Magnesium

What Is Magnesium?

Magnesium is a mineral the body cannot make. An adult carries about 25 g of it: half or more in the bones, most of the rest inside muscle and other soft tissue, and less than 1% in the blood. That last figure matters later, because it is the part a blood test measures.

What Magnesium Does in the Body

Magnesium is a helper for more than 300 enzyme systems. Without it the body cannot turn food into usable energy, build protein, or copy DNA and RNA when cells divide. It is also needed to make glutathione, one of the body's own antioxidants.

Its second job is moving other minerals. Magnesium helps carry calcium and potassium in and out of cells, and that flow is what lets a nerve fire, a muscle contract and relax, and the heart keep a steady rhythm. In bone it is part of the structure itself.

How the Body Absorbs and Keeps Magnesium

Of the magnesium in a meal, the gut typically absorbs about 30 to 40%. The kidneys then decide how much stays: when intake is low they pass less into urine, which is why a short stretch of poor eating rarely causes symptoms.

The same control is why a blood test can look normal while the body's stores run low. The blood level is held in a narrow range, 0.75 to 0.95 mmol/L, by drawing on bone and tissue. A normal result rules out a severe shortage, not a mild one.

Where Magnesium Comes From

Green leaves are a source for a simple reason: chlorophyll, the pigment that makes them green, has a magnesium atom at its center. Seeds, nuts, beans and whole grains are richer still, because magnesium is concentrated in the parts of a seed that feed the new plant.

Those parts are what refining removes. Cooked brown rice has about 39 mg per 100 g and cooked white rice about 12 mg, under a third. The same goes for white flour against whole grain. Tap and mineral water add some magnesium too, but how much depends on where it comes from, so check the label on bottled water.

A Day of Food That Meets the Magnesium RDA

One ordinary day built only from foods in the Caltide library, in usual portions, with no supplements.

Three foods do most of the work: black beans, almonds and oats give 56% of the total.

Who Is Most Likely to Be Low in Magnesium

US food surveys find that many people eat less magnesium than recommended, with teenagers and men over 70 the lowest. A few groups also lose more or absorb less: people with gut conditions such as Crohn's or celiac disease, people with type 2 diabetes, whose kidneys pass more magnesium into urine, people who drink heavily over a long time, and older adults.

How Magnesium Works With Other Nutrients

A long shortage of magnesium can pull down blood levels of calcium and potassium as well, because the body needs magnesium to keep both in balance. In the other direction, very high doses of zinc from supplements, around 142 mg a day, have been shown to reduce how much magnesium is absorbed. Normal amounts of zinc from food do not.

Common Magnesium Mistakes

Ranking foods only per 100 g. Seeds win per 100 g, but nobody eats 100 g of chia. The per portion table above is closer to real life.

Expecting fruit to help. Bananas are often named as a magnesium food, yet a medium banana gives about 34 mg, under a tenth of the daily reference.

Reading the compound, not the mineral. On a supplement label, 500 mg of magnesium citrate is the weight of the whole compound. The magnesium in it is the elemental figure, which is much lower.