Magnesium: which form actually does what
Glycinate, citrate, threonate, oxide — they're not interchangeable, and the label rarely tells you why.
Magnesium is one of the supplements for which the label on the bottle matters more than people realize. “Magnesium 400 mg” is not a useful description, because the form the magnesium is bound to determines how much gets absorbed, where it goes in the body, and what side effects it causes. Here’s the short guide.
Why form matters
Elemental magnesium is a metal. You can’t swallow it. In a supplement it’s always bound to another molecule — an amino acid, an organic acid, a simple salt — and the binding partner controls three things: bioavailability (how much you absorb), tissue distribution (where it ends up), and GI effects (whether it gives you diarrhea).
Magnesium is also the most common thing on the shelf where the cheap form is the one that mostly doesn’t work.
The forms, ranked by usefulness
Magnesium glycinate (a.k.a. bisglycinate) — magnesium bound to the amino acid glycine. High absorption, gentle on the gut, glycine itself is mildly calming. This is the default “I want the benefits of magnesium without caveats” option. If you’re taking it for sleep, muscle cramps, or general repletion, this is the one. Slightly more expensive than the cheap forms, worth it.
Magnesium citrate — magnesium bound to citric acid. Well absorbed, cheap, widely available. The catch: at higher doses it has a laxative effect, which is the reason it’s sold in drugstores as a gentle laxative. For normal supplementation (200–400 mg elemental) most people tolerate it fine. If you’re sensitive, switch to glycinate.
Magnesium L-threonate — magnesium bound to threonic acid. The selling point is that it’s the only form shown in animal studies to meaningfully raise magnesium levels in the brain, specifically in cerebrospinal fluid. The human evidence for cognitive benefits is thinner than the marketing suggests, but it’s the form to try if your interest is specifically cognitive, not general health. Expensive.
Magnesium malate — magnesium bound to malic acid. Well absorbed, sometimes recommended for fatigue and fibromyalgia, though the evidence there is weak. A reasonable alternative to citrate if citrate gives you GI trouble.
Magnesium taurate — bound to taurine. Interest here is cardiovascular; taurine itself has some heart rhythm evidence. Niche but reasonable.
Magnesium oxide — the cheap, high-dose-on-the-label form. Very poorly absorbed (maybe 4% bioavailability versus 40% for glycinate). The 400 mg on the label delivers roughly the same absorbed magnesium as 40 mg from a good form. It’s also the form most likely to cause diarrhea. Avoid it unless the goal is laxation. Most gas-station and warehouse-store magnesium is oxide.
Epsom salts (magnesium sulfate) — for baths and oral laxative use. Skin absorption from baths is negligible; the relaxation effect of a hot bath is from the hot bath.
Practical version
- For sleep, general repletion, muscle cramps: magnesium glycinate, 200–400 mg elemental in the evening.
- For constipation or as a gentle laxative: magnesium citrate.
- For specifically cognitive interest: L-threonate, but with tempered expectations.
- Never: magnesium oxide, unless you’re explicitly using it as a laxative.
Read the label. If it just says “magnesium,” or only shows the percent DV without specifying the form, put it back.