Magnesium forms

What are the types of magnesium supplements?

Magnesium supplements come in several forms, such as oxide, citrate and chloride. ^[1] This guide also summarizes the available evidence on bisglycinate (diglycinate, glycinate), malate, threonate, lactate and sulfate. No brand or product recommendations are made.

What does the amount on the label show?

As the NIH states, the Supplement Facts section on a supplement label reports the amount of elemental magnesium, not the weight of the whole compound. ^[1] The full weight of the compound may be written elsewhere on the label, for example right next to the product name, and the two can be confused.

The elemental proportion varies widely from form to form. According to a theoretical calculation from molecular weights (hydrate and purity vary by product), the approximate elemental magnesium in 100 mg of compound is as follows:

These proportions are stoichiometric calculations; a product may contain blends, hydration and fillers (for example, in the nonahydrate form of citrate the proportion falls to about 12%). What matters is the “elemental magnesium (mg)” line on the label. The upper limits (NIH 350 mg; EFSA 250 mg) are also interpreted for the amount of elemental magnesium, so they should not be compared with the compound weight of the product. ^[1]

Why is magnesium oxide different?

Lindberg and colleagues reported that magnesium oxide is almost insoluble in water, dissolving only 43% even under simulated maximal acid secretion, whereas citrate showed high solubility even in water. In volunteers, the rise in urinary magnesium after 25 mmol of citrate was significantly higher than with oxide. ^[4] In the study by Firoz and Graber, the fractional absorption of oxide was found to be 4%; chloride, lactate and aspartate were absorbed significantly more and equivalently to one another. ^[2]

In the 60-day double-blind study by Walker and colleagues in 46 healthy people (300 mg elemental magnesium per day), organic forms (citrate and amino acid chelate) were absorbed more than oxide according to 24-hour urinary magnesium; serum magnesium rose most with citrate; there was no difference between oxide and placebo. ^[3]

The difference in absorption does not mean oxide is a bad product. Oxide is common on labels because its elemental proportion is high, and the unabsorbed portion may contribute to a laxative effect. The NIH notes that the forms most often reported with diarrhea are carbonate, chloride, gluconate and oxide. ^[1]

What is magnesium citrate?

Citrate is a salt that magnesium forms with citric acid. In two studies comparing it directly with oxide, it was found to be more soluble and better absorbed. ^{[3] [4]} The NIH also reports that citrate is absorbed more completely than oxide. ^[1] High absorption, in addition to raising blood magnesium, does not remove the fact that the unabsorbed portion may also have an osmotic effect; diarrhea is reported at high amounts.

What is magnesium glycinate (bisglycinate)?

Glycinate or bisglycinate is the chelate form in which magnesium is bound to two glycine (amino acid) molecules. In the double-blind crossover study by Schuette and colleagues in 12 patients with ileal resection, isotope-labeled diglycinate and oxide at a 100 mg dose were absorbed similarly overall (23.5% vs 22.8%). In the four patients whose absorption of oxide was most impaired, absorption from glycinate was higher (23.5% vs 11.8%), and the product was better tolerated by all patients. The authors suggested that part of the glycinate may be absorbed intact as a dipeptide. ^[5] Because the sample was very small and the patients had ileal resection, the results cannot be carried over to the general population.

Regarding sleep, in 155 adults reporting poor sleep, 250 mg of elemental magnesium (bisglycinate) was reported to produce a small additional decrease in insomnia score over four weeks (effect size 0.2); 93% of participants reported no side effects, and side effects were reported less often in the magnesium group than with placebo. ^[6] This single study does not show that glycinate is superior to citrate or oxide; it contained no comparison.

For an existing page that places the two popular forms side by side: magnesium glycinate vs citrate comparison.

Magnesium malate, threonate and others

• Malate: Human data comparing the absorption of the malate form with oxide or citrate are limited; there is not enough evidence for a benefit on “energy” or “fatigue.”
• Threonate: In a single study, 109 healthy adults used a formula containing magnesium L-threonate with phosphatidylserine and vitamins C and D, or placebo, for 30 days; an improvement over placebo was reported on memory tests. The study was funded by the formula’s manufacturer. Because the formula is multi-component, the effect cannot be attributed to magnesium threonate; the study’s introduction bases the claim about brain levels on earlier research, but this study did not measure brain magnesium. ^[7]
• Chloride, lactate, aspartate: Absorbed better than oxide in small absorption studies. ^{[1] [2]}
• Zinc taken together: Whatever the form, the NIH states that very high amounts of supplemental zinc (142 mg per day) may impair magnesium absorption. ^[1]
• Enteric-coated products: In the study by Fine and colleagues, absorption from enteric-coated magnesium chloride was much lower than from magnesium acetate; that is, the coating may impair absorption. In the same study, absorption from almonds, a food source, was found to be equivalent to soluble magnesium acetate. ^[8]

What can and cannot be said about each form?

• Can be said: Highly soluble salts were absorbed more than oxide in small studies in healthy volunteers. ^{[1] [2] [3]}
• Cannot be said: It has not been shown in direct comparisons that the better-absorbed form improves sleep, cramps or any other outcome more.
• Absorption percentage is not fixed: Fine and colleagues showed that as the amount of magnesium given with a meal increased, fractional absorption fell from 65% to 11%; that is, the absorption of the same product also varies with the amount. ^[8]
• Digestion: With large amounts, diarrhea, nausea and abdominal cramps may occur with all forms; the mechanism is the osmotic effect of unabsorbed salts. ^[1]

This guide does not recommend a personal product or amount. In the presence of kidney disease, medication use (bisphosphonate, tetracycline, quinolone) or pregnancy, form selection and timing are evaluated together with a physician or pharmacist.