Magnesium oxide and the form question
Two labels can both say 250 mg of magnesium and deliver noticeably different amounts of it. The difference is in one word most buyers skip.
NIH states that magnesium in the aspartate, citrate, lactate and chloride forms is absorbed more completely than magnesium oxide. Three published comparisons found citrate outperforming oxide on urinary and serum measures.
The widely repeated claim that oxide is 'only 4% absorbed' could not be traced to a source we could open, so this article does not repeat it. Oxide is worse than citrate; a precise universal percentage is not established.
The word most people skip
Before this post went live, Dr. James Bateman III, MD, MPH, a physician, checked it against the CogniMary label and the sources it cites. A supplement panel does not just say “magnesium”. It says magnesium, and then in brackets, the salt it came from: oxide, citrate, glycinate, chloride, malate, threonate.
The milligram figure on the panel is the elemental magnesium — the magnesium itself, not the weight of the compound. So 250 mg is 250 mg of magnesium whichever salt it is. What changes is how much of it crosses from your gut into your blood.
That is the form question, and it is the difference between two labels that look identical.
What the comparisons found
NIH's own fact sheet states the position plainly: magnesium in the aspartate, citrate, lactate and chloride forms is absorbed more completely and is more bioavailable than the oxide and the sulfate. It gives no percentages.
Three published studies put numbers to the comparison.
| Study | Design | Finding |
|---|---|---|
| Lindberg et al., 1990 | Single 25 mmol oral load, citrate against oxide | Markedly higher urinary magnesium recovery from citrate at multiple timepoints; attributed to citrate's far higher solubility |
| Walker et al., 2003 | Randomised double-blind, 60 days, citrate against oxide and a chelate | Plasma magnesium rose significantly on citrate and did not move on oxide; salivary magnesium also higher on citrate |
| Kappeler et al., 2017 | Randomised crossover, single dose, citrate against oxide | Higher 24-hour urinary excretion on citrate and higher serum magnesium from two to six hours |
Scroll this table sideways →
All three point the same way. All three are also small, short, and use urinary or serum magnesium as a proxy rather than a clinical outcome, which is worth holding in mind.
The figure the internet repeats
Search for this topic and you will find, within about two results, the claim that magnesium oxide is “only 4% absorbed”.
We went looking for the source of that number and could not open one. The studies above show oxide performing worse than citrate; they do not establish a single universal absorption percentage for oxide, and a precise-sounding figure with no traceable source is exactly the kind of thing this site tries not to repeat.
What we will say instead
Oxide is less well absorbed than citrate, on three published comparisons and on NIH's own statement. How much less, in a given person on a given day, is not something we can put a number on.
That is a less satisfying sentence than “only 4%”. It is also one we can defend.
When the form actually matters
Form matters more in some situations than others, and it is worth being proportionate.
It matters if you are correcting a shortfall
If a clinician has told you your magnesium is low, absorption is the whole point and the better-absorbed forms are the sensible choice.
Form matters most here
It matters if you have a sensitive gut
NIH names oxide among the forms most likely to cause diarrhoea, nausea and cramping. Unabsorbed magnesium stays in the gut, which is exactly why magnesium oxide is used as a laxative.
The practical consequence
It matters less if it is a small part of a multi
A 60 mg top-up inside a multivitamin is not the thing standing between you and adequacy either way.
Proportion is worth keeping
It always tells you something about the formulator
Oxide is the cheapest common form. In a product sold as premium, choosing it is a decision that says something.
A signal, not a verdict
Reading a magnesium label
- Find the word in brackets after “Magnesium”. That is the form.
- Check the milligram figure is elemental magnesium, which it is on a US Supplement Facts panel.
- Check the total against the 350 mg upper intake level for supplemental magnesium, and add in anything else you take.
- If the form is oxide and you want a better-absorbed one, citrate and glycinate are the usual alternatives and are not expensive.
- If the product disagrees with you in the first week, the magnesium row is the first thing to look at.
