Magnesium: the mineral you're probably not getting enough of

It's involved in hundreds of processes in your body, about half of U.S. adults fall short, and the form you buy changes how it works.

The short version

• Magnesium supports muscle and nerve function, energy production, blood sugar control and bone health.
• About half of U.S. adults don't get enough from food. Seeds, nuts, beans and leafy greens are the best sources.
• The form matters: glycinate is gentle, citrate is well absorbed, oxide is mostly a laxative.
• Keep supplemental magnesium at or below 350 mg a day unless your doctor advises otherwise.

Magnesium doesn't get the attention that vitamin C or vitamin D does, but it may matter just as much. It's a helper for more than 300 enzyme systems in your body, and a large share of adults don't get enough of it from what they eat.

Here's what magnesium does, how to tell whether you're low, and how to choose a supplement if you need one.

What magnesium does

Magnesium works behind the scenes in a lot of everyday functions:

• Energy. The ATP your cells run on has to bind to magnesium to be usable.
• Muscles and nerves. Magnesium helps muscles relax after they contract and helps regulate nerve signaling.
• Blood sugar. It plays a role in how your body responds to insulin.
• Heart rhythm and blood pressure. It helps keep electrical signals in the heart steady.
• Bones. About 50 to 60% of your body's magnesium is stored in your skeleton.

Why so many people fall short

National survey data show that roughly half of Americans consume less magnesium than they need. A few reasons stack up. Processed and refined foods lose much of their magnesium. Many people eat few of the foods that are richest in it. And certain things increase how much you lose, including heavy alcohol use, some diuretics, long-term use of acid-reducing medications called proton pump inhibitors, and poorly controlled type 2 diabetes. Older adults also tend to absorb less and excrete more.

The recommended daily amount for adults is 400 to 420 mg for men and 310 to 320 mg for women, with slightly more during pregnancy.

Food first

Some of the best sources are foods you can add without much effort:

• Pumpkin seeds: about 150 mg per ounce
• Chia seeds: about 110 mg per ounce
• Almonds: about 80 mg per ounce
• Cooked spinach: about 80 mg per half cup
• Cashews: about 75 mg per ounce
• Black beans: about 60 mg per half cup
• Dark chocolate (70% or more): about 65 mg per ounce

A handful of pumpkin seeds on your salad and a serving of beans at dinner gets you a long way.

Can a blood test tell you if you're low?

Not very well. Less than 1% of your body's magnesium is in your blood, and your body works hard to keep that level steady, pulling from bones and tissues if needed. That means a standard serum magnesium test can look normal even when your overall stores are low. If you're concerned, talk to your doctor about your diet, symptoms and medications, which often say more than the test does.

Signs of low magnesium can include muscle cramps or twitches, fatigue and weakness. These are common and have many possible causes, so they're a reason to ask questions, not a diagnosis.

Choosing a form

Supplements pair magnesium with another compound to make it stable. That partner affects how well it's absorbed and how it feels in your gut. When you read a label, look for the elemental magnesium per serving, which is the amount of actual magnesium you're getting.

• Magnesium glycinate (bisglycinate). Bound to the amino acid glycine. Well absorbed and gentle on the stomach, which makes it a popular choice for evening use.
• Magnesium citrate. Well absorbed and widely available. At higher doses it has a mild laxative effect, which some people find helpful.
• Magnesium malate. Bound to malic acid. Well tolerated and often chosen for daytime use.
• Magnesium L-threonate. Studied for its ability to raise magnesium levels in the brain. Promising research, higher price, lower dose of elemental magnesium.
• Magnesium oxide. Packs a lot of magnesium into a small pill, but very little of it is absorbed. It's mostly used as a laxative or antacid.

Magnesium and sleep

Magnesium is often recommended for sleep, and there's a reasonable basis for it: it's involved in the calming side of the nervous system and in the activity of GABA, a neurotransmitter that helps you wind down. Some small trials, especially in older adults, have found improvements in how quickly people fall asleep and how well they sleep. The evidence isn't strong enough to call it a sleep aid, but if your intake is low, correcting it may help. Many people take glycinate an hour or so before bed.

How much to take, and safety

For adults, the upper limit for magnesium from supplements is 350 mg a day. This limit doesn't include magnesium from food, which your kidneys handle easily. Going above it mainly causes diarrhea and stomach cramps. Much higher doses can be dangerous, especially for people with reduced kidney function.

A few other things to know:

• Take magnesium at least two hours apart from certain antibiotics (tetracyclines and fluoroquinolones) and from osteoporosis drugs called bisphosphonates, since it can reduce their absorption.
• If you have kidney disease, talk to your doctor before taking any magnesium supplement.
• Splitting your dose and taking it with food can make it easier on your stomach.

The bottom line

Magnesium is easy to overlook and easy to fix. Start with food, and if you need a supplement, choose a well-absorbed form like glycinate or citrate and stay at or under 350 mg a day.

Sources

1. NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals.
2. Rosanoff A, et al. Suboptimal magnesium status in the United States: are the health consequences underestimated? Nutr Rev. 2012.
3. Abbasi B, et al. The effect of magnesium supplementation on primary insomnia in elderly: a double-blind placebo-controlled clinical trial. J Res Med Sci. 2012.
4. USDA FoodData Central, nutrient values for listed foods.

*These statements have not been evaluated by the Food and Drug Administration. This content is for general education and isn't medical advice. Talk with your doctor before starting a supplement, especially if you're pregnant, nursing, take medication or have a medical condition.