Magnesium deficiency: how it is diagnosed and what to do

Only a doctor can confirm magnesium deficiency, usually with a blood test and a look at your diet, medicines and health history. Tiredness, cramps or poor concentration are common and have many causes, so they cannot tell you that magnesium is the reason. What you can do on your own is check how much magnesium your food gives you and, if you want to top it up, choose a supplement with the dose in mind.

This guide explains how a deficiency is identified, who is more likely to run low, which foods are rich in magnesium, and how to choose a supplement without going over the safe limit.

How magnesium deficiency is diagnosed

The standard test is serum magnesium, a simple blood test. It has a built-in limit, which the US National Institutes of Health (NIH) points out: less than 1% of the magnesium in the body is in the blood, and most of it sits in bones and soft tissues. A normal result therefore does not always rule out low stores, and a doctor reads the number together with kidney function, medicines and other results.

What	Value (NIH)	Note
Share of body magnesium found in the blood	under 1%	most is in bones and soft tissues
Normal serum magnesium	0.75 to 0.95 mmol/L	lab ranges differ slightly, so compare with the range printed on your result
Low serum magnesium	below 0.75 mmol/L	a doctor decides the cause and what to do

Who is more likely to run low

The NIH lists several groups in which magnesium status is more often a question. Whether it applies to you is for your doctor to judge:

• people with digestive conditions that reduce absorption, such as Crohn's disease or coeliac disease
• people with type 2 diabetes
• people who drink heavily or have alcohol dependence
• older adults, because absorption tends to fall and kidney losses can rise with age
• people who take some medicines for a long time, for example proton pump inhibitors (acid-reducing medicines) or diuretics

An everyday diet low in magnesium is a separate and much more common situation. In healthy people with normal kidney function the kidneys limit losses, so a true deficiency is uncommon. A low intake is not the same thing as a diagnosed deficiency.

How much magnesium you need

EFSA sets an adequate intake of 350 mg a day for men and 300 mg for women, from food and supplements together. The reference intake printed on labels (NRV) is 375 mg. EFSA's tolerable upper level for magnesium taken as supplements is 250 mg a day for adults, which does not count the magnesium in food. It is based on a gut effect: loose stools and diarrhoea.

Magnesium in food

Seeds, nuts, legumes and green leafy vegetables are the main sources. The values below are from the NIH fact sheet:

Food	Portion	Magnesium
Pumpkin seeds, roasted	about 30 g	156 mg
Chia seeds	about 30 g	111 mg
Almonds, dry roasted	about 30 g	80 mg
Spinach, boiled	half a cup	78 mg
Cashews, dry roasted	about 30 g	74 mg
Black beans, cooked	half a cup	60 mg

What to do if you suspect low magnesium

1. Book a visit with your family doctor and ask about a serum magnesium test.
2. Bring a list of your medicines and supplements, with doses. Acid-reducing medicines, diuretics and alcohol habits matter to the doctor.
3. Look at your plate for a week. Count how many days include seeds, nuts, beans or green leafy vegetables.
4. Follow the doctor's advice if the result is low. Do not replace it with a higher supplement dose of your own.
5. If the result is normal and you still want to top up, pick a dose within the limit below.

When a supplement makes sense

A supplement is a way to raise intake when food falls short, not a treatment for a diagnosed deficiency. If a doctor has found low magnesium, follow their advice on the product and dose, and have it rechecked. If you simply want to top up your intake, the points below keep it sensible.

How to choose a magnesium supplement

1. Look for the line "magnesium" or "of which magnesium" and its % NRV. The compound weight in the product name is not the dose.
2. Compare that figure with the 250 mg upper level for supplements. Many products give 375 to 420 mg a day, so a lower dose or splitting the dose may suit you better.
3. Take your medicines into account. Ask a pharmacist if you use antibiotics, osteoporosis medicines (bisphosphonates), diuretics or acid-reducing medicines.
4. Pick a form your stomach likes. Our guide to the best form of magnesium compares them by elemental magnesium.
5. Check the daily dose: how many capsules, tablets or scoops make it.
6. Work out the price per day, not per pack.

