Walk down any supplement aisle and you'll see a dozen bottles that all say the same thing: Magnesium. Same word, wildly different products.
That's because magnesium can't be sold on its own — it has to be bound to something. And the “something” (oxide, citrate, glycinate, threonate…) determines two things the front of the label never tells you: how much elemental magnesium you actually get, and how much of it your body can absorb.
The absorption problem
The most common form in cheap supplements and multivitamins is magnesium oxide. It looks great on paper — about 60% of its weight is elemental magnesium, the highest of any form. The problem is what happens after you swallow it: in one of the most cited comparison studies, the fractional absorption of magnesium oxide was measured at roughly 4%.1
In other words, the “500mg of magnesium” on the label can translate to a small fraction of that actually reaching your bloodstream. The rest mostly stays in the gut — which is why high-dose oxide is better known as a laxative than as a supplement.
Magnesium citrate, by contrast, repeatedly outperforms oxide in absorption studies, including a randomized trial that found citrate more bioavailable than both oxide and a chelated form over 60 days of supplementation.2
Form by form
| Form | Absorption | Best known for | Watch out for |
|---|---|---|---|
| Oxide | Poor (~4% in studies) | Being cheap | Laxative effect; inflated label numbers |
| Citrate | Good | All-round use, constipation | Can loosen stools at higher doses |
| Glycinate (bisglycinate) | Good | Evening use, sensitive stomachs | Bulky — real doses need big capsules |
| Malate | Good | Daytime use | Less studied than citrate |
| L-Threonate | Moderate | Brain-related marketing claims | Very low elemental magnesium per gram; expensive |
| Chloride / lactate | Good | Liquid formats | Uncommon in capsules |
The trick to spot on any label
Look for the words “elemental magnesium.” A label that says “Magnesium bisglycinate 2,000mg” is describing the whole compound — the elemental magnesium inside is roughly 200mg, or 10%. A label that says “Magnesium (as bisglycinate) 200mg” is telling you the real number. Same molecule, very different honesty.
This is the same “compound vs. active” gap we keep running into across every category we test — it's rarely an accident, and it's the single most useful thing to check before comparing prices.
Bottom line
- For most people: citrate or glycinate, dosed by elemental magnesium (the RDA for adult men is 400–420mg/day from all sources, food included).3
- Oxide is fine as a laxative. As a magnesium supplement, you're mostly buying label numbers.
- Threonate is interesting but young science at a premium price — buy it for the research you've read, not the front of the box.
- Split doses (morning/evening) absorb better than one large dose.
3 sources
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001.
- Walker AF, et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003.
- NIH Office of Dietary Supplements. Magnesium — Fact Sheet for Health Professionals.