What does magnesium do in the body?
Magnesium doesn't get the airtime that iron or vitamin D do, which is a bit unfair given how much of the background work it's doing. It acts as a cofactor in more than 300 enzyme systems, turning up in processes as varied as protein synthesis, muscle and nerve function, and energy production.1
The NHS describes its job by saying that magnesium helps turn the food you eat into energy, and helps your parathyroid glands, which make hormones that matter for bone health, work normally.3
Your body holds roughly 25 grams of magnesium at any one time, with 50% to 60% of it stored in your bones and most of the remainder sitting in soft tissue.1 Less than 1% is in your blood, which is worth knowing if you ever have it measured, because a blood test is only seeing a sliver of the total.1 Food supplements should not be used as a substitute for a varied, balanced diet and healthy lifestyle.
How much magnesium do you need a day?
For adults aged 19 to 64, the NHS puts it at 300mg a day for men and 270mg a day for women, and says you should be able to get all of it by eating a varied and balanced diet.3
If you're taking a supplement rather than relying on food, the NHS says 400mg or less a day is unlikely to cause any harm.3
Foods high in magnesium
Magnesium isn't rare in food, which is part of why running short of it is less common than the internet suggests. Good sources include green leafy vegetables, legumes, nuts, seeds, and wholegrains, and it's added to some breakfast cereals and other fortified foods.1 The NHS singles out spinach, nuts, and wholemeal bread.3
Your body absorbs roughly 30% to 40% of the magnesium you eat, so the figure on a food label isn't the figure that reaches you.1
Signs you might be low in magnesium
Running genuinely short of magnesium is uncommon in otherwise healthy people, because your kidneys are good at holding on to it when your intake drops.1 When it does happen, the early signs are loss of appetite, nausea, vomiting, tiredness, and weakness, none of which points obviously at magnesium.1 If a deficiency gets worse, it can cause numbness, tingling, muscle cramps and contractions, personality changes, and abnormal heart rhythms.1
Some groups are more likely to be short of it than others, including older adults, people with type 2 diabetes, people with gastrointestinal conditions, and people with alcohol dependence.1
Do you need a magnesium supplement?
If you eat a balanced diet, you probably don't need one. A supplement is there to top up your intake if you're not getting enough magnesium from food.
There are several forms on the shelf and they behave differently, mostly because of what the magnesium is bound to:1
Magnesium glycinate: magnesium bound to the amino acid glycine. It tends to be gentle on the stomach.
Magnesium citrate: magnesium bound to citric acid. It's well absorbed, and at higher doses it can loosen stools.
Magnesium oxide: less well absorbed than the other two, and more likely to upset your stomach.
As for timing, you can take magnesium at any time of day, and taking it consistently matters more than taking it at a particular hour.
Who should check first
If you have kidney disease, don't take magnesium supplements unless a doctor has told you to, because your body may not clear the excess. Magnesium also interferes with several medicines, and in most cases the fix is spacing rather than avoidance:1
Bisphosphonates for osteoporosis, such as alendronate.
Tetracycline and quinolone antibiotics, such as doxycycline and ciprofloxacin.
Diuretics. Long-term use of loop and thiazide diuretics increases the magnesium you lose in urine, while potassium-sparing diuretics reduce it.
Proton pump inhibitors. Long-term use can lower magnesium levels.
If you take anything regularly, ask a pharmacist before you start.
Numan supplements
You can see the full range in the Numan supplement range.
Food supplements should not be used as a substitute for a varied, balanced diet and healthy lifestyle.
Magnesium side effects: can you have too much?
From food alone, almost never. Your kidneys are good at filtering out what you don't need.1
Supplements are a different matter, as are magnesium-based laxatives and antacids taken too freely. The NHS says that taking high doses, meaning more than 400mg, for a short time can cause diarrhoea, and that there isn't enough evidence to say what high doses do over a long period.3 So aim for a varied diet first, and don't take more from supplements than you need.3
Common questions
What is magnesium good for?
Magnesium contributes to normal muscle function, normal functioning of the nervous system, normal energy-yielding metabolism, and the maintenance of normal bones and teeth.2
Is it OK to take magnesium every day?
Yes, within the NHS guidance of 400mg or less a day from supplements, unless a doctor has told you otherwise.3 If you have kidney disease, only take magnesium if a doctor advises it.
Is 500mg of magnesium too much?
It's above the 400mg a day the NHS says is unlikely to cause harm, and higher doses can cause diarrhoea.3
What are the signs your body needs magnesium?
The early signs are vague ones: loss of appetite, nausea, tiredness, and weakness.1
When should I take magnesium?
There's no single best time, so pick one you'll stick to.
Who should not take magnesium?
Anyone with kidney disease, unless a doctor advises it, and anyone taking medicines that interact with magnesium should speak to a doctor or pharmacist first.1
The Numan take
Magnesium might not be flashy, but it's foundational. Magnesium contributes to normal muscle function, normal psychological function, and normal functioning of the nervous system.2 The best way to keep your levels steady is the least exciting one: a varied diet built on whole, unprocessed food, with leafy greens, nuts, seeds, grains, and pulses doing most of the work.1,3
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