How much magnesium per day do you actually need? The UK answer is 300mg a day for men and 270mg for women, and that figure counts everything: the nuts, the wholegrains, the greens and the tap water, not just what comes from a capsule. Most people eating a reasonably varied diet land close to it without thinking. That simple picture sits awkwardly next to a supplement market where 375mg, 400mg and 500mg doses are standard and magnesium is pitched as the answer to sleep, stress, cramps and everything between. This article reconciles the two: where the UK numbers come from, why the bottles say 400mg, what happens above that, and which forms are worth buying if you have a genuine reason to supplement.
What is the UK recommended intake?
The reference nutrient intake, the amount judged sufficient for nearly everyone, is 300mg a day for men and 270mg for women aged 19 to 64, per NHS guidance on minerals. Food gets you there faster than the supplement industry implies: a serving of pumpkin seeds carries around 150mg, a portion of brown rice or a couple of slices of wholemeal bread add 40 to 80mg, and spinach, black beans, almonds, peanut butter and even dark chocolate all contribute meaningfully. National diet surveys do show average intakes running somewhat below the reference figure, particularly in teenagers and young women, but a modest shortfall from food is not the same as a deficiency needing correction; the body also regulates magnesium well, absorbing a higher fraction when intake is lower. The starting question, as with any supplement decision, is whether food can do the job first.
Why do supplements say 400mg?
Mostly because the supplement industry is global and America's numbers are bigger. The US recommended dietary allowance runs to 400 to 420mg a day for men and 310 to 320mg for women, and the EU labelling reference value is 375mg, so products formulated for those markets arrive on UK shelves carrying doses that overshoot the UK reference intake by a third. There is no evidence that healthy people benefit from pushing intake above the UK figure. Helpfully, the NHS position on supplement doses lands near the same number from the safety side: 400mg a day or less from supplements is unlikely to cause harm. The key point is that the supplement sits on top of your food intake. Someone eating well and taking a 400mg capsule is consuming roughly double the reference intake, which is usually pointless rather than dangerous, with the excess handled by the kidneys and, less comfortably, the bowel.
Is 500mg of magnesium too much?
From food alone, no: the body simply absorbs less and excretes more, and there is no established harm from dietary magnesium in people with healthy kidneys. From supplements, 500mg crosses the line where problems begin. The first and most reliable is diarrhoea, because unabsorbed magnesium salts draw water into the bowel, the same mechanism that makes some magnesium compounds effective laxatives. Cramping and nausea follow at higher doses. Genuine magnesium toxicity, with low blood pressure, muscle weakness and heart rhythm disturbance, is rare and almost always involves very large doses or impaired kidneys, which is why anyone with kidney disease should not take magnesium supplements without medical advice. If a 500mg product appeals, splitting it across the day or taking it with food softens the gut effect, but a more sensible response is usually to question why the dose is that high at all.
Which form of magnesium should you take?
The form matters less for outcomes than the marketing suggests and more for your gut than the label admits. Magnesium oxide is the least expensive and most concentrated but poorly absorbed, which is why it doubles as a laxative and why it produces the most digestive complaints. Citrate is better absorbed and a reasonable default. Glycinate (bisglycinate) is the gentlest on the stomach, at a higher price. Malate, taurate, threonate and the rest carry claims that outrun their evidence; our full guide to magnesium types ranks them honestly. As for what supplementing achieves, the strongest everyday case is sleep, where trials show modest benefit; we cover the doses and evidence in magnesium for sleep. For leg cramps, the trial evidence is largely disappointing, whatever the packaging implies.
Who genuinely runs low on magnesium?
Deficiency worth treating is concentrated in identifiable groups rather than scattered through the population. Long-term use of acid-suppressing proton pump inhibitors can drain magnesium, as can some diuretics. Heavy alcohol use, poorly controlled type 2 diabetes, and gut conditions that impair absorption, such as coeliac disease and Crohn's disease, all raise the risk. Older adults absorb less and often eat less of the right foods. In these situations magnesium status is a medical question, sometimes confirmed by blood testing, and supplementation may be genuinely indicated. Timing has one practical catch: magnesium can reduce the absorption of some antibiotics, thyroid medication and bisphosphonates when taken together, so separate the doses by a few hours; our guide to supplement and medicine interactions has the details.
The summary is undramatic, as mineral advice usually should be. Aim for 300mg a day for men and 270mg for women, mostly from food. If you supplement, stay at or below 400mg a day on top of a normal diet, pick citrate or glycinate if your stomach objects to oxide, and treat 500mg doses as a laxative experiment rather than a health upgrade. Reserve real concern for the situations that earn it: certain medicines, heavy alcohol use, diabetes and gut disease, where a clinician rather than a label should set the dose.
