The forms of magnesium: element or salt?
Magnesium is one of the best-selling supplements, and one of the most misunderstood. The reason comes down to one word: form. Between oxide, citrate and bisglycinate, the amount actually provided and the tolerability change markedly. Here's how to find your way.
Elemental magnesium and magnesium salt
Pure magnesium is a metal: you don't swallow it as such. In a supplement, it is always bound to something else to form a salt : oxide, citrate, bisglycinate, lactate, malate… What your body uses is the portion of elemental magnesium contained in that salt, not the total weight of the salt3.
This is the key distinction. A serious label states the elemental magnesium content, the only one that is comparable from one product to another. When it shows only the weight of the salt, it's easy to overestimate what you're really getting.
Why oxide shows big numbers
Magnesium oxide is cheap and concentrated: it contains a high proportion of elemental magnesium relative to its weight. This makes it possible to display big numbers on a small capsule, which explains its presence in many entry-level products.
Its downside is well known: oxide is poorly soluble, part of it passes through the digestive tract without being absorbed, and it is the form most likely to speed up transit1,2. Hence the frequent gap between the dose shown and the dose actually used by the body.
At equal elemental magnesium content, a better-absorbed form is often worth more than a large dose of oxide of which part will be lost.
Citrate, bisglycinate: better absorbed, better tolerated
The magnesium citrate is more soluble than oxide and generally better tolerated1. The bisglycinate, or glycinate, binds magnesium to an amino acid, glycine; it is reputed to be gentle on transit and well absorbed4, which makes it a form that is well liked, often a little more expensive.
Other forms exist, such as malate or lactate, each with its supporters. There is no universally perfect form: the right choice depends on each person's digestive tolerance and budget. A simple rule: at equal elemental magnesium content, a better-absorbed form is often worth more than a large dose of oxide that is partly lost.
Reading the real content on the box
To compare two magnesiums honestly, look for three pieces of information: the form (in parentheses after the word magnesium), the elemental magnesium content per daily dose, and the number of capsules to take to obtain it3.
Then bring everything back to the same basis. A product that provides 150 mg of elemental magnesium in two well-absorbed capsules can be worth more than another showing 300 mg of oxide in one capsule, part of which will not be retained. The percentage of the NRVs, shown alongside, helps you situate it: the NRV for magnesium is counted in hundreds of milligrams.
The question of tolerance
The most common sign of a poorly tolerated or overdosed magnesium is accelerated transit. It often eases by changing form, by splitting the intake across the day, or by taking it during a meal.
Magnesium is one of the minerals whose intake from supplements is framed by safety guideposts5. It is therefore pointless, and sometimes counterproductive, to multiply the doses. In the event of an ongoing treatment, kidney failure or doubt, advice from the pharmacist or doctor helps you choose the appropriate form and dose.
What to remember
Remember three ideas. First, only elemental magnesium counts, not the weight of the salt. Next, oxide shows big numbers but is poorly absorbed, whereas citrate and bisglycinate are better tolerated. Finally, an honest comparison is made at equal elemental content, doses included.
With these guideposts, a magnesium label stops being a marketing argument and becomes information again. The same reasoning applies, moreover, to most minerals.
References
- Walker AF, et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnesium Research, 2003. pubmed.ncbi.nlm.nih.gov
- Higher bioavailability of magnesium citrate as compared to magnesium oxide, shown by urinary excretion and serum levels. BMC Nutrition, 2017. link.springer.com
- Magnesium bioavailability from magnesium citrate and magnesium oxide. Journal of the American College of Nutrition, 1990. pubmed.ncbi.nlm.nih.gov
- Comparative clinical study on magnesium absorption after oral intake of different magnesium sources. Nutrients, 2024. ncbi.nlm.nih.gov
- Magnesium, Health Professional Fact Sheet. Office of Dietary Supplements, National Institutes of Health, accessed 2026. ods.od.nih.gov