Abstract

Background: Magnesium is supplied as several salts that differ in solubility and elemental content. The choice of salt and format may affect absorption and gut tolerability.

Objective: To review human studies on the absorption and gastrointestinal tolerability of magnesium salts and formats, with reference to powder sachet and effervescent products.

Methods: PubMed was searched on 24 October 2026 using ten phrasings, restricted to human clinical evidence. Human studies comparing magnesium salts or formats, or reporting gastrointestinal effects at supplement-type doses, were included. Records were assessed from their published abstracts.

Results: Eight records were included: one systematic review and seven human studies, with 12 to 46 participants where reported. Three studies comparing magnesium citrate with magnesium oxide each reported greater absorption for citrate (PMIDs 2407766, 32162607), or for the organic forms citrate and amino-acid chelate (PMID 14596323). In one crossover study, an effervescent magnesium oxide tablet gave a larger rise in urinary magnesium than capsules of the same salt (PMID 20862466). In a 100 mg crossover in patients with ileal resection, absorption from magnesium diglycinate and magnesium oxide did not differ overall, but the chelate was better tolerated by all patients (PMID 7815675). A 2024 crossover reported fewer gastrointestinal side effects with a proprietary microencapsulated magnesium ingredient than with the other sources tested (PMID 39770988). Gastrointestinal tolerability was addressed in few records.

Conclusion: In the human studies, magnesium citrate was absorbed more than magnesium oxide, and an effervescent magnesium oxide tablet gave greater urinary excretion than capsules of the same salt. On tolerability, PMID 39770988 reports fewer side effects on intestinal motility and gastric heaviness for a microencapsulated magnesium ingredient than for magnesium oxide, citrate and bisglycinate. The evidence is graded B.

Review methodology

Database and search date: PubMed (United States National Library of Medicine), searched 24 October 2026.

Search terms, word for word:

• magnesium bioavailability
• magnesium oxide versus magnesium citrate absorption randomised
• magnesium gastrointestinal tolerability diarrhoea supplementation
• magnesium salts adverse effects gastrointestinal randomized
• magnesium lactate or magnesium chloride or magnesium sulfate oral absorption human
• magnesium oxide bioavailability human trial
• effervescent magnesium tablet bioavailability
• magnesium bisglycinate versus magnesium oxide gastrointestinal side effects randomized
• magnesium supplement laxative effect diarrhea dose
• magnesium citrate tolerability adverse events

Included: human studies of oral magnesium at supplement-type doses that compared two or more magnesium salts, compared two presentations of one salt, or reported gastrointestinal effects of a magnesium product. One systematic review is included as context; it is not counted as trial evidence.

Excluded, and why:

• PMID 38608850: the solubility comparison was in vitro, and the tolerability trial tested a multimineral product against placebo, reporting adverse events rather than gastrointestinal effects.
• Bowel-cleansing trials (for example PMIDs 32221332, 23566639, 26945825): low-volume bowel preparations for colonoscopy, not nutritional supplementation.
• PMID 33767108: laxative therapy for constipation, not supplement absorption or tolerability.
• PMID 36558392: a magnesium L-threonate (Magtein) based formula compared with placebo, with cognitive function as the outcome.
• PMID 27412366: a single-product pharmacokinetic study of oral magnesium hydroxide, with no oral salt comparator.

Assessment: Records were assessed from their published abstracts rather than from full texts.

Summary of study findings

Pardo et al., 2021, systematic review (PMID 34111673)

Design: systematic review of magnesium forms; 14 studies selected from 433 records. Outcome: “Inorganic formulations appear to be less bioavailable than organic ones, and the percentage of absorption is dose dependent.”

Lindberg et al., 1990 (PMID 2407766)

Design: controlled clinical trial with an in vitro solubility study and an in vivo absorption study. Participants: normal volunteers. Dose and form: 25 mmol oral magnesium load as citrate or oxide. Duration: urinary magnesium over 4 hours, then a further 2 hours. Outcome: “The increment in urinary magnesium following magnesium citrate load (25 mmol) was significantly higher than that obtained from magnesium oxide load.” The abstract concludes: “magnesium citrate was more soluble and bioavailable than magnesium oxide.”

Walker et al., 2003 (PMID 14596323)

Design: randomised, double-blind, placebo-controlled, parallel study over 60 days. Participants: 46 healthy individuals. Dose and form: 300 mg elemental magnesium daily as amino-acid chelate, citrate or oxide. Outcome: “supplementation of the organic forms of Mg (citrate and amino-acid chelate) showed greater absorption (P = 0.033) at 60 days than MgO, as assessed by the 24-h urinary Mg excretion.” The abstract adds: “Mg oxide supplementation resulted in no differences compared to placebo.”

