What Is Magnesium? The Mineral, the 375mg NRV and the Claims
What is magnesium?
An essential mineral, the fourth most abundant in your body, and a cofactor for hundreds of enzyme systems. It's also the most over-claimed supplement in Britain — so this page sets out what magnesium is, and separates the nine things it's actually authorised to be sold on from the five it isn't.
What is magnesium — explainer summary
Magnesium is an essential dietary mineral, meaning the body cannot manufacture it and it must be obtained from food or supplements. It is the fourth most abundant mineral in the human body. The majority is held in bone, most of the remainder sits inside cells, and only a very small fraction circulates in blood — which is why a blood magnesium measurement is only a partial guide to total body magnesium. Magnesium acts as a cofactor for a large number of enzyme systems, commonly cited as several hundred, including enzymes involved in energy-yielding metabolism and protein synthesis. A cofactor is a non-protein molecule that an enzyme requires in order to function. Dietary sources include nuts, seeds, legumes, wholegrains, leafy green vegetables, dark chocolate and some mineral waters. Refining removes much of the magnesium from grains, so a diet built largely on refined foods tends to supply less than one built on wholegrains and vegetables. In supplements, magnesium is never sold alone, because elemental magnesium is a reactive metal that cannot be encapsulated on its own. It is always bound to another molecule, so the milligram figure on the front of a pack is the weight of the whole compound rather than the weight of magnesium. Elemental magnesium, declared in the nutrition panel, is the fraction that is actual magnesium and the only figure comparable across forms and brands. Three regulatory figures govern the category, all referring to elemental magnesium. The United Kingdom Nutrient Reference Value is 375 milligrams per day, a labelling reference rather than a personal target. The 15 percent threshold is 56.25 milligrams of elemental magnesium per serving, the minimum before a product may be described as a source of magnesium or carry any authorised magnesium health claim. UK guidance sets a safe upper level of 400 milligrams per day for supplemental magnesium, separate from and additional to magnesium obtained from food. Nine authorised Great Britain and European Union health claims exist for magnesium: contribution to a reduction of tiredness and fatigue, normal energy-yielding metabolism, normal functioning of the nervous system, normal muscle function, normal protein synthesis, normal psychological function, maintenance of normal bones and teeth, electrolyte balance, and a role in the process of cell division. Claims relating to sleep, muscle cramps, anxiety, migraine and restless legs are not authorised for magnesium in the United Kingdom. No authorised claim distinguishes one magnesium form from another. Magnesium can reduce the absorption of certain antibiotics including tetracyclines and quinolones, and can interact with bisphosphonates. People with reduced kidney function should seek medical advice before supplementing, because the kidneys are the main route by which excess magnesium is cleared.
Essential, meaning you have to eat it.
"Essential" has a precise meaning in nutrition: your body can't manufacture it, so it has to come in from outside. Magnesium is essential in that sense, and it's the fourth most abundant mineral in the human body.
Its main job is as a cofactor — a molecule that enzymes need in order to work. Magnesium is a cofactor for a large number of enzyme systems, commonly cited as several hundred, including enzymes involved in energy-yielding metabolism and protein synthesis.
That breadth is why magnesium ends up attached to so many marketing claims. Being involved in hundreds of processes is not the same as fixing hundreds of problems, and the gap between those two statements is where most of the category's over-claiming lives.
Almost none of it is in your blood.
This is the detail that makes magnesium status genuinely hard to assess, and it's rarely mentioned.
The majority of the body's magnesium is held in the skeleton, functioning partly as a reservoir.
Intracellular, where the enzyme work happens. Not something a routine blood draw reaches.
Only a very small fraction circulates in serum — so a normal-looking reading tells you less than it appears to.
The practical upshot: serum magnesium is a partial picture. A blood level can look unremarkable while total body stores differ. Interpreting that is a clinical judgement — and emphatically not something a supplement company should be making for you.
If you're concerned about your magnesium status, that's a conversation for a doctor, not a product page. We're not going to tell you that you're deficient, and you should be wary of any brand that does — particularly one selling the remedy.
Spread widely across plant foods.
Magnesium isn't concentrated in one or two exotic foods. It's distributed fairly broadly, mostly across plants:
| Source | Notes |
|---|---|
| Nuts and seeds | Among the most commonly cited sources — pumpkin seeds, almonds, cashews |
| Legumes | Beans, lentils, chickpeas |
| Wholegrains | Wholegrain rather than refined — see below |
| Leafy green vegetables | Magnesium sits at the centre of the chlorophyll molecule |
| Dark chocolate | Genuinely a source, though not a strategy |
| Some mineral waters | Content varies enormously by source — check the label |
Refining is the thing worth knowing. Processing grain removes much of its magnesium, so a diet built largely on refined foods tends to supply less than one built on wholegrains, nuts and vegetables. That's a compositional fact about foods — not a claim that you personally are short of magnesium, which we've no way of knowing.
375, 56.25, 400.
