Magnesium: who is actually low, and what the evidence says

Magnesium supplements are sold for sleep, stress, cramps, headaches, and more. The facts are narrower. About 48 percent of Americans eat less magnesium than the estimated requirement, but eating less than the target is not the same as being deficient, and deficiency with symptoms is uncommon in otherwise healthy people. It occurs mainly in people with digestive diseases, type 2 diabetes, heavy alcohol use, or long-term use of certain medications (National Institutes of Health, Office of Dietary Supplements).

As for benefits, the trial evidence is strongest, though modest, for blood pressure, where a meta-analysis of 34 randomized trials found a reduction of about 2 mm Hg. A professional guideline rates magnesium as probably effective for preventing migraine. The evidence for sleep is low quality, the evidence for leg cramps in older adults is reasonably good and negative, and the evidence for mood rests on one unblinded trial.

This article is for adults wondering whether they are low in magnesium and whether a supplement is worth taking.

How much is needed and how much people get

The recommended intake is 400 to 420 milligrams a day for adult men and 310 to 320 milligrams for adult women, with slightly higher figures in pregnancy. Pumpkin seeds, chia seeds, almonds, spinach, cashews, peanuts, and black beans are among the richest sources; one ounce of roasted pumpkin seeds provides about 156 milligrams, an ounce of almonds about 80, and a half cup of cooked spinach about 78. About 30 to 40 percent of what is eaten is absorbed (National Institutes of Health, Office of Dietary Supplements).

In survey data from 2013 to 2016, 48 percent of Americans consumed less than the estimated average requirement from food and drink, with men over 70 and teenagers most likely to fall short. Average intake from food was about 268 milligrams a day for men and 234 for women not taking supplements (National Institutes of Health, Office of Dietary Supplements).

Who is actually at risk of being low

A diet somewhat below the target does not usually cause symptoms, because the kidneys cut losses in urine and bone holds a reserve. The groups at genuine risk are people with Crohn's disease, celiac disease, or intestinal surgery; people with type 2 diabetes or insulin resistance, who lose more magnesium in urine; people with alcohol dependence; and older adults, who eat less, absorb less, and excrete more. Loop and thiazide diuretics increase urinary loss, and proton pump inhibitors taken for a year or more can cause low magnesium (National Institutes of Health, Office of Dietary Supplements).

Early signs of deficiency are nonspecific: loss of appetite, nausea, fatigue, and weakness. More severe depletion can cause numbness, tingling, muscle cramps, abnormal heart rhythms, and low calcium or potassium. Testing is unsatisfying. Less than 1 percent of the body's magnesium is in blood serum, so a normal serum level does not rule out low tissue stores. The U.S. national health survey has not measured serum magnesium since 1974, so there are no current data on how many people are truly deficient (National Institutes of Health, Office of Dietary Supplements). For a healthy adult, the practical question is about diet rather than a lab number. For someone in a risk group, it is a question for a doctor.

Blood pressure, diabetes, and heart disease

The best randomized evidence concerns blood pressure. A meta-analysis of 34 double-blind placebo-controlled trials in 2,028 adults found that magnesium supplements, at a median dose of 368 milligrams a day for a median of three months, lowered systolic blood pressure by 2.00 mm Hg and diastolic by 1.78 mm Hg (Zhang et al., 2016). A 2 mm Hg reduction is real but small, and not a reason to replace blood pressure treatment.

The diabetes and heart findings come from observational cohorts and are weaker evidence. A meta-analysis of 40 prospective cohorts with more than a million participants found that each additional 100 milligrams a day of dietary magnesium was associated with a 19 percent lower risk of type 2 diabetes, a 22 percent lower risk of heart failure, a 7 percent lower risk of stroke, and a 10 percent lower risk of death from any cause, but no association with coronary heart disease or cardiovascular disease overall (Fang et al., 2016). These studies measured magnesium from food, and people who eat more nuts, beans, and greens differ in many other ways, so the figures do not describe the effect of a pill.

