Magnesium Explained: Benefits, Deficiency Signs, Best Foods and Supplement Types

Magnesium has become one of the most popular supplements in recent years, promoted for everything from sleep to muscle cramps to stress. It is genuinely important: the body uses it in hundreds of chemical reactions. But what does the evidence actually say, and do you really need a supplement? In Part 4 of Vitamins and Minerals Explained, we look at magnesium.

What does magnesium do?

Magnesium is a mineral involved in many processes in the body. According to the US National Institutes of Health (NIH), it helps:

• Muscles and nerves work properly.
• Regulate blood sugar levels.
• Regulate blood pressure.
• Make protein, bone and DNA.

It also plays a role in energy production and in keeping the heart's rhythm steady. Around half of the body's magnesium is stored in bones, with most of the rest in muscles and soft tissues; only a small amount is in the blood.

How much magnesium do you need?

The NIH recommends the following daily amounts:

Best magnesium-rich foods

Magnesium is widely found in plant foods, especially nuts, seeds, legumes, whole grains and leafy greens. The table below shows top sources from the NIH (the US Daily Value is 420 mg).

Other good sources include whole-grain bread, brown rice, oats, avocado, bananas, yoghurt and dark chocolate. Refined grains lose much of their magnesium during processing, so choosing whole grains helps. Tap and mineral water can also provide some magnesium, depending on the source.

Do most people get enough?

According to the NIH, many people in the US get less magnesium from food than recommended, with men over 70 and teenagers most likely to fall short. However, when supplements are included, total intakes are generally above recommendations. The kidneys are good at conserving magnesium when intake is low, so true deficiency is less common than low intake.

Who is at risk of low magnesium?

• People with digestive diseases such as Crohn's disease or coeliac disease, which reduce absorption.
• People with type 2 diabetes, who may lose more magnesium in urine.
• People with long-term alcohol use disorder.
• Older adults, who tend to eat less magnesium, absorb less and lose more through the kidneys, and are more likely to take medicines that affect magnesium.
• People taking certain medicines, such as some diuretics and long-term acid-reducing drugs.

Signs of magnesium deficiency

In the short term, low intake usually causes no obvious symptoms. When deficiency develops, the NIH lists possible signs including:

• Loss of appetite
• Nausea and vomiting
• Fatigue and weakness

Extreme deficiency can cause:

• Numbness and tingling
• Muscle cramps
• Seizures
• Personality changes
• Abnormal heart rhythms

Low magnesium can also lead to low calcium and potassium in the blood. Because blood tests do not always reflect total body magnesium, doctors interpret results alongside symptoms and other tests.

Magnesium supplements: which type is best?

Magnesium supplements come in many forms. The NIH notes that forms that dissolve well in liquid, such as magnesium aspartate, citrate, lactate and chloride, are absorbed more completely than magnesium oxide and sulfate. Popular forms include:

Magnesium is also found in some laxatives and antacids, which can contribute large amounts.

Can you take too much magnesium?

Magnesium from food is not considered harmful, because healthy kidneys remove excess. But high doses from supplements or medicines can cause problems. The NIH sets these upper limits for supplements and medicines only (not food):

These limits are lower than the recommended total intake because the recommendations include food. High supplemental doses commonly cause diarrhoea, nausea and abdominal cramping. Extremely high intakes, often from large doses of magnesium-containing laxatives or antacids, can cause irregular heartbeat and even cardiac arrest. People with kidney disease are at higher risk and should not take magnesium supplements without medical advice.

What does the research say?

Blood pressure and heart health

The NIH notes that magnesium supplements may lower blood pressure only slightly. People who eat more magnesium-rich foods tend to have a lower risk of some heart disease and stroke, but this may partly reflect other healthy nutrients in those foods.

Type 2 diabetes

Higher dietary magnesium is associated with a lower risk of developing type 2 diabetes. Whether supplements help people who already have diabetes is still being studied.

Bones

Higher magnesium intake is linked to higher bone mineral density, which may help older women, but more research is needed to know whether supplements prevent or treat osteoporosis.

Migraine

People with migraines sometimes have low magnesium levels, and small studies found supplements modestly reduced migraine frequency. The NIH advises using magnesium for migraine only under the care of a health professional.

Sleep, anxiety and muscle cramps

Magnesium is widely promoted for sleep, stress and night-time leg cramps. Some small studies suggest possible benefits, especially in people with low magnesium, but overall evidence is limited and mixed. Reviews of magnesium for typical leg cramps in older adults have generally found little benefit. A balanced diet, good sleep habits (see our Sleep Science series) and medical advice for persistent symptoms are better first steps.

Medicine interactions

• Bisphosphonates (osteoporosis drugs) are poorly absorbed if taken too close to magnesium.
• Some antibiotics, including tetracyclines and quinolones, may not be absorbed properly if taken close to magnesium.
• Diuretics can either increase or decrease magnesium loss, depending on the type.
• Long-term acid-reducing drugs (proton pump inhibitors) can lower blood magnesium.
• Very high-dose zinc supplements can interfere with magnesium absorption.

Practical tips

1. Snack on a handful of nuts or seeds, such as pumpkin seeds or almonds.
2. Add beans, lentils or edamame to meals several times a week.
3. Choose whole grains instead of refined grains.
4. Eat leafy greens like spinach regularly.
5. If you consider a supplement, stay within 350 mg a day from supplements unless your doctor advises otherwise, and avoid it if you have kidney disease.

Frequently asked questions

When is the best time to take magnesium?

There is no strict rule. Many people take it in the evening, and taking it with food can reduce stomach upset.

Does magnesium help you sleep?

Evidence is limited. It may help some people with low levels, but it is not a proven sleep treatment for everyone.

Can magnesium cause diarrhoea?

Yes, especially citrate, oxide and high doses. Reducing the dose or switching forms may help.

In Part 5, we look at vitamin C: what it really does for colds, skin and immunity.

Sources

• NIH Office of Dietary Supplements – Magnesium (consumer fact sheet)
• NIH Office of Dietary Supplements – Magnesium (health professional fact sheet)

Vitamins and Minerals Explained: the full series

• Part 1: Vitamin D
• Part 2: Vitamin B12
• Part 3: Iron
• Part 4: Magnesium (you are here)
• Part 5: Vitamin C
• Part 6: Calcium
• Part 7: Zinc
• Part 8: Omega-3 Fatty Acids
• Part 9: Folate and Folic Acid
• Part 10: Vitamin A
• Part 11: Vitamin E
• Part 12: Vitamin K
• Part 13: Potassium
• Part 14: Iodine
• Part 15: Biotin
• Part 16: Do You Need Supplements?

This article is for general information only and is not medical advice. Recommended amounts shown are US figures from the NIH Office of Dietary Supplements; guidance in other countries may differ slightly. Talk to a doctor or pharmacist before starting supplements, especially if you are pregnant, take medicines or have a health condition.