Why two magnesium bottles can both say 500 mg and mean different things

Magnesium never turns up in a supplement on its own. It's a reactive metal, so it always comes attached to something else — oxide, citrate, malate, or an amino acid like glycine. That partner is what separates one magnesium product from the next, and it's also why the numbers on two packs can look the same and mean quite different things.

The catch is that the weight on the front of a pack might refer to the whole compound, or to the magnesium inside it. Magnesium oxide is around 60% magnesium by weight; citrate about 16%; bisglycinate roughly 14%. So a capsule saying 500 mg of magnesium bisglycinate is actually giving you about 70 mg of the mineral. The number that matters is the second one — and in the EU it's the one that has to appear in the nutrition table, next to its percentage of the daily reference intake.

The forms themselves are genuinely different things. Bisglycinate holds the magnesium between two molecules of glycine, an amino acid, wrapped around it like a pair of rings. Citrate pairs it with the sharp-tasting compound we know from citrus fruit. Oxide is the plain combination of magnesium and oxygen. They dissolve differently, and they carry very different amounts of actual magnesium for the same weight.

Magnesium is one of the nutrients the EU keeps an approved list of claims for. Magnesium contributes to the reduction of tiredness and fatigue, to normal muscle function, to normal psychological function, to the normal function of the nervous system and to normal energy-yielding metabolism. Those claims may be made for any product supplying at least 15% of the nutrient reference value per daily portion — which, again, means the elemental figure rather than the compound weight.

So when you're comparing two magnesium products, three numbers tell you most of what you need: how much actual magnesium is in a daily portion, what percentage of the daily reference intake that comes to, and which form the mineral is in. We print all three on every product page. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle.