Magnesium labels mix two different numbers: the weight of the compound (magnesium bound to something else) and the weight of elemental magnesium in it. Only the second one counts toward your intake.
The arithmetic
The share of elemental magnesium depends on what it is bound to. Using standard molar masses (magnesium is 24.3 g/mol):
| Form | Formula | Molar mass | Elemental Mg |
|---|---|---|---|
| Magnesium oxide | MgO | 40.3 | about 60% |
| Magnesium citrate (anhydrous) | Mg3(C6H5O7)2 | 451.1 | about 16% |
| Magnesium bisglycinate | Mg(C2H4NO2)2 | 172.4 | about 14% |
So 500 mg of bisglycinate compound carries about 70 mg of magnesium, and 500 mg of oxide carries about 300 mg. Real products vary (hydrates, buffering with oxide), so read the "elemental" or "as magnesium" line on the label.
More magnesium on paper, less absorbed
Oxide packs the most magnesium per gram but absorbs poorly. In an absorption study of four US commercial preparations, magnesium oxide had a fractional absorption of about 4%. Chloride, lactate and aspartate were absorbed significantly better and about equally. The same study notes that inorganic salts are not automatically worse: chloride did as well as the organic forms.
For glycinate, a small study in people with ileal resection found it was partly absorbed intact and better tolerated than oxide.
The upper limit applies to supplements
EFSA set a tolerable upper intake level of 250 mg a day of supplemental magnesium for adults, based on the risk of mild diarrhoea. Food magnesium does not count toward it. That number is elemental magnesium, which is why the arithmetic above matters.
"Threonate crosses into the brain"
The claim traces back largely to a 2010 study in rats, where magnesium L-threonate raised brain magnesium and improved learning and memory. That is a mechanism study, not evidence that it does the same in people.
Two safety notes
- Magnesium binds tetracycline and quinolone antibiotics in the gut and cuts their absorption. Separate them.
- With kidney failure, oral magnesium can build up to dangerous levels; fatal cases have been reported.
Sources
- Firoz M, Graber M. 2001. Bioavailability of US commercial magnesium preparations. Magnes Res 14(4):257-62. PubMed 11794633
- Schuette SA, Lashner BA, Janghorbani M. 1994. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN. PubMed 7815675
- EFSA Scientific Committee on Food. 2006. Tolerable Upper Intake Levels for Vitamins and Minerals. PDF
- Slutsky I et al. 2010. Enhancement of learning and memory by elevating brain magnesium. Neuron 65(2):165-77. PubMed 20152124
- Gugler R, Allgayer H. 1990. Effects of antacids on the clinical pharmacokinetics of drugs. An update. PubMed 1969784
- Schelling JR. 2000. Fatal hypermagnesemia. Clin Nephrol. PubMed 10661484
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