Schuette et al., 1994 (PMID 7815675)

Design: double-blind, randomised crossover trial. Participants: 12 patients with ileal resection, a group the abstract describes as at risk of magnesium depletion. Dose and form: 100 mg isotope-labelled magnesium diglycinate chelate compared with magnesium oxide. Outcome: “26Mg absorption was low but was not different for the two supplements (23.5% vs 22.8% for magnesium chelate and MgO, respectively).” In the four patients most impaired on magnesium oxide, absorption was “substantially greater from the chelate (23.5% vs 11.8%; p < .05)”. The chelate “was better tolerated by all patients”.

Pajuelo et al., 2024 (PMID 39770988)

Design: double-blind, randomised crossover study. Participants: 40 healthy women and men, on a low-magnesium diet for 7 days beforehand. Dose and form: a microencapsulated magnesium ingredient (MAGSHAPE microcapsules), magnesium oxide, magnesium citrate and magnesium bisglycinate, each taken orally. Duration: plasma magnesium at 0, 1, 4 and 6 hours. Outcome: “the Mg-MS microencapsulation technology significantly increased Mg bioavailability compared to the non-microencapsulated MgO,” and it “reduced adverse side effects commonly associated with Mg supplementation, specifically with regard to increased intestinal motility and sensations of gastric heaviness.” For magnesium bisglycinate, “no significant increase in Mg levels was observed.”

Werner et al., 2019 (PMID 32162607)

Design: classified in PubMed as a randomised controlled trial. Participants: 14 healthy males. Dose and form: five days of 400 mg magnesium daily to saturate magnesium pools, then a single 400 mg dose of citrate or oxide. Duration: 24-hour urine and plasma to 24 hours. Outcome: “Single-dose MgC supplementation led to a significant (P < 0.05) increase in 24 h urinary Mg excretion, but this was not significant following MgO.” The abstract concludes: “MgC shows higher bioavailability compared with MgO.”

Siener et al., 2011 (PMID 20862466)

Design: crossover study on a standardised diet. Participants: 13 healthy male volunteers aged 22 to 31 years, all of whom completed. Dose and form: 450 mg magnesium as magnesium oxide in capsules, compared with 450 mg as magnesium oxide in effervescent tablets. Duration: fractional urine collections on the test days; 24-hour urines at baseline and in the weekly follow-up phase. Outcome: “urinary magnesium excretion increased by 40% after ingestion of the effervescent tablets, and by only 20% after intake of the capsules.” The authors say this “may be attributed to” the tablets being dissolved before ingestion so that “magnesium becomes ionized”.

Swetha et al., 2026 (PMID 42091979)

Design: randomised, open-label pharmacokinetic study. Participants: 24 healthy male volunteers aged 18 to 45 years. Dose and form: a single oral dose of 750 mg or 2000 mg magnesium oxide. Duration: serum magnesium up to 12 hours. Outcome: “The study showed non-dose-proportional systemic exposure, with no proportional increase in Cmax or AUC at the higher dose.” On tolerability: “Both doses were well tolerated without serious adverse events or gastrointestinal side effects.” The abstract states the findings “should not be extrapolated to clinical efficacy or perioperative dosing.”

Evidence-quality assessment

Grade B. Human randomised trials are present, but the evidence is limited in size, in the formats it covers and in its tolerability measures.

Consistency. The finding that citrate, or the organic forms, were absorbed more than oxide is consistent across three studies (PMIDs 2407766, 14596323, 32162607), two of them randomised. Elsewhere the picture is mixed: diglycinate did not differ from oxide overall (PMID 7815675), and the 2024 study found a significant plasma rise for oxide and citrate but none for bisglycinate (PMID 39770988). The endpoints also differ (urinary excretion, plasma or serum magnesium, isotope-labelled absorption), so the results cannot be pooled.

Design and size. Five human studies are randomised. Participant numbers range from 12 to 46, where reported. None measured gastrointestinal tolerability over several weeks across more than one salt.

Directness. Only PMID 20862466 compares two presentations of the same salt in a format relevant to effervescent products. PMID 39770988 is a directly relevant comparison, but it tests a proprietary ingredient against unmodified salts.

Transfer. Most participants were healthy adults, and one study was in ileal resection patients. Single-dose data predominate. A multi-week comparison (PMID 14596323) used 300 mg elemental magnesium daily. Tolerability was reported favourably in three records.