Sleep, cramps, migraine, and mood

Sleep is the most common reason people buy magnesium and the area with the thinnest evidence. A systematic review found only three randomized trials, with 151 older adults in total, comparing magnesium with placebo for insomnia. Pooled, magnesium shortened the time to fall asleep by about 17 minutes; total sleep time was not significantly different. All three trials were at moderate to high risk of bias and the evidence was rated low to very low quality (Mah and Pitre, 2021).

For leg cramps, the evidence is better and the answer is no, at least for the common nighttime cramps of older age. A Cochrane review of 11 trials in 735 people found that in adults with idiopathic cramps, magnesium made no meaningful difference to cramp frequency, intensity, or duration, with moderate to high certainty. Trials in pregnancy were at high risk of bias and conflicting. Diarrhea and other gut side effects affected up to 37 percent of people taking magnesium (Garrison et al., 2020).

Migraine prevention has more support. The American Academy of Neurology and American Headache Society guideline rates magnesium as probably effective, a Level B rating, based on small placebo-controlled trials using doses up to 600 milligrams a day (Holland et al., 2012). Those doses exceed the upper limit for supplements, so the NIH advises using magnesium for migraine only under a doctor's supervision (National Institutes of Health, Office of Dietary Supplements).

For depression, the most cited study is a randomized crossover trial in 126 primary care patients with mild to moderate symptoms, in which six weeks of 248 milligrams of magnesium a day improved scores on a standard depression questionnaire by 6 points compared with no treatment (Tarleton et al., 2017). The trial was open-label, meaning participants knew whether they were taking magnesium, and the comparison was no treatment rather than placebo, so expectation effects cannot be separated from the mineral.

If you decide to supplement

• Try food first. A handful of pumpkin seeds or almonds, a serving of beans, and a portion of leafy greens each day will close most of the gap for a typical adult.
• Stay at or below 350 milligrams a day from supplements, the official upper limit for adults. The limit applies to supplements and medicines, not food, and exists because higher doses commonly cause diarrhea, nausea, and cramping (National Institutes of Health, Office of Dietary Supplements).
• Form matters somewhat. Small studies suggest citrate, chloride, lactate, and aspartate are absorbed better than oxide, and oxide and carbonate are most often linked to diarrhea.
• Separate magnesium from oral bisphosphonates by at least two hours, and take it two hours before or four to six hours after tetracycline or quinolone antibiotics, since it reduces their absorption.

Who should be careful

People with impaired kidney function should not take magnesium supplements without medical advice, because the kidneys clear excess magnesium and very high levels can be dangerous. Anyone on diuretics, a proton pump inhibitor, or diabetes treatment who has persistent fatigue, cramps, or palpitations should ask their doctor about checking magnesium rather than self-treating, since those symptoms have many causes. Pregnant women should discuss any supplement with their provider.

Sources

• Magnesium: Fact Sheet for Health Professionals (National Institutes of Health, Office of Dietary Supplements) — U.S. government reference on requirements, intake data, risk groups, safety, and interactions
• Effects of magnesium supplementation on blood pressure: a meta-analysis of randomized double-blind placebo-controlled trials (Zhang et al., Hypertension, 2016) — meta-analysis of 34 randomized trials, n=2,028
• Dietary magnesium intake and the risk of cardiovascular disease, type 2 diabetes, and all-cause mortality (Fang et al., BMC Medicine, 2016) — dose-response meta-analysis of 40 prospective cohorts, more than 1 million participants
• Oral magnesium supplementation for insomnia in older adults: a systematic review and meta-analysis (Mah and Pitre, BMC Complementary Medicine and Therapies, 2021) — systematic review of 3 randomized trials, n=151
• Magnesium for skeletal muscle cramps (Garrison et al., Cochrane Database of Systematic Reviews, 2020) — Cochrane review of 11 randomized trials, n=735
• Evidence-based guideline update: NSAIDs and other complementary treatments for episodic migraine prevention in adults (Holland et al., Neurology, 2012) — American Academy of Neurology and American Headache Society clinical guideline
• Role of magnesium supplementation in the treatment of depression: a randomized clinical trial (Tarleton et al., PLoS One, 2017) — open-label randomized crossover trial, n=126, 6 weeks