Magnesium

Magnesium is a quiet workhorse. It is involved in over 300 enzyme reactions in the human body. It helps convert food into energy (it is needed for ATP, the cell's energy currency). It works with calcium to make muscles contract and relax (calcium triggers a muscle contraction; magnesium helps the muscle release). It plays a role in nerve transmission, in keeping the heartbeat steady, in DNA synthesis, in protein synthesis, and in maintaining bone structure (around half the body's magnesium is in bone).

The body absorbs around a third of the magnesium in a typical meal. Bowel and kidney losses are tightly regulated. Mild low-magnesium intake from food rarely causes obvious problems because the body adapts: absorption rises and kidney loss falls. Symptomatic deficiency usually requires a sustained dietary shortfall combined with another factor (gut disease, certain water tablets, heavy alcohol intake, severe vomiting or diarrhoea).

Magnesium has also become a popular over-the-counter supplement for cramps, restless legs, and sleep. The evidence base is mixed: for confirmed low magnesium, a supplement at the doses below helps. For people with normal magnesium status, the benefit on cramps and sleep is uncertain, and tolerance is limited by gut side effects at higher doses.

Magnesium is concentrated in the parts of plants the body uses for growth: seeds, nuts, the bran of wholegrains, and dark green leaves. The list of richest sources looks similar to the iron list, which is one of the reasons "more plant foods" tends to fix several nutrient gaps at once.

Values per 100g come from USDA SR Legacy and McCance and Widdowson 7th edition; the site's food entries currently store macros only.

Practical: a small handful of nuts or seeds a day, a pulse-based meal a couple of times a week, dark greens often, and wholegrain rather than refined staples covers the magnesium requirement without thinking about it.

Pumpkin seeds and sunflower seeds are among the densest magnesium sources by weight; both are widely available in UK supermarkets. They are not currently in the site's food encyclopedia but are a strong addition to a small-handful-a-day habit.

UK Reference Nutrient Intake (RNI) for magnesium is set by SACN (1991). Values are mg per day.

Severe magnesium deficiency is uncommon in healthy UK adults eating a varied diet. Mild low intakes are more widespread, particularly in diets light on plant foods, nuts, seeds, and wholegrains. Symptoms of clinical low magnesium include:

• Loss of appetite
• Nausea, vomiting
• Fatigue and weakness
• Muscle cramps and tremor
• Tingling or numbness
• An irregular heartbeat in severe cases
• Personality changes or low mood in long-running deficiency

Who is at higher risk in the UK

• People with gastrointestinal disease. Crohn's disease, coeliac disease, and prolonged diarrhoea reduce absorption.
• People with type 2 diabetes. Magnesium is lost in the urine when blood sugar is high.
• People on certain water tablets (loop and thiazide diuretics).
• People on long-term proton pump inhibitors (for reflux).
• Heavy alcohol intake. Reduces absorption, increases losses.
• Older adults. Absorption tends to fall with age, and several common medications interact.
• People with very plant-light diets. Magnesium is concentrated in seeds, nuts, leaves, pulses, and wholegrains.

If symptoms are persistent or there is a risk factor, a GP can order a blood test. Note that serum magnesium is a relatively crude indicator (the body holds most of its magnesium in bone and cells, not blood), so symptoms in a risk group can warrant a trial of dietary or supplement change even with a normal blood result.

Magnesium from food is not a realistic risk. The risk is from supplements at high doses.

NHS supplement guidance: taking 400mg or less a day of magnesium from supplements is unlikely to cause any harm.

Above 400mg from supplements, the typical effect is osmotic diarrhoea (the magnesium pulls water into the bowel). This is dose-related and reverses on stopping. Severe excess in people with kidney disease can cause more serious problems (low blood pressure, muscle weakness, irregular heartbeat) because the kidneys cannot clear it.

The 400mg upper applies to magnesium from supplements only, not magnesium naturally present in food.

Magnesium-containing laxatives (such as Epsom salts) and antacids that contain magnesium can deliver doses well above this threshold; they are not intended for regular long-term use and instructions on the pack should be followed.

Most UK adults can meet the magnesium reference from food without thinking about it: small daily handful of nuts or seeds, pulses a couple of times a week, dark green leaves often, wholegrain rather than refined staples.

NHS over-the-counter guidance: 400mg or less a day of magnesium from supplements is unlikely to cause any harm.

When a supplement may be sensible (ideally after a GP conversation):

• Confirmed low magnesium on blood test.
• Conditions or medicines that reduce magnesium (see deficiency section).
• Pregnancy-related leg cramps, where some evidence supports a short trial; discuss with the antenatal team.

Common forms in UK supplements: magnesium citrate and magnesium glycinate are typically better tolerated than magnesium oxide, which is cheap but more likely to cause loose stools. Whatever the form, stay at or below 400mg a day of supplemental magnesium without medical advice.

Topical magnesium (sprays, flakes, oil) is widely sold for cramps and sleep. The evidence that meaningful amounts of magnesium cross the skin is weak. If something is going to make a real difference, it is dietary change or an oral supplement at the doses above.

• The same plant foods cover several nutrient gaps at once: Iron, Potassium, and magnesium overlap heavily in their UK food sources.
• Vitamin D needs magnesium to activate: Vitamin D.
• Best food sources: Cashew nuts, Spinach, Lentils, Oat bran.

• NHS. Vitamins and minerals: Others (calcium, magnesium, etc). nhs.uk/conditions/vitamins-and-minerals/others. Adult RNI and supplement guidance.
• SACN. Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. Department of Health Report 41 (1991).
• Public Health England. McCance and Widdowson's The Composition of Foods, 7th summary edition (2015).
• USDA Agricultural Research Service. FoodData Central, SR Legacy release. fdc.nal.usda.gov.

This page is reference information for UK shoppers. It is not medical advice.