Limitations

Records were assessed from their published abstracts rather than from full texts.

• Samples are small (12 to 46 where reported), and several are single-sex.
• Most studies measured magnesium at one or a few time points after a single dose, so absorption over a full dosing interval is not described.
• Gastrointestinal tolerability is reported in three records (PMIDs 7815675, 39770988, 42091979).
• The 2024 tolerability finding concerns a proprietary ingredient.

Conclusion

The human records support the following: in the studies that compared them, magnesium citrate was absorbed more than magnesium oxide (PMIDs 2407766, 32162607), and the organic forms citrate and amino-acid chelate were absorbed more than magnesium oxide (PMID 14596323); an effervescent tablet of magnesium oxide gave greater urinary excretion than capsules of the same salt (PMID 20862466); and absorption from magnesium diglycinate and magnesium oxide did not differ overall in ileal resection patients (PMID 7815675).

The records do not establish that any magnesium salt is better tolerated than another at supplement doses on a defined gastrointestinal measure. They do not show that absorption from magnesium oxide is dose-proportional. Nothing here claims that any magnesium salt treats, prevents or cures any condition.

For a formulator choosing a salt for a powder sachet or effervescent product, the human evidence is limited. On absorption it points toward citrate and the effervescent presentation. Gastrointestinal tolerability is the less-supported part of that choice. For help with salt selection for a specific format, please contact our nutrition specialist.

References

1. Pardo MR, Garicano Vilar E, San Mauro Martín I, Camina Martín MA. Bioavailability of magnesium food supplements: A systematic review. Nutrition (Burbank, Los Angeles County, Calif.). 2021;89:111294. PMID: 34111673. doi:10.1016/j.nut.2021.111294
2. Lindberg JS, Zobitz MM, Poindexter JR, Pak CY. Magnesium bioavailability from magnesium citrate and magnesium oxide. Journal of the American College of Nutrition. 1990;9(1):48-55. PMID: 2407766. doi:10.1080/07315724.1990.10720349
3. Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium research. 2003;16(3):183-91. PubMed: 14596323
4. Schuette SA, Lashner BA, Janghorbani M. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN. Journal of parenteral and enteral nutrition. 1994;18(5):430-5. PMID: 7815675. doi:10.1177/0148607194018005430
5. Pajuelo D, Meissner JM, Negra T, Connolly A, Mullor JL. Comparative Clinical Study on Magnesium Absorption and Side Effects After Oral Intake of Microencapsulated Magnesium (MAGSHAPETM Microcapsules) Versus Other Magnesium Sources. Nutrients. 2024;16(24). PMID: 39770988. doi:10.3390/nu16244367
6. Werner T, Kolisek M, Vormann J, Pilchova I, Grendar M, Struharnanska E, et al. Assessment of bioavailability of Mg from Mg citrate and Mg oxide by measuring urinary excretion in Mg-saturated subjects. Magnesium research. 2019;32(3):63-71. PMID: 32162607. doi:10.1684/mrh.2019.0457
7. Siener R, Jahnen A, Hesse A. Bioavailability of magnesium from different pharmaceutical formulations. Urological research. 2011;39(2):123-7. PMID: 20862466. doi:10.1007/s00240-010-0309-y
8. Swetha RK, Gayathri B, Kumar V, Sananthya K. Randomized, open-label study of the short-term pharmacokinetics of oral magnesium oxide in healthy volunteers. Scientific reports. 2026;16(1). PMID: 42091979. doi:10.1038/s41598-026-50427-z

Disclosure of commercial interest

Bionutricia Extract, part of the same group, is the ingredient supplier and supplies magnesium citrate, magnesium oxide and magnesium glycinate as ingredients. Magnesium glycinate is the same material this review calls magnesium bisglycinate or magnesium diglycinate; these names refer to one salt. Bionutricia, the contract manufacturer, makes powder sachet and effervescent formats, the formats this review addresses. The company therefore has a commercial interest in the subject of this review through those formats. The review was written from the published literature listed in the references, and its findings and evidence grade were not altered to favour any product or any salt. The review was not commissioned or paid for by a third party. Nothing in this review is medical advice, and nothing in it claims that any product treats, prevents or cures any condition.

Author biography

Vitthia Rama Murti, Registered Pharmacist (RPh), RPh 15632. Subject scope: formulation, excipients, stability, tolerability and safety.

Review history

• Date written: 24 October 2026.
• Reviewed and signed off by Dr Yong Yi Yi, 26 October 2026.
• Date reviewed: 26 October 2026.

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