What are the signs of magnesium deficiency?

Early signs of magnesium deficiency are loss of appetite, nausea, vomiting, fatigue and weakness.

Who is most likely to run low

48% of Americans take in less magnesium from food and drink than the estimated average requirement.

What this means for you

The early signs of magnesium deficiency are the least specific in this whole section: loss of appetite, nausea, vomiting, fatigue and weakness. Nothing there points at magnesium on its own. As it gets worse, the picture sharpens into numbness, tingling, muscle cramps, tremor and abnormal heart rhythms.

Worth being straight about the scale of the problem. Around 48 per cent of Americans take in less magnesium from food and drink than the estimated average requirement, and most of those people have no symptoms and no diagnosable deficiency. Low intake and clinical deficiency are different things, and the supplement industry works hard to blur them.

Established. The gap between those two is built into the label. The 420 mg figure is designed to cover 97 to 98 people in every hundred, and the agency that sets it says plainly that it is usually not the best estimate of any one person's requirement. It was never placed at the point where a shortfall starts. So a number under the Daily Value tells you where you sit against a deliberately generous population target and nothing about whether anything is wrong with you.

Myth. Muscle cramps are the symptom most often blamed on magnesium. A Cochrane review of five trials in 307 older adults with night cramps found the number of cramps a week barely moved, with a mean difference of 0.18 cramps against placebo on moderate-certainty evidence. The pregnancy version has been tested too. Leg cramps hit 30 to 50 per cent of pregnancies twice a week or more, and pooling four randomised trials magnesium changed neither how often they came nor how well women recovered from them. If your calves seize at night, magnesium is a cheap thing to try and a poor thing to count on.

The blood test will not answer this

The obvious move when you suspect a shortage is to ask for a blood test, and magnesium is the nutrient where that move works least well. In a group of adults whose eating was recorded with three-day food diaries, neither serum magnesium nor the ionised magnesium in whole blood tracked how much magnesium people were actually taking in. Average intake was about 304 mg a day with a wide spread around it, and the blood did not follow it either way.

Unsettled. It fails in the direction that matters most. In metabolic ward experiments, people whose serum magnesium sat comfortably inside the normal range, as high as 0.85 millimoles per litre, still responded to magnesium supplements, particularly if their usual intake had been under 250 mg a day. A normal result does not rule a shortfall out. There is no good test to ask for, which is exactly why the vague symptom list above is such a poor tool on its own.

Who is genuinely at risk

People with gut diseases that block absorption, such as Crohn's or coeliac disease. People with type 2 diabetes, who lose more in urine. People with long-term alcohol dependence. Older adults, who absorb less and excrete more. And people on long-term proton pump inhibitors, which can drive magnesium down far enough to matter.

Two of those groups now have a size attached to them. Pooling the observational studies of people on a proton pump inhibitor, low blood magnesium turned up in 19.4 per cent of users against 13.5 per cent of non-users, an odds ratio of 1.71. A high dose roughly doubles the odds again against a low one. In type 2 diabetes the pooled prevalence of low blood magnesium is about 32 per cent. Those are the two settings where looking for this is worth the trouble.

Not for everyone. If you supplement, know two things. High doses cause diarrhoea, nausea and cramping, which is the body's way of refusing the excess. Splitting the dose across the day and taking it with food both make that less likely, and neither instruction is on the label. And if your kidney function is reduced, magnesium accumulates instead of clearing, which turns a harmless-looking supplement into a real risk. That one is a conversation with a doctor.

Food carries no such ceiling. Nuts, seeds, beans, whole grains and leafy greens are the sources, and magnesium from food and drink has never needed a limit.

The pattern across this nutrient is worth carrying away, because it explains why magnesium looks miraculous in some studies and inert in others. In 38 randomised trials, magnesium lowered blood pressure by 7.68 mmHg in people already on medication and by 5.97 in people with low blood magnesium, and did nothing measurable in people whose pressure was normal. It does something when there is something to fix. More of that evidence here.

Two medicines belong on the risk list above, and they are among the most prescribed there are. Long-term loop or thiazide diuretics increase magnesium losses in urine. Prescription proton pump inhibitors taken for more than about a year can push blood magnesium below normal. If the vague symptoms above have been going on and you take either, that is worth raising with the prescriber rather than treating yourself.

Low mood belongs on this page with a careful label. In seven randomised trials in 325 adults who already had a depressive disorder, magnesium lowered depression scores. The same authors call for larger trials before anyone treats that as clinical advice, and none of those trials asked whether the people in them were short of magnesium to begin with.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Where to get it from food

• Pumpkin and squash seed kernels, roasted — 649 mg per 1 cup (155% DV)
• Sesame seed kernels, dried — 517.5 mg per 1 cup (123% DV)
• Sesame seeds, whole, dried — 505.4 mg per 1 cup (120% DV)
• Brazil nuts, dried, unblanched — 500.1 mg per 1 cup, whole (119% DV)
• Sunflower seed kernels, dry roasted — 454.7 mg per 1.0 cup (108% DV)

All magnesium sources ranked →

Full guide: Magnesium: widely under-eaten, and widely over-promised