Magnesium deficiency: symptoms, why a blood test says little, and magnesium-rich foods

Magnesium deficiency symptoms

Signs of low magnesium are non-specific: the same complaints come with anaemia, thyroid problems, poor sleep and stress. So treat the list below as a reason to talk to your GP, not a way to diagnose yourself.

According to NIH ODS, early signs include:

• loss of appetite;
• nausea and vomiting;
• tiredness and weakness.

With more severe deficiency, you may also see:

• numbness and tingling;
• muscle contractions and cramps;
• personality changes;
• abnormal heart rhythms;
• seizures.

Severe deficiency also disturbs calcium and potassium, so their blood levels fall too. That’s a medical condition to be treated by doctors, not with supplements.

Get urgent help for palpitations or an irregular heartbeat, a seizure, severe weakness or confusion. This matters especially if you take diuretics or proton pump inhibitors, or drink heavily.

Low magnesium: causes and who is at risk

In healthy people, diet alone rarely causes a deficiency: when intake drops, the kidneys excrete less. Low magnesium is more often linked to health conditions, medicines or lifestyle.

If you’ve been on a proton pump inhibitor such as omeprazole for a long time, your doctor may check magnesium before starting and from time to time during treatment, as the US FDA advises. In about a quarter of reported cases, supplements didn’t help and the medicine had to be stopped.

Is a magnesium blood test reliable?

An adult body holds about 25 g of magnesium. 50–60% is in bone and most of the rest sits inside soft-tissue cells. Serum holds less than 1%, and the body keeps that level steady by drawing on its stores. So a normal result doesn’t prove your stores are fine.

What the tests can and can’t do:

• Serum magnesium. The normal range is 0.75–0.95 mmol/L; below 0.75 is hypomagnesaemia. It reliably picks up only marked deficiency.
• Red blood cell, saliva, urine and ionised magnesium. Used in research and some clinics, but no single method is considered satisfactory.
• Loading (tolerance) test, which measures how much magnesium is passed in urine after a dose. Experts disagree on whether it suits adults.

The Nordic Nutrition Recommendations 2023 make the same point: there’s no adequate functional marker of magnesium status. So the decision rests on the whole picture: diet, symptoms, conditions, medicines and any tests your GP orders. Other nutrient tests are covered in our tests and deficiencies section.

Magnesium-rich foods

Seeds, nuts, beans, wholegrains and leafy greens give the most magnesium. Values below are from the NIH ODS table. Cup measures are US cups, about 240 ml.

Compare foods by group:

Food magnesium doesn’t count towards the supplement limit, and in healthy people the kidneys clear any excess from food.

An example day without supplements: 28 g of pumpkin seeds (156 mg), half a cup of black beans (60 mg), half a cup of brown rice (42 mg), a pot of yoghurt (42 mg) and a banana (32 mg). That’s about 330 mg, above the NHS figure for both women and men.

Easy ways to eat more magnesium:

• sprinkle seeds or nuts on porridge and salads;
• eat beans, lentils or chickpeas 3–4 times a week;
• choose wholemeal bread and brown rice over white;
• add leafy greens, especially spinach.

Food or supplement?

If your diet is low in these foods, try adding them first. A supplement makes sense if changing your diet is hard or you have a risk factor from the table above.

• A supplement dose is 100–200 mg of elemental magnesium a day.
• EFSA’s supplement limit is 250 mg a day. The NHS says 400 mg or less is unlikely to cause harm. Food magnesium doesn’t count.
• Glycinate is gentler on the gut; citrate is cheaper and mildly laxative.
• After 1–2 months, check with your GP whether you still need it.

If you’re unsure where you stand, work through the check below, from your plate to a supplement:

The forms are compared in detail in magnesium glycinate, magnesium citrate and types of magnesium compared. If you’re considering a magnesium product with vitamin B6, check the B6 dose first: see magnesium and vitamin B6.

Who shouldn’t take magnesium without advice

• Kidney disease. The kidneys clear excess magnesium; if they don’t work well, it builds up.
• Tetracycline and quinolone antibiotics. Take the antibiotic at least 2 hours before or 4–6 hours after magnesium.
• Bisphosphonates. Separate by at least 2 hours.
• Pregnancy and breastfeeding. Needs are higher, but check any supplement with your midwife or GP.
• Children. Doses depend on age and weight; follow a paediatrician.

Our vitamin compatibility chart shows which supplements and medicines to keep apart from magnesium.

Too much magnesium from supplements causes loose stools, nausea and stomach cramps. Severe weakness, low blood pressure or an irregular heartbeat mean you should stop and get medical advice.

How to choose magnesium on iHerb

1. Look at your diet first: seeds, nuts and beans may be enough.
2. If you need a supplement, pick a form: glycinate for a sensitive gut, citrate if you’re prone to constipation.
3. In the Supplement Facts, check elemental magnesium per serving and how many capsules make it.
4. Keep supplements to 100–200 mg of magnesium a day.
5. Check multivitamins and electrolyte drinks, which may contain magnesium too.

Our Supplement Facts guide explains label reading and comparing cost per dose in £ or €.