The short version
- A 2017 review of magnesium absorption concluded the type of magnesium salt appears less relevant than is often thought; dose, existing magnesium status and the food it arrives with matter more.
- Magnesium oxide showed roughly 4% fractional absorption in a bioavailability study, and in a 60-day randomised trial at 300mg elemental daily it produced no differences compared with placebo on any marker measured.
- In that same 60-day trial in 46 healthy adults, magnesium citrate produced the greatest mean serum magnesium of the forms tested, after both a single dose and 60 days of use.
- Magnesium glycinate is only about 14% magnesium by weight, so a 500mg capsule delivers roughly 70mg of elemental magnesium; its advantage is tolerability rather than proven superior absorption.
- The cognitive evidence for magnesium L-threonate rests on a single 12-week trial of a proprietary formulation in 44 adults aged 50 to 70 who already had cognitive impairment.
"Which magnesium should I take?" is the question we get more than any other, and the honest answer starts with a sentence that isn't good for business: less rides on the form than the internet suggests.
Schuchardt and Hahn's 2017 review of magnesium absorption puts it plainly. The type of magnesium salt "appears less relevant than is often thought." Some studies find organic salts slightly better absorbed than inorganic ones under standardised conditions; others don't. What consistently drives how much magnesium you retain is the dose, your existing magnesium status, and the food it arrives with. Relative absorption is also higher from several small doses across the day than one large one — an unglamorous fact that no brand puts on a bottle.
That said, "less relevant than you think" is not "irrelevant." There are real differences, and they are mostly about two things: how much elemental magnesium a form carries, and what it does to your digestion.
Magnesium oxide: the one with the worst data
Oxide is roughly 60% magnesium by weight, which makes it cheap to hit a big number on the front of a bottle. It is also the form with the clearest evidence of poor absorption. Firoz and Graber measured urinary magnesium excretion after dosing four US commercial preparations and found fractional absorption of magnesium oxide at about 4%, against significantly higher and roughly equivalent absorption from magnesium chloride, lactate and aspartate.
Walker and colleagues ran a 60-day randomised, double-blind trial in 46 healthy adults comparing citrate, an amino-acid chelate and oxide, all at 300mg of elemental magnesium daily. Magnesium oxide produced no differences compared with placebo on any marker they measured.
Oxide has one genuine use: it draws water into the bowel, which is why it turns up in laxatives. If that is what you want, it works. If you are trying to raise magnesium status, it is the wrong tool, and it is the form you are most likely to be sold by accident.
Magnesium citrate: the best-supported absorption, with a caveat you can feel
In the same Walker trial, citrate led to the greatest mean serum magnesium after both a single dose and 60 days of daily use, and the highest salivary magnesium at 60 days. On the head-to-head bioavailability data, citrate is the strongest performer of the commonly sold forms.
The caveat is the same mechanism that makes it useful for some people and unpleasant for others: citrate is mildly osmotic. At higher doses it loosens stool. Plenty of people take it happily; plenty of people quietly stop after a week and conclude magnesium "doesn't agree with them," when what didn't agree with them was that salt.
Magnesium glycinate: chosen for tolerability, not for a bioavailability trophy
This is the form we use, so here is the honest version. Glycinate — magnesium bound to two glycine molecules — is well tolerated at the doses people actually take, and it is markedly less likely to cause the loose stool that citrate and oxide can. That is a real advantage, because the supplement you tolerate is the one you keep taking, and magnesium status responds to consistency rather than to any single dose.
What we are not going to tell you is that glycinate out-absorbs citrate. We haven't seen head-to-head human data that supports that claim, and Walker's trial — the one usually cited on this topic — found citrate ahead of an amino-acid chelate on serum magnesium. If we ever see a good direct comparison that goes the other way, we'll say so.
One practical note: glycinate is only about 14% magnesium by weight. A capsule advertising 500mg is delivering roughly 70mg of elemental magnesium. Read the elemental line, not the headline.