Three figures govern magnesium supplements in the UK, and all three refer to elemental magnesium — not the compound weight on the front of a pack.
| Figure | What it is | What it is not |
|---|---|---|
| 375mg — the NRV | A labelling reference, so a serving can be shown as a % | Not a personal target, and doesn't account for food |
| 56.25mg — 15% of NRV | The minimum per serving to be called a source of magnesium or carry a claim | Not a recommended dose |
| 400mg — upper level | UK safe upper level for magnesium from supplements | Not a total-intake ceiling — food is separate |
Why elemental magnesium matters so much: magnesium is a reactive metal and can't be put in a capsule alone, so it's always bound to something — glycine, citric acid, an oxide. That partner is most of the weight. A pack saying "Magnesium Citrate 1000mg" might contain around 110mg of actual magnesium. Read the panel, not the front. Our forms guide has the full elemental ladder, and the dosage guide covers adding up your total across products.
Nine authorised claims. Five that aren't.
Magnesium is unusual in having a generous set of authorised claims — and it's also among the most heavily over-claimed ingredients on the UK market. Knowing which is which tells you a great deal about a brand.
| Claim | Status in Great Britain |
|---|---|
| Reduction of tiredness and fatigue | Authorised — requires ≥15% NRV |
| Normal energy-yielding metabolism | Authorised |
| Normal functioning of the nervous system | Authorised |
| Normal muscle function | Authorised |
| Normal protein synthesis | Authorised |
| Normal psychological function | Authorised |
| Maintenance of normal bones and teeth | Authorised |
| Electrolyte balance | Authorised |
| Role in the process of cell division | Authorised |
| Sleep | Not authorised. The most common unauthorised magnesium claim in Britain |
| Muscle cramps | Not authorised. "Normal muscle function" is permitted; preventing cramp is a different claim |
| Anxiety and stress | Not authorised. "Normal psychological function" is permitted; treating anxiety is not |
| Migraine | Not authorised. A disease claim — not permitted for any food supplement |
| Restless legs | Not authorised. Also a disease claim |
Look at how close some of those pairs sit. "Contributes to normal muscle function" is permitted; "prevents cramp" is not. "Contributes to normal psychological function" is permitted; "helps with anxiety" is not.
That isn't pedantry. It's the line between describing a nutrient's role in normal physiology and promising to fix a problem — and it's precisely the line most magnesium marketing steps over. A brand that crosses it here is likely crossing it elsewhere, including on the parts of the label you can't independently verify.
Why there are so many of them.
Because magnesium has to be bound to something, and there are many candidates. Each pairing gives a different elemental percentage, a different solubility and a different tolerability profile.
What's important to be clear about: no authorised UK or EU health claim distinguishes one magnesium form from another. Nobody may lawfully tell you glycinate is better for sleep, taurate better for the heart, or threonate better for the brain. What genuinely differs is:
| Difference | Example |
|---|---|
| Elemental content | Oxide is ~60% elemental magnesium; chelates such as bisglycinate are around 14% |
| Absorption | Separate from elemental content — oxide is highest in elemental magnesium and among the least well absorbed |
| Tolerability | Citrate has a laxative effect at higher intakes; chelates are commonly chosen by people who find other forms unsettling |
| Cost | Simpler salts are cheaper per mg of elemental magnesium than chelates |
| Serving size | Low-elemental forms need more capsules to reach the same magnesium |
The full elemental ladder across twelve forms, the "oxide trap" and the buffered-chelate arithmetic check are all in magnesium forms explained. For the two forms people most often weigh against each other, see glycinate vs citrate.
Ready to choose one?
Our buying guide ranks the range on elemental magnesium and %NRV — the figures this page is about — with the trade-offs stated plainly.
Common questions.
Keep going.
- BuyingBest magnesium supplement UK — ranked on elemental magnesium
- BuyingBest magnesium glycinate UK — the glycinate options ranked
- ReferenceMagnesium forms explained — elemental content of every form
- DosageMagnesium dosage — the 375mg NRV and 400mg upper level
- CompareMagnesium glycinate vs citrate
- ShopMagnesium supplements — the full UK range
- QualityQuality, testing & sourcing — UK manufacture and batch CoAs
- EditorialEditorial standards — how we research and review
Last reviewed 17 August 2026 by the Vitality Supplements Editorial Team, written to our published editorial standards and quality & testing policy. We are a UK supplement manufacturer and we sell magnesium, which we disclose openly. Two editorial decisions worth naming: we have set out the nine authorised magnesium claims alongside the five commonly marketed ones that are not authorised, rather than quietly using the permitted wording and letting readers infer the rest; and we have been explicit that we cannot tell you whether you are low in magnesium, because serum magnesium is a partial picture and that judgement belongs to a doctor rather than to a company selling the supplement.
Food supplements should not be used as a substitute for a varied, balanced diet and a healthy lifestyle. Do not exceed the stated recommended daily dose. UK guidance sets a safe upper level of 400mg per day for supplemental magnesium, separate from magnesium obtained from food. Authorised GB and EU health claims for magnesium require a serving providing at least 15% of the 375mg Nutrient Reference Value; no authorised claim distinguishes between magnesium forms, and claims relating to sleep, muscle cramps, anxiety, migraine or restless legs are not authorised and are not made here. If you have reduced kidney function, are pregnant, breastfeeding, taking medication or under medical care, consult a healthcare professional before use — magnesium can interact with certain antibiotics and other medicines. This page is general information, not medical advice, and is not intended to diagnose, treat or prevent any condition. Every Vitality batch is independently tested. UK manufactured. Free UK delivery. 60-day satisfaction guarantee on first purchase. See our full disclaimer · Privacy · Terms · info@vitality-supplements.co.uk