUC0-infinity, CL/F, MRT) could not be estimated within the 12-hour sampling window; single-dose (not chronic) administration; sample size n=24 (12 per dose group).

2. Comparative Clinical Study on Magnesium Absorption and Side Effects After Oral Intake of Microencapsulated Magnesium (MAGSHAPE™ Microcapsules) Versus Other Magnesium Sources.

   Nutrients · 2024 · PMID 39770988

   Design

   Double-blind, randomized, cross-over clinical study in 40 healthy women and men. Participants followed a low-magnesium diet for 7 days and fasted 8 hours, then received a single oral dose of each of four products in cross-over fashion: microencapsulated magnesium (Mg-MS / MAGSHAPE), magnesium oxide (MgO), magnesium citrate (Mg-C), and magnesium bisglycinate (Mg-BG). Blood was drawn by digital puncture before dosing (0 h) and at 1, 4, and 6 hours after each product.

   What it measured

   Plasma magnesium concentration at 0, 1, 4, and 6 hours after oral intake of each of the four products; gastrointestinal side effects (intestinal motility, sensations of gastric heaviness) associated with each product.

   Limits

   Single-dose exposure per product, not chronic supplementation; sample size n=40; a specific proprietary microencapsulation technology was tested, which may not generalize to other encapsulation methods; plasma magnesium was the only absorption marker used (no urinary or tissue measurement).

3. Effects of short-term magnesium supplementation on ionized, total magnesium and other relevant electrolytes levels.

   Biometals : an international journal on the role of metal ions in biology, biochemistry, and medicine · 2022 · PMID 35041150

   Design

   Short-term, 10-day supplementation study in 61 healthy young female adults, comparing three magnesium supplement forms: magnesium oxide (powder/granulate), magnesium citrate, and magnesium carbonate (effervescent tablets).

   What it measured

   Ionized magnesium concentration and its percentage of total magnesium; serum total magnesium concentration; other serum electrolyte levels — each assessed at baseline and after the 10-day intervention.

   Limits

   Healthy young female adults only, no male or older/deficient populations studied; short 10-day intervention with no longer-term follow-up; three specific product formulations were tested, so results may not generalize to other products of the same salts; the abstract does not report the per-arm breakdown of the 61 participants.

4. Dose-Dependent Absorption Profile of Different Magnesium Compounds

   Biological Trace Element Research · 2020 · PMID 30761462

   Design

   Absorption of several magnesium compounds profiled across doses rather than at a single dose.

   What it measured

   Absorption is treated as dose-dependent, so a comparison made at one dose need not hold at another — which is why single-dose head-to-head results disagree.

   Limits

   Compound coverage and sample size vary by arm; read the paper before carrying any single figure across.

5. Timeline (Bioavailability) of Magnesium Compounds in Hours: Which Magnesium Compound Works Best?

   Biological trace element research · 2019 · PMID 29679349

   Design

   Single-dose administration of 400 mg/70 kg magnesium as one of five compounds (magnesium sulfate, magnesium oxide, magnesium acetyl taurate, magnesium citrate, magnesium malate) to Sprague Dawley rats, with a control group, followed by time-dependent tissue and serum sampling.

   What it measured

   Time-dependent magnesium absorption profile (area under the curve) in serum; magnesium tissue penetration and concentration, including brain tissue; behavioral indicators (e.g., anxiety-related behavior) recorded in the animals.

   Limits

   Animal model (Sprague Dawley rats), not humans — extrapolation of pharmacokinetic and behavioral findings to people is not established; single dose only, no chronic-dosing data; per-compound group sample size is not stated in the abstract; behavioral endpoints were measured only in rodents. The tested acetyl taurate is distinct from the unmodified taurate in the chemistry table.

6. Comparison of magnesium status using X-ray dispersion analysis following magnesium oxide and magnesium citrate treatment of healthy subjects

   Magnesium Research · 2012 · PMID 22433473

   Design

   Oral magnesium citrate versus magnesium oxide in healthy subjects; intracellular magnesium measured by X-ray dispersion analysis, alongside platelet function.

   What it measured

   The comparison used intracellular magnesium rather than serum magnesium, which changes what the read-out means.

   Limits

   Healthy subjects, small sample; intracellular magnesium by X-ray dispersion is not a routine clinical measure.

7. Study of magnesium bioavailability from ten organic and inorganic Mg salts in Mg-depleted rats using a stable isotope approach

   Magnesium Research · 2005 · PMID 16548135

   Design

   Eighty male Wistar rats fed a magnesium-depleted diet for three weeks, then randomised into ten groups, each receiving a different magnesium salt labelled with a stable isotope.

   What it measured

   Stable isotopes allow absorption and retention to be traced directly rather than inferred from urine, across ten salts at once — the widest head-to-head comparison in the literature.

   Limits

   Rats, not people, and magnesium-depleted rats at that: depletion itself raises absorption. The breadth of the comparison is its value; the species is its limit.

8. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study

   Magnesium Research · 2003 · PMID 14596323

   Design

   Randomised, double-blind, placebo-controlled parallel study in 46 healthy individuals, 300 mg of elemental magnesium per day, comparing an amino-acid chelate, citrate and oxide.

   What it measured

   Relative bioavailability of the three preparations was compared at an equal elemental dose.

   Limits

   46 participants, healthy, 300 mg per day; “amino-acid chelate” is a class rather than one defined compound, so the chelate arm is not a single substance.

9. Bioavailability of US commercial magnesium preparations

   Magnesium Research · 2001 · PMID 11794633

   Design

   Four commercially available US preparations given to healthy volunteers; bioavailability measured as the increment in urinary magnesium excretion.

   What it measured

   The study set out to test the claim that organic magnesium salts are more easily absorbed than inorganic ones, using urinary excretion as the read-out.

   Limits

   Healthy volunteers, small sample, urinary excretion as a proxy for absorption rather than a direct measurement of retention.

10. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection

    JPEN: Journal of Parenteral and Enteral Nutrition · 1995 · PMID 7815675

    Design

    Comparison of magnesium diglycinate (a chelate) with magnesium oxide in patients who had undergone ileal resection.

    What it measured

    The work followed in vitro and in situ findings suggesting diglycinate is absorbed by a different route than inorganic salts, and tested that in patients with impaired absorption.

    Limits

    Patients after ileal resection are a specific clinical population with poor absorption to begin with; results do not transfer to people with an intact gut.

11. Magnesium bioavailability from magnesium citrate and magnesium oxide

    Journal of the American College of Nutrition · 1990 · PMID 2407766

    Design

    In vitro solubility of 25 mmol of each salt across hydrochloric acid concentrations mimicking achlorhydric to peak acid secretion, plus in vivo gastrointestinal absorbability.

    What it measured

    Magnesium oxide was virtually insoluble across the acid range tested, while citrate dissolved; absorbability was then compared in vivo.

    Limits

    Solubility in a beaker is not absorption in a person; the in vivo arm is small. Published 1990.

12. Effect of magnesium citrate and magnesium oxide on the crystallization of calcium salts in urine

    The Journal of Urology · 1990 · PMID 2299712

    Design

    Seven normal subjects and four patients with recurrent calcium oxalate stones; 486 mg of magnesium per day for two weeks, taken on an empty stomach and again with food.

    What it measured

    Taken on an empty stomach, urinary magnesium rose by only 77–79 mg per day out of 486 mg given — and the increase changed when the dose was taken with food.

    Limits

    Eleven subjects; the endpoint is urinary chemistry in stone formers, not absorption in general. Published 1990.

Compare oxide with other salts

13 published comparisons include magnesium oxide as of 2026-09-13; each row gives the other salt's magnesium fraction and compound mass for 100 mg elemental magnesium from the chemistry model.

• Magnesium oxide vs magnesium citrate: 11.3–16.16% magnesium by mass; 619–885 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium glycinate: 11.83–14.1% magnesium by mass; 709–845 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium malate: 13.2–15.54% magnesium by mass; 644–758 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium aspartate: 8.42% magnesium by mass; 1,188 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium carbonate: 24.78–28.83% magnesium by mass; 347–404 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium gluconate: 5.86% magnesium by mass; 1,706 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium chloride: 11.96–25.53% magnesium by mass; 392–836 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium taurate: 8.85% magnesium by mass; 1,130 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium hydroxide: 41.68% magnesium by mass; 240 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium l-threonate: 8.25% magnesium by mass; 1,212 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium orotate: 7.27% magnesium by mass; 1,376 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium lactate: 12.01% magnesium by mass; 833 mg compound per 100 mg elemental magnesium.
• Magnesium oxide vs magnesium sulfate: 9.86–20.19% magnesium by mass; 495–1,014 mg compound per 100 mg elemental magnesium.

Excipients most common in oxide labels

5 published excipient facets below have the highest frequencies among 124 labels mentioning magnesium oxide in the NIH DSLD records, counted 2026-09-13; each label enters an ingredient count once.

• magnesium stearate: 65 of 124 labels.
• silicon dioxide: 61 of 124 labels.
• cellulose: 54 of 124 labels.
• microcrystalline cellulose: 42 of 124 labels.
• stearic acid: 30 of 124 labels.