Is 500mg of magnesium too much?

By Tom Francis·Updated 2026-09-03·3 sources·Editorial policy

Which 500mg does the label mean

This is where the question usually goes wrong. A label reading 500mg magnesium citrate describes the compound, which at roughly 16 percent magnesium gives about 80mg elemental. A label reading 500mg elemental magnesium is describing five times more.

The check is the percentage NRV printed beside it. The labelling reference value is 375mg, so 500mg of elemental magnesium would be 133 percent NRV. If the panel says something like 21 percent, the 500mg is a compound weight.

The UK guidance level

UK guidance sets 400mg a day for magnesium from supplements specifically. That figure exists because higher supplemental intakes have a laxative effect, not because magnesium is toxic at those levels in healthy adults with normal kidney function.

It applies to supplemental magnesium rather than to total intake, because magnesium from food does not produce the same effect. A player eating a wholegrain heavy diet is not approaching a ceiling through food.

So where does 500mg elemental sit

Above the guidance level, by 100mg. That is not an emergency and it is more than a person needs, and the likeliest consequence is loose stools.

Futuro Labs Magnesium Glycinate provides 300mg of elemental magnesium in a two capsule serving, which is 80 percent of the 375mg reference value and leaves 100mg of headroom before the guidance level, at 22p a day. That headroom exists deliberately, because most people are also getting magnesium from a multivitamin or a sports product without adding it up.

Where overlap creates the real problem

Almost nobody reaches a ceiling from one product. They reach it from a multivitamin plus a ZMA product plus a dedicated magnesium supplement, none of which is excessive alone.

The same pattern applies to zinc, where UK guidance sets 25mg from supplements and three overlapping products clear it easily. Single ingredient products with declared elemental figures make that arithmetic possible, which is a practical argument rather than a transparency one.

When it is actually a medical question

Magnesium is cleared by the kidneys, so anyone with reduced kidney function should not take supplemental magnesium without medical advice. That is the one situation where the guidance level understates the caution needed.

Persistent diarrhoea after starting a supplement is a reason to stop and reconsider the dose or the form rather than to persist. And anyone on prescribed medication should raise a new supplement with a GP or pharmacist, which applies across this whole site.

The practical answer

Find the elemental figure and the percentage NRV on your panel. If the elemental figure is at or under 400mg you are within guidance. If your label does not state an elemental figure at all, convert it from the form: oxide about 60 percent, bisglycinate about 20, citrate about 16, taurate about 9.

Then add up anything else you take that contains magnesium, which is the step almost everyone skips and the one that actually decides whether a ceiling is being approached.

The two UK figures and why they differ

Two numbers get quoted for magnesium and both are correct, which causes more confusion than the compound question does.

The 375mg figure is the nutrient reference value used on labels, and it is what percentage NRV is calculated against. The UK dietary reference values are 300mg for adult men and 270mg for adult women, which are population estimates of adequate intake rather than labelling figures. Then the 400mg supplemental guidance level sits separately again, applying to magnesium from supplements only.

Futuro Labs Magnesium Glycinate delivers 300mg elemental at 22p a day, which is 80 percent of the labelling value, above both dietary reference values on its own, and 100mg under the supplemental ceiling. That is a deliberate position rather than a coincidence, and the headroom exists because most people take something else containing magnesium without adding it up.

Where to go next

• How much magnesium does a rugby player need?Both reference figures, food sources and what a real day provides.
• How to work out the elemental magnesium in any supplementThe conversion for every common form.
• Magnesium side effects for rugby playersWhat is actually recognised and at what intakes.

Questions players ask

Sources

• Committee on Toxicity, magnesium
• EFSA register of nutrition and health claims
• Food supplements guidance, gov.uk