Not all magnesium is the same: why the form matters more than the dose
Not all magnesium is the same: why the form matters more than the dose
Magnesium supplements are not interchangeable. The form decides how much of the mineral dissolves in your gut and reaches your blood. A 2019 Ghent University study tested 15 commercial magnesium products and found very wide differences in how well they dissolved and absorbed. When two of them were given to 30 healthy adults, the tablet carrying more than twice as much elemental magnesium barely moved blood levels, while the lower-strength product raised them clearly.[1]
The reason is simple chemistry. Magnesium has to dissolve before the intestine can take it up. Magnesium oxide packs a lot of magnesium into each tablet but dissolves poorly, so much of it passes straight through the bowel, which is why it is also used as a laxative.[10] More soluble forms tend to absorb better, and a 2021 systematic review concluded that inorganic forms are generally less bioavailable than organic ones.[5]
The practical point: the milligrams on the label tell you what is in the tablet, not what reaches your cells. The form, the health of your gut, your medications and your current magnesium status all shape the result. A standard serum test reflects less than 1% of the body’s magnesium and can look normal while stores run low,[14] which is why I use red blood cell magnesium instead. We assess first, choose a form to suit the person, and set the dose individually rather than reading it off a bottle.
What happened when researchers tested 15 magnesium products
Most people pick a magnesium supplement by price or by the biggest number on the label. Researchers at Ghent University in Belgium wanted to know whether that number predicts anything. They ran 15 commercial magnesium products through a simulated stomach and small intestine, and through a standard dissolution test.[1] Results varied widely.
The best and worst performers then went into a crossover trial with 30 healthy men and women, with blood magnesium measured for six hours after one tablet. The worst, a straight magnesium oxide tablet, raised serum magnesium no more than the normal daily swing seen after a placebo. The best, a mix of oxide and the more soluble magnesium glycerophosphate, gave a clear rise: roughly 20 times more total uptake. The oxide tablet held more than twice as much elemental magnesium.[1]
The authors' conclusion was that how well a supplement dissolves matters more than how much magnesium it carries. Two caveats. These were single doses in healthy young adults, and no purely organic form was tested in people. A manufacturer supplied the products but had no role in the design or analysis.
Dissolving comes before absorbing
Magnesium is absorbed as a dissolved ion. Most of it slips between the cells of the small intestine, and a smaller amount moves through dedicated channels further down the bowel.[3] On a typical diet, people absorb around 30% of the magnesium they eat, and the body absorbs a larger share when its stores are low.[1][4] Magnesium that never dissolves cannot use either route.
That is the problem with magnesium oxide. It has the highest share of magnesium by weight of the common forms, so it is cheap and the label number looks impressive. But it dissolves poorly. In a US study that tracked urinary magnesium, fractional absorption from oxide was about 4%.[6] In a 60-day randomised trial, magnesium citrate and an amino-acid chelate both absorbed better than oxide, and oxide performed no differently from placebo.[7] Trials comparing citrate with oxide, from 1990 to 2019, show the same pattern.[8][9]
The organic label isn't the whole story, though. The same US study found magnesium chloride, an inorganic salt that dissolves readily, absorbed about as well as the organic lactate and aspartate forms.[6] A 2021 systematic review of 14 studies still found inorganic forms less bioavailable overall.[5] Solubility is the better guide.
Why more magnesium isn’t absorbed in proportion
Absorption has a ceiling. The channels in the bowel wall fill up, so the percentage absorbed drops as the amount taken at once rises.[4][5] In the Ghent trial, taking two tablets of the better-absorbed product raised blood magnesium no further than taking one.[1]
What isn't absorbed stays in the bowel and pulls water in. That is how magnesium oxide works as a laxative.[10] So loose stools after starting magnesium usually mean some of it is going straight through. A better-absorbed form or splitting intake across the day usually helps, but how much and when depends on the person.
Common magnesium forms, side by side
What the research shows about how well each form is absorbed, and where it tends to fit.
| Form | What the research shows | In practice |
|---|---|---|
| Oxide | Poorly soluble. About 4% fractional absorption in one study, and no better than placebo in a 60-day trial.[6][7] | High number on the label, low uptake. Widely used as a laxative.[10] |
| Citrate | Absorbed better than oxide across several randomised trials.[7][8][9] | Can loosen stools, which suits some people and not others. |
| Glycinate | An amino-acid chelate. Absorbed better than oxide in people with the most impaired absorption after bowel surgery, and better tolerated.[11] | Usually gentler on the gut. Direct human comparisons are still limited. |
| Chloride | Inorganic but soluble. Absorbed about as well as organic salts in one study.[6] | A good illustration that solubility matters more than the organic label. |
| L-threonate | Raised brain magnesium and improved memory measures in rats.[12] | Promising, but human evidence is still early. |
Most of these comparisons come from short studies in healthy adults. They show which forms get in, not which form suits a particular person.
Who absorbs magnesium poorly to begin with
The form is one variable. The gut is another. Bowel surgery, chronic diarrhoea and inflammatory bowel disease all reduce magnesium uptake.[3] In people who had part of the lower small intestine removed, a glycinate form absorbed twice as well as oxide in the four patients with the poorest absorption.[11] If digestion is a weak point, magnesium is worth checking as part of gut health and IBS care.
Medications matter too. A meta-analysis of 16 studies covering more than 131,000 patients found low blood magnesium in 19.4% of people taking proton pump inhibitors, compared with 13.5% of non-users.[13] If you take one long term, our piece on low stomach acid and reflux is worth a read. Some diuretics, regular alcohol and older age also lower absorption or raise losses through the kidneys.[3][4]
Then there's intake: 31.3% of Australians had an inadequate magnesium intake in 2023, barely better than 34.9% in 2011–12.[2]
Why a normal blood test doesn’t settle it
Most of the body’s magnesium sits in bone and muscle. Less than 1% is in the blood, and the kidneys and bone work to hold that amount steady,[14] so standard serum magnesium can stay in range while stores run down. Even the serum reference range is contested, and researchers argue that suboptimal magnesium is probably under-recognised.[16][17]
That is why I use red blood cell (RBC) magnesium instead. It measures the magnesium held inside your red cells rather than the small amount in the fluid around them. Red cells live for around four months, so RBC magnesium moves slowly and reflects the past few months, not this morning’s breakfast. Reviews note it tracks magnesium status best over longer periods of low or high intake.[15]
It has a research track record too. In a Lancet study, people with chronic fatigue syndrome had lower RBC magnesium than matched healthy controls, and levels returned to normal with weekly magnesium injections, which bypass the gut entirely.[19] A 2022 study found RBC magnesium a better marker than plasma magnesium in people with migraine.[20] No test is perfect,[15] so I read RBC magnesium alongside diet, medications and symptoms through functional testing. Retesting after a change of form shows whether the new one is actually getting in.
Where magnesium fits in clinic
Food comes first. Leafy greens, legumes, nuts, seeds and whole grains are the richest everyday sources. When a supplement makes sense, I choose the form for the person: a sensitive gut, constipation, a proton pump inhibitor or reduced kidney function each changes the answer. Magnesium also interacts with some medicines, so timing needs checking against what else you take.[10]
Plenty of people take magnesium for stress or sleep. A systematic review found the anxiety evidence suggestive but low quality.[18] In my experience it helps most when there's a real shortfall to correct. There's more on the stress link in magnesium and stress resilience and on our hormones and stress page. Dosage is determined by individual assessment, since it depends on your intake, gut, kidneys and the form used.
Key Insights
Frequently asked questions
Which form of magnesium is best absorbed?
There is no single best form for everyone. Soluble forms such as citrate, glycinate and chloride consistently absorb better than magnesium oxide, which showed fractional absorption of about 4% in one study. The right form also depends on your gut, your medications and why you are taking it, so it is worth assessing before you choose.
Is magnesium oxide a waste of money?
Not always, but it is a poor choice for raising your magnesium. Oxide carries a lot of magnesium per tablet yet dissolves poorly, so much of it stays in the bowel. That suits it to constipation more than to topping up stores.
Why does magnesium give me diarrhoea?
Magnesium that is not absorbed stays in the bowel and draws water in, which loosens stools. This is more likely with poorly absorbed forms such as oxide and with larger single amounts, because the gut can only absorb so much at once. Switching to a better-absorbed form or spreading intake across the day often helps, but the right approach depends on your gut health and the reason you are taking magnesium.