Magnesium L-threonate: an interesting trial, a small one
Threonate is the form marketed for cognition, on the strength of one human trial. Liu and colleagues randomised 44 adults aged 50–70 with cognitive impairment to a proprietary magnesium L-threonate formulation (MMFS-01) or placebo for 12 weeks. Overall cognitive ability, measured as a composite across four domains, improved significantly against placebo (p = 0.003, Cohen's d = 0.91). Executive function, where this group was most impaired, improved most.
Read the rest of the paper too. Forty-four people is small. The formulation is proprietary and the trial was tied to it. And the authors state directly that placebo effects on sleep and anxiety were strong enough that they could not determine whether the supplement did anything for either. Threonate is also the least magnesium-dense form on the shelf at roughly 8% by weight, so the gram counts on those labels are large for a reason.
It is a genuinely interesting result in a specific population. It is not a reason to buy threonate for general use.
Magnesium malate: thin
Malate is usually sold for energy and muscle soreness, on the logic that malic acid feeds into the citric acid cycle. The logic is fine. The human evidence for magnesium malate specifically is thin — we could not find controlled trials of the form that we would be comfortable citing to you, and we would rather say that than dress up a mechanism as a finding.
What we'd actually tell you
Pick on tolerability and elemental dose, in that order. If your digestion is unbothered and you want the best absorption data, citrate is defensible. If citrate loosens your stool, glycinate is the sensible switch and the reason we make it. If you have been buying a big-number oxide product, that is the one worth changing.
Check the elemental amount on the label, take it consistently rather than heroically, and split it across the day if you are taking a larger amount. And if you have kidney disease or take medications that affect magnesium handling, this is a doctor conversation before it is a shopping one.
Good questions
Which form of magnesium is best absorbed?
Citrate, on the head-to-head human data. In a 60-day randomised trial in 46 adults taking 300mg of elemental magnesium daily, citrate produced the greatest mean serum magnesium after both a single dose and 60 days, plus the highest salivary magnesium at 60 days. The trade-off is that citrate is mildly osmotic and loosens stool in some people.
Is magnesium glycinate better absorbed than citrate?
No published head-to-head trial supports that, and glycinate is the form we sell. In the trial usually cited on this topic, citrate came out ahead of an amino-acid chelate on serum magnesium. The genuine case for glycinate is tolerability: it is markedly less likely to loosen stool, and the supplement you tolerate is the one you keep taking.
Why does magnesium give some people diarrhoea?
Because some magnesium salts draw water into the bowel. Oxide does this strongly enough that it is sold as a laxative, and citrate is mildly osmotic at higher doses. Plenty of people quietly stop after a week and conclude that magnesium does not agree with them, when what did not agree with them was that particular salt.
Is magnesium malate worth buying?
We could not find controlled human trials of magnesium malate that we would be comfortable citing to you. It is usually sold for energy and muscle soreness on the logic that malic acid feeds into the citric acid cycle. The logic is fine. The human evidence for that specific form is thin, and a mechanism is not a finding.
How do I know how much magnesium is actually in a capsule?
Read the elemental line rather than the headline number. Glycinate is about 14% magnesium by weight, so a 500mg capsule delivers roughly 70mg of elemental magnesium. Threonate is about 8%, which is why those labels carry large gram counts. Oxide is about 60%, which is exactly why it appears on big-number bottles despite the worst absorption data.
Can I take magnesium if I have kidney problems?
Talk to your doctor before you start. Kidney disease changes how magnesium is handled, and some medications affect it too, which makes this a clinical conversation rather than a shopping decision. For everyone else, the practical approach is to pick on tolerability and elemental dose, take it consistently rather than heroically, and split larger amounts across the day.
Sources
- Schuchardt JP, Hahn A. Intestinal Absorption and Factors Influencing Bioavailability of Magnesium — An Update. Curr Nutr Food Sci, 2017. View study
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res, 2001. View study
- 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. View study
- Liu G, et al. Efficacy and Safety of MMFS-01, a Synapse Density Enhancer, for Treating Cognitive Impairment in Older Adults: A Randomized, Double-Blind, Placebo-Controlled Trial. J Alzheimers Dis, 2016. View study
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
