Magnesium: a complete guide to the forms

What does magnesium do in the body?

Magnesium is a cofactor in more than 300 enzyme reactions, including the ones that make and use ATP, the cell's energy currency; inside cells, ATP exists mostly bound to magnesium [1][2]. It is also needed to build proteins, for normal nerve signaling, for muscle contraction and relaxation, for the body's handling of glucose, and for bone, where 50 to 60 percent of the body's magnesium is stored [1].

Magnesium works as a counterweight to calcium: calcium drives muscle contraction, and magnesium supports the relaxation that follows. That balance is why low magnesium is linked with muscle cramps and twitching, and why it matters for the steady rhythm of the heart muscle [1].

How much do you need, and who runs low?

The recommended dietary allowance is about 400 to 420 mg a day for adult men and 310 to 320 mg for adult women, a little higher in pregnancy [1]. Outright deficiency is uncommon in healthy people because the kidneys conserve magnesium, but low intake is common: in one US national survey, almost half of the population took in less than the required amount from food [3].

Some people are more likely to run low: those whose digestion absorbs poorly or who have chronic diarrhea, people with persistently high blood sugar, older adults, and heavy drinkers. Long-term use of some medicines also lowers magnesium, notably proton pump inhibitors (stomach-acid blockers) and certain diuretics [1]. If that is you, raise it with your doctor rather than guessing.

A standard blood test can look normal even when body stores are low, because less than 1 percent of the body's magnesium is in the blood serum; the rest sits in bone and inside cells [1].

How do the magnesium forms compare?

Every magnesium supplement is magnesium bound to something else, and that partner shapes how well it absorbs and how your gut reacts. Forms that dissolve well are absorbed more completely: small studies summarized by the NIH found aspartate, citrate, lactate and chloride absorbed more completely than oxide [1].

In a randomized, double-blind trial in 46 healthy adults taking 300 mg of elemental magnesium a day for 60 days, citrate and an amino-acid chelate were absorbed better than oxide, citrate produced the highest blood levels, and oxide did no better than placebo [5].

Which forms suit which goal?

Citrate is the workhorse for closing an everyday gap: well absorbed, widely available and affordable [5]. It also draws water into the bowel, so higher doses loosen stools; at modest doses that is usually a non-issue, and splitting the dose helps [1]. That makes it a good fit if you also want help with regularity, and a less ideal one if you already tend toward loose stools.

Glycinate is magnesium bound to the amino acid glycine. It is usually described as one of the gentlest forms on the gut, and amino-acid chelates were absorbed better than oxide in the 60-day trial, though glycinate itself has not been tested head to head against citrate [5]. It is a common evening pick for sleep and calm.

Threonate was developed to raise magnesium in the brain. In rats it raised brain magnesium, increased the density of synapses in the hippocampus, a region central to memory, and improved learning and memory [4]. Human research is earlier and smaller, so think of it as a promising, brain-focused form with developing evidence. It also delivers less elemental magnesium per dose, which makes it an expensive way to close a general shortfall.

Oxide is cheap and very concentrated, about 60 percent magnesium by weight, but poorly absorbed; much of it stays in the gut, where it acts as an osmotic laxative [1][5]. It is a poor way to raise body stores and a practical one when occasional constipation is what you want help with.

Magnesium chloride is a soluble salt sold as an oral supplement, where it is absorbed more completely than oxide [1], and in sprays and bath flakes. Epsom salts are magnesium sulfate. A 2017 review found the claim that meaningful amounts of magnesium cross intact skin from baths or sprays scientifically unsupported [6]. Enjoy the soak, but do not count on it to raise your magnesium.

Whatever the form, read the elemental dose, not the compound weight. Pure magnesium bisglycinate is only about 14 percent magnesium, so a capsule labeled 1,000 mg magnesium glycinate gives around 140 mg of magnesium, while some products sold under that name are blends cut with oxide that deliver more. The elemental number is usually in the Supplement Facts panel [1].

How much should you take, and when?

Common supplemental doses run about 100 to 350 mg of elemental magnesium a day, on top of food. The tolerable upper intake level for magnesium from supplements is 350 mg a day for adults; it does not apply to magnesium from food [1]. The first sign of too much is usually loose stools or diarrhea, most often reported with carbonate, chloride, gluconate and oxide [1].

• Match the form to the goal: citrate for value, glycinate for evenings, citrate or oxide for regularity, threonate as a brain-focused add-on with realistic expectations.
• Count food first. Total intake from diet plus supplements is what matters.
• Check the elemental dose on the Supplement Facts panel, not the compound weight on the front.
• Start low, and split the dose with food if your gut complains.
• Many people take a calming form such as glycinate in the evening.
• Stay at or below 350 mg of supplemental magnesium a day unless your doctor advises otherwise.

Who should be careful with magnesium?

In healthy people the kidneys clear extra magnesium, so the main effect of overdoing supplements is digestive. The important exception is reduced kidney function: people with impaired kidneys can build up magnesium to dangerous levels and should not take supplements without medical supervision [1].

Magnesium can also reduce the absorption of some medicines, including tetracycline and fluoroquinolone antibiotics and bisphosphonates for bone density; spacing the doses a few hours apart generally avoids the problem [1]. If you take prescription medicines, have reduced kidney function or are pregnant, check the dose and form with your doctor or pharmacist first.

Where does magnesium come from in food?

Supplements are a backstop, not a replacement for diet. Magnesium is concentrated in green leafy vegetables such as spinach, in nuts and seeds such as pumpkin seeds, almonds and cashews, and in legumes and whole grains [1].

If you are choosing one product, ask which form fits both your goal and your gut, and check that the label states the elemental magnesium plainly. A label that hides the elemental dose, or dresses up cheap oxide with a better-sounding name on the front, undermines any choice.