Can a blood test tell me if I'm low in magnesium?
It depends on the test. Standard serum magnesium reflects less than 1% of the body's magnesium, because most is stored in bone and muscle and the body keeps blood levels steady, so a normal result can sit alongside low stores. Red blood cell (RBC) magnesium measures what is held inside cells and reflects the past few months. It is the test I use, read alongside your diet, medications and symptoms.
Ready to find answers?
Taking magnesium and not noticing much? The form, your gut or your stores may be the missing piece. We can work out which.
References
- Blancquaert L, Vervaet C, Derave W. Predicting and testing bioavailability of magnesium supplements. Nutrients. 2019;11(7):1663. doi:10.3390/nu11071663
- Australian Bureau of Statistics. Usual nutrient intakes, 2023. Canberra: ABS. abs.gov.au
- de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev. 2015;95(1):1–46. doi:10.1152/physrev.00012.2014
- Schuchardt JP, Hahn A. Intestinal absorption and factors influencing bioavailability of magnesium: an update. Curr Nutr Food Sci. 2017;13(4):260–278. doi:10.2174/1573401313666170427162740
- Pardo MR, Garicano Vilar E, San Mauro Martín I, Camina Martín MA. Bioavailability of magnesium food supplements: a systematic review. Nutrition. 2021;89:111294. doi:10.1016/j.nut.2021.111294
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001;14(4):257–262. PMID:11794633
- Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003;16(3):183–191. PMID:14596323
- Lindberg JS, Zobitz MM, Poindexter JR, Pak CY. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990;9(1):48–55. doi:10.1080/07315724.1990.10720349
- Werner T, Kolisek M, Vormann J, et al. Assessment of bioavailability of Mg from Mg citrate and Mg oxide by measuring urinary excretion in Mg-saturated subjects. Magnes Res. 2019;32(3):63–71. doi:10.1684/mrh.2019.0457
- Mori H, Tack J, Suzuki H. Magnesium oxide in constipation. Nutrients. 2021;13(2):421. doi:10.3390/nu13020421
- Schuette SA, Lashner BA, Janghorbani M. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN J Parenter Enteral Nutr. 1994;18(5):430–435. doi:10.1177/0148607194018005430
- Slutsky I, Abumaria N, Wu LJ, et al. Enhancement of learning and memory by elevating brain magnesium. Neuron. 2010;65(2):165–177. doi:10.1016/j.neuron.2009.12.026
- Srinutta T, Chewcharat A, Takkavatakarn K, et al. Proton pump inhibitors and hypomagnesemia: a meta-analysis of observational studies. Medicine (Baltimore). 2019;98(44):e17788. doi:10.1097/MD.0000000000017788
- Jahnen-Dechent W, Ketteler M. Magnesium basics. Clin Kidney J. 2012;5(Suppl 1):i3–i14. doi:10.1093/ndtplus/sfr163
- Workinger JL, Doyle RP, Bortz J. Challenges in the diagnosis of magnesium status. Nutrients. 2018;10(9):1202. doi:10.3390/nu10091202
- Costello RB, Elin RJ, Rosanoff A, et al. Perspective: the case for an evidence-based reference interval for serum magnesium: the time has come. Adv Nutr. 2016;7(6):977–993. doi:10.3945/an.116.012765
- Rosanoff A, Weaver CM, Rude RK. Suboptimal magnesium status in the United States: are the health consequences underestimated? Nutr Rev. 2012;70(3):153–164. doi:10.1111/j.1753-4887.2011.00465.x
- Boyle NB, Lawton C, Dye L. The effects of magnesium supplementation on subjective anxiety and stress: a systematic review. Nutrients. 2017;9(5):429. doi:10.3390/nu9050429
- Cox IM, Campbell MJ, Dowson D. Red blood cell magnesium and chronic fatigue syndrome. Lancet. 1991;337(8744):757–760. doi:10.1016/0140-6736(91)91371-z
- Hoshino MT, Bochio MG, Spagnol Bonache J, et al. Plasma and erythrocyte magnesium levels: from validation of the method to analysis in volunteers diagnosed with migraine. Magnes Res. 2022;35(2):51–61. doi:10.1684/mrh.2022.0502
