Magnesium

How much you need

The official Recommended Dietary Allowance for adults, per day:

People aged 18 fall in the official 14–18 group — 410 mg a day for males and 360 mg for females, 400 mg in pregnancy and 360 mg when breastfeeding. The upper limit for supplemental magnesium is the same 350 mg. Pregnancy and breastfeeding change what you need — confirm yours with your midwife, obstetrician or doctor.

Upper limit

This limit applies only to magnesium from supplements and medicines — not to magnesium found naturally in food and drink.

The limit is the most magnesium from supplements and medicines a day that is unlikely to cause harm, not a target. It is lower than the recommended amount, and that is correct — the recommended amount counts magnesium from every source, while the limit counts only supplements and medicines. Magnesium from food poses no risk to healthy people, because the kidneys remove the excess.

What it does

Magnesium is a cofactor in more than 300 enzyme systems that regulate reactions throughout the body, including protein synthesis, muscle and nerve function, blood sugar control and blood pressure regulation ^[1]. It is required for energy production, contributes to the structure of bone, and is needed to make DNA, RNA and the antioxidant glutathione ^[1]. It also helps move calcium and potassium across cell membranes, which matters for nerve signals, muscle contraction and a normal heart rhythm ^[1]. An adult body holds about 25 grams, 50% to 60% of it in the bones ^[1].

Forms

Supplements contain magnesium in several forms, including oxide, citrate and chloride, and the label states the amount of magnesium itself, not the weight of the whole compound ^[1]. Forms that dissolve well in liquid are absorbed more completely: small studies have found the aspartate, citrate, lactate and chloride forms better absorbed than magnesium oxide and sulfate ^[1]. Magnesium is also a main ingredient in some laxatives and in some remedies for heartburn and indigestion. Phillips' Milk of Magnesia, for example, provides 500 mg per tablespoon — well above the upper limit, though some is not absorbed because of the laxative effect ^[1].

Where it comes from

1. Pumpkin seeds, roasted1 ounce156 mg
2. Chia seeds1 ounce111 mg
3. Almonds, dry roasted1 ounce80 mg
4. Spinach, boiled½ cup78 mg
5. Cashews, dry roasted1 ounce74 mg

How these five were chosen: The five foods with the most magnesium per serving in the NIH fact sheet's food table. Fortified foods are marked.

Getting the most from it

About 30% to 40% of the magnesium in food and drink is absorbed ^[1]. Refining grains in ways that remove the germ and bran substantially lowers their magnesium ^[1]. Tap, mineral and bottled water can add some, though the amount varies widely by source and brand, from 1 mg to more than 120 mg per liter ^[1]. One study found that very high doses of zinc from supplements — 142 mg a day — interfered with magnesium absorption and balance ^[1]. When magnesium status is low, the kidneys excrete less of it ^[1].

In the United States

Many Americans get less magnesium from food than they need: in national survey data from 2013–2016, 48% of people of all ages took in less than their estimated average requirement from food and drink, with men 71 and older and teenagers most likely to fall short ^[1]. In 2003–2006 data, adults who took a magnesium supplement also ate more from food — 350 mg a day for men and 267 mg for women, against 268 mg and 234 mg for those who did not — and their totals including supplements averaged 449 mg and 387 mg ^[1]. There are no current data on magnesium status in the United States: the national survey has not measured blood magnesium since 1974, and routine hospital and clinic blood tests do not include it ^[1].

Who is most likely to run low

• People with gastrointestinal diseases such as Crohn's disease or celiac disease, or who have had part of the small intestine removed or bypassed — chronic diarrhea and fat malabsorption deplete magnesium over time ^[1]
• People with type 2 diabetes or insulin resistance, who can lose more magnesium in their urine ^[1]
• People with alcohol dependence, in whom magnesium deficiency is common ^[1]
• Older adults, who eat less magnesium, absorb less of it and lose more in urine, and are more likely to have conditions or take medicines that affect it ^[1]

What deficiency looks like

Deficiency from diet alone is uncommon in otherwise healthy people, because the kidneys limit how much is lost in urine ^[1]. It can follow habitually low intakes, or losses caused by some health conditions, chronic alcoholism or certain medicines ^[1]. Early signs include loss of appetite, nausea, vomiting, fatigue and weakness; as it worsens, numbness, tingling, muscle cramps, seizures, personality changes, abnormal heart rhythms and coronary spasms can follow, and severe deficiency can cause low blood calcium or potassium ^[1]. Status is hard to measure because most magnesium is inside cells or in bone; blood levels are the usual test but reflect the body's total poorly ^[1].