That is a two-minute check that applies to a very large number of products, because magnesium is in almost everything.
On this label
CogniMary declares 250 mg of magnesium as the oxide, 60% of the Daily Value, under the 350 mg supplemental upper limit.
The amount is unremarkable and the form is the cheapest available. In a product carrying the word “Premium” on the front, that is a fair thing to notice, and it is the sort of decision that is invisible unless you read the brackets.
It is also, practically, the most likely reason someone would have a problem with this product. If you take it and your digestion objects in week one, that is the row to suspect, and taking it with food is the first thing to try.
- NIH Office of Dietary SupplementsMagnesium — Health Professional Fact Sheet — ods.od.nih.gov · source
- Lindberg, Zobitz, Poindexter & Pak — 1990Magnesium bioavailability from magnesium citrate and magnesium oxide — Journal of the American College of Nutrition · PMID 2407766 · source
- Walker, Marakis, Christie & Byng — 2003Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study — Magnesium Research · PMID 14596323 · source
- Kappeler, Heimbeck, Herpich et al. — 2017Higher bioavailability of magnesium citrate as compared to magnesium oxide — BMC Nutrition · DOI 10.1186/s40795-016-0121-3 · source
- Chen et al. — 2024Magnesium and Cognitive Health in Adults: A Systematic Review and Meta-Analysis — Advances in Nutrition · PMC11362647 · source
Why oxide is so common
If oxide is the worst absorbed of the common forms, its popularity needs explaining, and the explanation is not sinister.
It is the most concentrated
Magnesium oxide is around 60% elemental magnesium by weight. Citrate is roughly 16%. To deliver 250 mg of magnesium from citrate you need well over a gram of compound, which will not fit in two capsules alongside anything else.
A real formulation constraint
It is the cheapest
By a wide margin. In a formula competing on ingredient count, the magnesium row is an obvious place to save.
The commercial reason
It looks identical on the panel
250 mg of magnesium is 250 mg of magnesium whichever salt it came from. The panel does not rank forms, and most buyers do not read the brackets.
No penalty for choosing it
The first of those is a genuine engineering argument and worth taking seriously. In a two-capsule serving that also carries a 635 mg blend, there is very little room, and a better-absorbed magnesium salt would have forced something else out.
Which is itself a finding: the crowdedness of this formula is not only a dose problem for the blend, it is a form problem for the magnesium.
The forms, side by side
| Form | Roughly, elemental magnesium | Usually chosen for |
|---|---|---|
| Oxide | ~60% | Concentration and cost; also used as a laxative |
| Citrate | ~16% | Better absorption; the comparison studies' winner |
| Glycinate | ~14% | Tolerability; commonly chosen for sensitive stomachs |
| Chloride | ~12% | Absorption; NIH lists it among the better-absorbed forms |
| Lactate | ~12% | Absorption and tolerability |
| Threonate | ~8% | Marketed for cognition specifically; the trial evidence is thin |
Scroll this table sideways →
The percentages are approximate and are the reason the trade-off exists: the better-absorbed forms are bulkier, so they need bigger capsules or more of them.
Note the last row. Magnesium L-threonate is the form marketed specifically for brain function, and the 2024 systematic review found the trial evidence for magnesium and cognition insufficient to draw conclusions from — including for threonate.
What to do if you want better magnesium
- Check what you are already getting. Magnesium is in multivitamins, in some antacids and in a lot of powders.
- Add up the supplemental total against the 350 mg upper intake level. Food magnesium does not count towards it; supplements do.
- If you want a better-absorbed form, citrate and glycinate are widely available, inexpensive and published at real doses.
- Take it with food and in the evening if it upsets your stomach.
- If a product gives you loose stools in week one, the magnesium row is the first suspect, and oxide is the likeliest form behind it.
That is a genuinely useful five minutes for anyone who takes more than one supplement, and it applies well beyond this product.
It is also the kind of advice a product page is not supposed to give, because it points at a cheaper product somebody else sells. We think a site that will not say it has a bigger problem than a lost sale.