Magnesium: which form to choose? What the studies say

Citrate, oxide, bisglycinate, threonate, "marine"... Every label promises the best-absorbed form. Studies give a simpler answer: citrate passes better than oxide in most comparisons, but the dose and your baseline status matter more than the form. And the only major trial that compared three forms on a health effect found none that works better than placebo.

What matters before the form: the dose

An absorption study in healthy subjects measured it directly: the share of magnesium absorbed drops from 65% at the smallest dose to 11% at the largest. The more you take at once, the more the lost share increases. In the same study, magnesium from almonds was absorbed as well as a soluble salt, whereas a gastro-resistant magnesium chloride tablet was markedly less so.

A 2017 review on intestinal absorption draws the practical consequence: relative absorption is better with several small doses spread throughout the day. It also concludes that the type of salt matters less than believed: some studies find a slight advantage for organic salts, others do not, and lacking standardized tests, no one can say which form is best. The total quantity and what you already have in the body weigh heavier. A 2026 pharmacokinetic study confirms this for oxide: in 24 men, going from 750 to 2,000 mg did not proportionally increase exposure.

Organic or inorganic: an advantage, but modest

A 2021 systematic review selected 14 studies out of 433. Inorganic forms (oxide, for example) appear less bioavailable than organic ones (citrate, glycinate...), and absorption depends on the dose. But all magnesium supplements are enough to maintain a normal level in a healthy person who is not deficient; this is no longer guaranteed in the elderly or in someone whose level is already low. An older review recalls where the difference comes from: the anion to which magnesium is bound changes how it is released.

Citrate versus oxide: the most studied duel

Three trials give the advantage to citrate. In 1990, citrate dissolved at 55% in water when oxide was practically insoluble, and in healthy volunteers, urinary magnesium rose much more after citrate (0.22 versus 0.006 mg per mg of creatinine over four hours). A double-blind trial on 46 people, 300 mg per day for 60 days, finds the highest blood level with citrate, better absorption of organic forms (citrate and amino acid chelate), and no difference between oxide and placebo. In 14 men with already full reserves, 400 mg of citrate made urinary and blood magnesium rise, 400 mg of oxide did not.

A fourth trial says the opposite. In 41 volunteers, for a month, oxide made intracellular magnesium rise more and total cholesterol and LDL drop more — but it provided 520 mg of magnesium per day against 296 mg for citrate. The comparison is therefore not done at an equal dose.

The flaw of oxide lies mainly in its poor solubility. In 13 men, 450 mg of magnesium as oxide made urinary excretion rise by 40% in an effervescent tablet, dissolved in water before being drunk, compared to 20% in a capsule. Same salt, twice the effect: galenics matter as much as the name on the label.

Better absorbed, but more effective?

This is the question that matters, and only one trial in the batch answers it head-on. Its protocol, published in advance, planned to compare citrate, oxide and sulfate for 24 weeks, at 450 mg of magnesium per day, in overweight adults. The results, on 164 people: none of the three forms changed arterial stiffness or blood pressure compared to placebo. Citrate did make blood magnesium rise (+0.04 mmol/L) and especially urinary magnesium, more than oxide and sulfate; the rise in the blood, however, was similar from one form to another.

In other words, the better-absorbed form filled the bladder better, not the arteries. Superior bioavailability measured in urine does not automatically translate into a health effect.

The other forms: few direct comparisons

Glycinate (or bisglycinate). Its only direct comparison in the batch involves patients. In 12 patients who underwent intestinal surgery, 100 mg of diglycinate and oxide were absorbed the same way (23.5% versus 22.8%). In the four patients who absorbed oxide the worst, glycinate did twice as well (23.5% versus 11.8%), and it was better tolerated. This is a result in 12 patients, not in a healthy athlete.

"Marine" forms, extended-release, patented. In 20 young adults, a citrate and a hydroxide sourced from seawater both increased absorption by more than 97 % compared to placebo, with no difference between them once the dose was equalized. Two 500 mg tablets, one immediate-release, the other partly delayed, do not differ in absorption or tolerance. An extended-release lactate is 20 % absorbed when fasting and 12.5 % absorbed after a meal according to blood tests, and around 40 % according to urine. A so-called "sucrosomial" form outperformed oxide in blood and urine, and outperformed bisglycinate in red blood cells, in a study conducted on this patented product.

L-threonate. EFSA evaluated it in 2024 as a novel ingredient: up to 3,000 mg per day, representing about 250 mg of magnesium, it is deemed safe and constitutes a bioavailable source of magnesium. Incidentally, this figure highlights a labeling trap: 3,000 mg of threonate provide only about 250 mg of magnesium. This is the amount of magnesium that must be compared from one product to another.

Sleep and memory: trials, but no duel

We read everywhere that glycinate or threonate are "the sleep forms". In 155 adults who slept poorly, 250 mg of bisglycinate for four weeks lowered the insomnia score by 3.9 points against 2.3 for placebo: a modest effect (effect size 0.2), more pronounced in those who ate little magnesium. In 80 adults aged 35 to 55, 1 g of L-threonate per day for 21 days improved deep and REM sleep measured by a smart ring, as well as energy and mood felt during the day.

But these two trials compare one form to a placebo, never to another form. There is no evidence that a citrate at the same magnesium dose would perform less well. On the memory side, the trial on 109 Chinese adults tests a formula that adds phosphatidylserine and vitamins C and D to threonate: the measured memory improvement cannot be attributed to magnesium alone.

Every figure in this article comes from a study you can open: the underlined mentions lead to the summary of the work it is drawn from, written from the abstract alone. Most bioavailability studies involve 12 to 46 people and measure blood or urine magnesium, not a health effect. Informative content, which does not replace the advice of a healthcare professional.