Magnesium oxide vs citrate vs glycinate: which form

Magnesium oxide is the most concentrated and the least absorbed form, and it draws water into the intestine, which is why it is the one that helps constipation. Magnesium citrate is better absorbed and gentler on the stomach. Magnesium glycinate is the best tolerated and the least likely to loosen stools. Same mineral, three different jobs.

If constipation is the problem, oxide does the work. If you are topping up body magnesium and oxide upsets your stomach, citrate or glycinate is the better choice. Keel uses oxide plus citrate, which covers both jobs in one capsule.

Why does the magnesium form matter so much?

Magnesium is a mineral that has to dissolve before it can be absorbed, and what it is bound to decides both how much gets through and what happens to the rest. The three common forms differ on three axes at once: how much elemental magnesium they carry per gram, how well that magnesium is absorbed, and what the unabsorbed remainder does on its way through.

That last axis is the one that decides whether a supplement constipates you or loosens you, and it is the reason the same mineral appears in both a sleep formula and a constipation remedy.

How do the three forms compare?

Form	Elemental magnesium	Absorption	Effect on the bowel	Best for
Oxide	Highest, about 60% by weight	Lowest of the three	Draws water in, loosens stool	Constipation, and the most milligrams per capsule
Citrate	Middle, about 16% by weight	Good, clearly better than oxide	Mild loosening	General top-up, better tolerated than oxide
Glycinate	Lower, about 14% by weight	Good	Usually neutral, gentle	Stomachs that object to the others, and evening use

The percentages are elemental magnesium by weight, which is why a 500 mg capsule of oxide and a 500 mg capsule of glycinate are not the same amount of magnesium at all. A 500 mg oxide capsule carries roughly 300 mg of elemental magnesium; the same weight of glycinate carries about 70 mg. Reading only the front of the bottle tells you almost nothing.

How much magnesium do you actually need?

The adult reference intake is 310 to 420 mg a day depending on age and sex, from food and supplements together. There is also a supplemental tolerable upper intake of 350 mg a day for adults, which applies to magnesium taken as a supplement rather than to magnesium in food.

That supplemental ceiling exists because magnesium from supplements is the form most likely to cause diarrhoea, and it is the reason a sensible formula does not simply load up. Keel carries 200 mg as oxide plus citrate, which sits under the supplemental ceiling while still being a meaningful amount.

What does the research say about magnesium and zinc together?

Magnesium and zinc are commonly sold together, and there is a reason beyond marketing. A meta-analysis of mineral supplements including magnesium and zinc found improvements in insulin resistance, though the evidence base is mixed and the studies correct deficiencies rather than super-dose people whose levels are already fine. The caveat matters as much as the finding: these were corrections of deficiency, not super-doses in people whose levels were already fine.

If your levels are already adequate, the effect is smaller, which is the honest version of why a supplement helps some people and does nothing for others.

Who should pick which?

• Constipation, especially on a GLP-1 medication. Oxide, because the osmotic effect is the point. Build up gradually and with fluid.
• Topping up when oxide upsets your stomach. Citrate. Better absorbed, gentler, and the usual next step.
• Evening use, or a stomach that objects to everything. Glycinate. Often marketed for sleep, and while the sleep evidence is modest, the tolerability is real.
• Already getting plenty from food. Probably nothing. Nuts, seeds, whole grains and greens are the main sources, and most diets fall short, which is the actual case for supplementing.

What are the practical rules for taking magnesium?

Take magnesium with food if it sits heavily, and separate it from certain antibiotics and from levothyroxine by a few hours, because magnesium binds them and reduces absorption. A pharmacist can confirm the right gap for your specific medicine.

If you want the pair inside the rest of a habit, Keel carries 200 mg as oxide plus citrate alongside 20 mg zinc bisglycinate, two capsules a day with food. The full label is on the magnesium ingredient page.

The wider picture, including the calculators, is in our guide to eating and supplementing well on GLP-1 medications.

FAQ

Which magnesium is best for constipation? Oxide, because it is poorly absorbed and therefore draws water into the bowel. That is a feature here rather than a flaw. Increase gradually and drink plenty with it.

Is magnesium glycinate better than citrate? For tolerability, usually yes, and it is the gentlest on the stomach. For elemental magnesium per capsule, no, it carries less. Neither is better in absolute terms; they suit different jobs.

Can I take magnesium every day? Up to the supplemental tolerable upper intake of 350 mg a day for adults, yes. Above that, diarrhoea becomes the limiting effect rather than a serious risk, but there is no benefit to pushing through it.

Does magnesium help you sleep? The evidence is modest and mostly indirect. Magnesium glycinate is often sold for evening use because it is gentle, and because magnesium deficiency is associated with poor sleep, correcting a deficiency helps. It is not a sedative.

Sources

• NIH Office of Dietary Supplements, Magnesium fact sheet

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.