Two options with a lower dose per portion are the NOW Foods citrate (133 mg per capsule) and the NOW Foods glycinate (100 mg per capsule). You can browse every product in the magnesium range, or filter by magnesium citrate and magnesium bisglycinate.

Does the form of magnesium matter?

The EU claims apply to magnesium in any form, so bisglycinate, citrate and malate do not differ on claims. What differs is the share of pure magnesium in each capsule and how well the form suits your stomach. A 133 mg citrate capsule and a 375 mg bisglycinate dose in three capsules are very different doses, although both are called magnesium. Compare the elemental magnesium on the label, not the name.

What EU law lets us say about magnesium

Regulation (EU) No 432/2012 lists the authorised claims. They apply to products that provide at least 15% of the NRV (about 56 mg of magnesium) per daily dose, in any form:

• Magnesium contributes to a reduction of tiredness and fatigue.
• Magnesium contributes to electrolyte balance.
• Magnesium contributes to normal energy-yielding metabolism.
• Magnesium contributes to normal functioning of the nervous system.
• Magnesium contributes to normal muscle function.
• Magnesium contributes to normal psychological function.
• Magnesium contributes to the maintenance of normal bones.

There is no authorised claim linking magnesium to cramps or to sleep, and none that says a supplement corrects a deficiency.

Safety

The most common side effect of magnesium supplements is loose stools or diarrhoea. If that happens, lower the dose or split it between two meals. People with kidney disease should not take magnesium supplements without a doctor's approval, because the kidneys remove excess magnesium. Ask a doctor too if you are pregnant or breastfeeding, or if you plan to take more than 250 mg a day from supplements for a long time.

Magnesium can reduce the absorption of some antibiotics and osteoporosis medicines, so ask your pharmacist how far apart to take them. Taking magnesium with a meal also tends to be gentler on the stomach.

FAQ

How do I find out if I have a magnesium deficiency?

Ask your doctor for a serum magnesium blood test. Home checklists cannot confirm it, and a normal blood value does not always rule out low stores, so the doctor will read it with the rest of your picture.

What are the symptoms of magnesium deficiency?

Tiredness, muscle cramps and similar feelings are non-specific and shared by many conditions, so they cannot identify magnesium as the cause. We do not give a self-diagnosis list for that reason. If something worries you, see a doctor.

Does magnesium help with cramps?

The authorised claim is that magnesium contributes to normal muscle function. No claim covers cramps. If cramps keep coming back, tell your doctor.

Is magnesium good for sleep?

There is no authorised EU claim about sleep. The claims cover the nervous system and psychological function, and tiredness and fatigue.

Which foods are highest in magnesium?

Pumpkin seeds, chia seeds, almonds, cashews, cooked spinach and beans are among the richest, according to the NIH.

How much magnesium can I take from supplements?

EFSA's upper level for supplemental magnesium is 250 mg a day for adults. Many products give more, so check the label and ask a doctor before going above it for long.

Which magnesium should I take?

The one that gives a dose you can justify, within the 250 mg supplement limit unless a doctor advises otherwise, in a form you tolerate and at a price per day you accept.

Is magnesium with vitamin B6 a better choice?

Not by default. You get two nutrients with separate claims. If you choose a B6 combination, add up the B6 from all your supplements: EFSA set the tolerable upper level for adults at 12 mg a day in 2023, and one citrate and B6 product in our range gives 12 mg at its maximum dose.

Sources

• Regulation (EU) No 432/2012, authorised health claims
• EFSA: Scientific Opinion on Dietary Reference Values for magnesium (2015)
• NIH Office of Dietary Supplements: Magnesium fact sheet for health professionals

Content reviewed by Inese Kalniņa, dietitian.