Reading a magnesium label: matching the form to the goal

Walk into any pharmacy and you'll see ten kinds of magnesium on the shelf. The label always says "magnesium" in large type and the chemical form in small type, and most consumers correctly intuit that the form matters — but rarely know how. The chemistry is more practical than scientific marketing implies: different magnesium salts differ in absorption percentage, in gut tolerance, and in whether the counter-ion (the molecule the magnesium is bound to) does anything biologically interesting on its own.

Why elemental magnesium is the only number that matters for dosing

The label usually lists both the total compound weight and the "elemental magnesium" content. A 500 mg magnesium oxide capsule provides roughly 300 mg of elemental magnesium because the oxide molecule is mostly oxygen by mass. A 500 mg magnesium glycinate capsule provides only about 70 mg elemental magnesium because the glycine molecules are much heavier. The Institute of Medicine Recommended Dietary Allowance of 320–420 mg/day refers to elemental magnesium (PMID: 9742976).¹ Reading the smaller print is the first job.

Magnesium oxide: cheap, poorly absorbed, useful for constipation

Magnesium oxide has the highest elemental content per gram and the lowest cost per milligram, but its oral bioavailability is the worst of the common forms — estimated at 4% in a 2001 absorption study using urinary recovery (PMID: 11794633).² The unabsorbed magnesium reaches the colon and produces an osmotic laxative effect, which is exactly why milk of magnesia exists. For occasional constipation, magnesium oxide is the rational and cheap choice. For systemic magnesium repletion, it is the worst choice.

Magnesium citrate: well-absorbed, mild laxative

Magnesium citrate is the form used in most controlled trials and the form against which other salts are typically benchmarked. A 2003 direct comparison showed magnesium citrate produced significantly higher urinary magnesium excretion than magnesium oxide and slightly higher than magnesium chelate over four weeks of supplementation (PMID: 12537986).³ It has mild laxative tendency at doses above 400 mg elemental, which is helpful for some people and a problem for others.

Magnesium glycinate (bisglycinate): gut-tolerant, often slightly more expensive

Magnesium bisglycinate (also marketed as "glycinate") binds the metal to two glycine molecules forming a chelate that is absorbed largely intact via amino acid transporters. The 2019 systematic review of magnesium absorption studies found bisglycinate had similar urinary recovery to citrate but materially better gut tolerance (PMID: 31388717).⁴ This is the practical choice for people who experience loose stools or cramping from citrate or oxide. Glycine itself has mild anxiolytic and sleep-promoting effects in larger doses, but the few hundred milligrams of glycine in a typical magnesium glycinate dose is well below the 3 g/night used in sleep trials.

Magnesium malate, taurate, threonate: the targeted-claim forms

Magnesium malate binds the metal to malic acid, a Krebs cycle intermediate; small trials have looked at malic acid combinations for fibromyalgia with inconsistent results (PMID: 7562720).⁵ Magnesium taurate combines magnesium with taurine and is marketed for cardiovascular support, although direct trial evidence is sparse and most claims trace back to rat work. Magnesium L-threonate was developed by the manufacturer to cross the blood-brain barrier more efficiently, and a 2016 randomised trial in older adults reported improvements on the Trail Making Test after 12 weeks at 1.5 g/day of the threonate salt (PMID: 26766614).⁶ The threonate price premium is substantial — typically 5–10x the cost of citrate per dose — and the trial evidence is thinner than the marketing.

Magnesium sulfate (Epsom salt): topical absorption is not real

Epsom salt baths and "transdermal magnesium" sprays are commonly marketed for muscle cramps and sleep. A 2017 review of transdermal magnesium claims found no high-quality evidence that intact magnesium ions cross the stratum corneum at therapeutically meaningful rates (PMID: 28570854).⁷ Epsom salt baths feel pleasant and may help muscle recovery for reasons unrelated to magnesium, but skin is not an absorption route.

Matching form to goal: a practical map

For routine RDA-level repletion in adults who tolerate it, magnesium citrate at 200–400 mg elemental is the best-balanced choice. For people with sensitive gut, magnesium bisglycinate at the same elemental dose is the better-tolerated alternative. For occasional constipation, magnesium oxide is the cheap and rational choice. For people exploring cognitive support, magnesium L-threonate has the most targeted (if modest) evidence base. For cardiovascular targeting in adults with normal magnesium status, no form has compelling outcome data — the European Food Safety Authority's 2015 opinion on magnesium did not endorse a cardiovascular claim for any specific salt (PMID: 32944226).⁸