Made & designed in Sweden.Magnesium supplements never contain magnesium on its own. It is always bound to something else — citrate, glycine, oxide and so on. That partner decides how much magnesium you get per milligram, how easily it dissolves, and how it feels in your stomach. Here is how the three most common forms compare.
Why magnesium comes in different forms
Magnesium is a reactive mineral, so in a supplement it is always part of a compound: a magnesium salt (like citrate or oxide) or magnesium bound to an amino acid (like glycine). The EU lists which magnesium compounds may be used in food supplements, and citrate, oxide and bisglycinate are all on that list.
Two numbers matter when you compare them. The first is how much of the compound is actually magnesium — the elemental magnesium. The second is how much of that magnesium your body can absorb.
The three forms side by side
| Magnesium oxide | Magnesium citrate | Magnesium bisglycinate | |
|---|---|---|---|
| Bound to | Oxygen | Citric acid | The amino acid glycine |
| Elemental magnesium (approx.) | About 60% | About 11–16% | About 14% |
| Dissolves in water | Poorly | Well | Well |
| Human absorption studies | Many — generally low absorption | Several — higher than oxide | Few |
The share of elemental magnesium varies with how much water the compound contains, so treat the percentages as rough guides. What counts on a label is the amount of magnesium itself — for example “Magnesium (citrate) 100 mg” — not the weight of the whole compound.
Magnesium oxide: a lot of magnesium, poorly dissolved
Oxide packs the most magnesium into the smallest capsule, which is why it is common and cheap. The drawback is that it dissolves poorly. In a frequently cited study, magnesium from oxide was absorbed less than magnesium from citrate, and the authors linked this to oxide’s low solubility.
Magnesium citrate: soluble and well studied
Citrate is magnesium bound to citric acid, the same acid found in citrus fruit. It dissolves readily, and studies comparing it with oxide have found higher absorption from citrate. In a randomised, double-blind study, magnesium citrate led to higher blood magnesium levels than both oxide and an amino-acid chelate, after a single dose and after 60 days.
Because each milligram of citrate contains less magnesium than oxide, a citrate supplement needs more powder — or more capsules — for the same amount of magnesium.
Magnesium glycinate: popular, less researched
Magnesium bisglycinate (often just called glycinate) is magnesium bound to two molecules of glycine. It is often described as gentle on the stomach and well absorbed. There are fewer controlled human studies on glycinate than on citrate and oxide, so the evidence for how it compares is thinner than its popularity suggests.
What about the stomach?
Magnesium that isn’t absorbed draws water into the gut, which is why high doses of magnesium can loosen the stomach. For this reason the EU’s scientific advisers set an upper limit of 250 mg per day for magnesium from supplements — a level based on avoiding that effect, not on toxicity.
For context, EFSA considers 350 mg per day an adequate intake for men and 300 mg for women, from all sources. Most of that should come from food: whole grains, legumes, nuts, seeds, dark green vegetables and dark chocolate are good sources.
What the EU allows anyone to say about magnesium
In the EU, a supplement may only make health claims that have been scientifically assessed and approved. For magnesium, the approved claims include that it:
- Contributes to a reduction of tiredness and fatigue
- Contributes to normal energy-yielding metabolism
- Contributes to normal muscle function
- Contributes to normal functioning of the nervous system
- Contributes to normal psychological function
- Contributes to electrolyte balance
- Contributes to the maintenance of normal bones
- Contributes to the maintenance of normal teeth
These claims belong to magnesium as a nutrient, whichever form it comes in. No form has an approved claim of its own. If you see a specific form marketed for a specific effect — sleep, for example — that is not a claim the EU has approved.
So which is best?
If you want the most magnesium in the fewest capsules, oxide wins on paper but is the least soluble. If you want a form with good solubility and solid comparative research, citrate is a well-documented choice. Glycinate is a reasonable option too, but with less direct evidence behind it.
Whatever the form, check the amount of elemental magnesium on the label and how it fits with what you already get from food.
Sources
- Lindberg JS et al. Magnesium bioavailability from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990;9(1):48–55
- Walker AF et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003;16(3):183–191
- EFSA NDA Panel. Scientific Opinion on Dietary Reference Values for magnesium. EFSA Journal 2015;13(7):4186
- NIH Office of Dietary Supplements — Magnesium fact sheet for health professionals
- European Commission — EU Register of nutrition and health claims
This article is general information about nutrition and is not medical advice. Food supplements should not be used as a substitute for a varied diet and a healthy lifestyle. If you have a medical condition, are pregnant or take medication, talk to your doctor before taking supplements.
