When you choose a magnesium supplement, two things matter more than the name on the pack: how much magnesium the dose actually gives you and in what form. Organic forms such as magnesium citrate or bisglycinate are generally absorbed better than magnesium oxide. For supplements, the upper limit set by the EU's scientific committee is 250 mg of magnesium a day for adults.
Why are there so many forms of magnesium?
Magnesium never comes on its own. In a supplement it is always bound to something else, which keeps it stable and affects how easily it dissolves in the gut. The most common forms on the shelf:
- Magnesium oxide. A lot of magnesium in a small tablet and cheap, but it does not dissolve easily.
- Magnesium citrate. Dissolves easily. At higher doses it has a well-known laxative effect.
- Magnesium bisglycinate (also sold as glycinate or "chelated" magnesium). Magnesium bound to the amino acid glycine. Many people prefer it because they find it gentler on the stomach.
- Magnesium lactate, chloride and aspartate. Less common, but also soluble forms.
Which form is absorbed best?
A 2021 systematic review in the journal Nutrition (Pardo and colleagues) brought together 14 studies and concluded that inorganic forms, such as oxide, appear to be less bioavailable than organic ones. The US National Institutes of Health Office of Dietary Supplements says something similar: small studies have found that magnesium as aspartate, citrate, lactate and chloride is absorbed more completely than magnesium oxide and sulfate.
Two caveats:
- The studies are small and differ a lot from one another. The differences between the good organic forms are not large.
- The same review notes that in healthy people without a deficiency, all forms can maintain normal levels.
So in practice the form is a matter of tolerance and preference, not a dramatic difference. If you have a sensitive stomach, a bisglycinate is a sensible place to start. If not, a citrate does the job too.
What should you check on the label?
1. The mg of magnesium, not of the compound. This is the most common mix-up. A tablet may say "1000 mg magnesium citrate" on the front, but magnesium itself is only part of that weight. Going by the chemistry of each compound, magnesium makes up roughly:
- 60% of magnesium oxide
- 16% of (anhydrous) magnesium citrate
- 14% of magnesium bisglycinate
EU labels must show the amount of magnesium per daily dose in the nutrition table. That is the number to compare.
2. The %NRV. The EU reference value for magnesium is 375 mg. A dose with 100 mg gives about 27% NRV, a dose with 200 mg about 53%.
3. How many tablets make up the dose. If the daily dose is "2 tablets", the figures in the table are for both together.
4. What else is in it. Some products add vitamin B6, zinc or other nutrients. If you already take a multivitamin, add them up.
How much magnesium do I need, and how much is too much?
EFSA has set an adequate intake of 350 mg a day for men and 300 mg a day for women, from the whole diet. Most of this comes from food: pulses, nuts, wholegrain cereals and green leafy vegetables.
For magnesium from supplements, the EU Scientific Committee on Food set an upper limit of 250 mg a day for adults, including during pregnancy and breastfeeding. This limit covers magnesium from supplements, water and fortified foods, not the magnesium naturally present in food. The main side effect of going over it is diarrhoea.
Are there medicines that clash with magnesium?
Yes, a few. According to the NIH, magnesium can reduce the absorption of some antibiotics (tetracyclines and quinolones) and of bisphosphonates used for osteoporosis, so they usually need to be taken at different times of day. Ask your doctor or pharmacist before starting magnesium if you take any medication, if you have a kidney problem, or if you are pregnant or breastfeeding.
What does magnesium actually do?
The EU has authorised several health claims for magnesium. Some of them, as worded in the official register:
- Magnesium contributes to a reduction of tiredness and fatigue.
- Magnesium contributes to normal muscle function.
- Magnesium contributes to normal functioning of the nervous system.
- Magnesium contributes to the maintenance of normal bones.
- Magnesium contributes to normal energy-yielding metabolism.
Claims such as "cures cramps" or "fixes insomnia" are not allowed and are not supported by the register.
FAQ
Is magnesium bisglycinate the "best" form?
There is no single best form for everyone. Organic forms such as bisglycinate and citrate are generally absorbed better than oxide. The differences between them are small, so choose based on how your stomach copes and on the mg of magnesium per dose.
Why does magnesium give me diarrhoea?
Magnesium that is not absorbed draws water into the gut. This is more likely with larger doses and with certain forms. Lower the dose, split it across the day or try a different form, and speak to your pharmacist if it carries on.
Can I take magnesium with vitamin D?
Many people take them together. Check that the total from all your products stays below the upper limits: 250 mg for magnesium from supplements and 100 µg for vitamin D. If you take medication or have a health condition, ask your doctor.
Sources: EFSA, Scientific Opinion on Dietary Reference Values for magnesium (EFSA Journal 2015, 13(7):4186), Scientific Committee on Food, Opinion on the Tolerable Upper Intake Level of magnesium (2001), as summarised in EFSA's tables of upper intake levels, EU Regulation 1169/2011 (Annex XIII), EU Regulation 432/2012 (list of authorised health claims), Pardo MR et al., Bioavailability of magnesium food supplements: A systematic review, Nutrition 2021, 89:111294, NIH Office of Dietary Supplements, Magnesium Fact Sheet for Health Professionals.