What the evidence supports, and what it doesn't

Established. Magnesium is essential: more than 300 enzyme systems depend on it, and it is needed to build bone ^[1]. The fact sheet states that diets providing the recommended amounts enhance bone health ^[1].

Not supported by trials. Blood sugar control in diabetes: the American Diabetes Association states that the evidence is insufficient to support routine use of magnesium to improve glycemic control ^[3], and the few small, short trials have had conflicting results ^[1]. Blood pressure: trials find only a marginal effect — a Cochrane review of 12 trials in 545 people with high blood pressure, who took about 243 to 973 mg a day for 8 to 26 weeks, found a reduction of only 2.2 mmHg in diastolic pressure ^[2]. The FDA allows foods and supplements to say that adequate magnesium may reduce the risk of high blood pressure, but only together with its own conclusion that the evidence is inconsistent and inconclusive ^[1].

Mixed. Heart disease and stroke: in several long-term observational studies, people with higher magnesium intakes or blood levels had lower rates of sudden cardiac death, heart disease or stroke, but such studies cannot rule out other dietary factors, and a large, well-designed trial is still needed ^[1]. Type 2 diabetes risk: in a meta-analysis of cohort studies, each extra 100 mg a day was associated with a 15% lower risk, with the same limitation ^[1]. Bone: higher intakes are associated with greater bone density, and a few small studies suggest more magnesium might increase it in postmenopausal women, but its role in preventing osteoporosis needs more research ^[1]. Migraine: three of four small, short trials found modest reductions in how often migraines occurred, and the American Academy of Neurology and the American Headache Society concluded that magnesium is probably effective for prevention ^[4] — but the amounts typically used exceed the upper limit, so the fact sheet says this should be done only under a health care provider's direction and supervision ^[1].

Too much

Magnesium from food is not a risk for healthy people, because the kidneys remove the excess in urine ^[1]. High doses from supplements or medicines often cause diarrhea, sometimes with nausea and abdominal cramping — most commonly reported with magnesium carbonate, chloride, gluconate and oxide ^[1]. Very large doses from laxatives and antacids, typically more than 5,000 mg a day, have caused magnesium toxicity, including deaths ^[1]. Its signs include low blood pressure, nausea, vomiting, facial flushing, urine retention and lethargy, progressing to muscle weakness, difficulty breathing, irregular heartbeat and cardiac arrest ^[1]. The risk is greater when kidney function is impaired, because the kidneys cannot remove the excess ^[1].

Medicines that interact

Magnesium-rich supplements or medicines can reduce the absorption of oral bisphosphonates for osteoporosis, such as alendronate (Fosamax); taking them at least 2 hours apart minimises this ^[1]. Magnesium binds tetracycline antibiotics such as doxycycline and quinolones such as ciprofloxacin, which should be taken at least 2 hours before or 4 to 6 hours after a magnesium supplement ^[1]. Long-term use of loop and thiazide diuretics, such as furosemide and hydrochlorothiazide, increases the magnesium lost in urine, while potassium-sparing diuretics such as spironolactone reduce it ^[1]. Proton pump inhibitors such as esomeprazole and lansoprazole can cause low blood magnesium when taken for long periods, typically more than a year; in the cases the FDA reviewed, supplements often corrected it but in a quarter did not and the drug had to be stopped, and the FDA advises doctors to consider checking magnesium before and during long-term treatment ^[5]. If you take any of these regularly, talk to your doctor or pharmacist before taking a magnesium supplement.