Magnesium

The usual blood tests, which measure magnesium in serum, do not reflect the body's real status well: less than 1% of the body's magnesium circulates in the blood. Half lives in the bones and the rest in muscles and other tissues. We will come back to this.

The primary source of magnesium should not be a supplement but food: pumpkin seeds, almonds, green leaves, legumes, dark chocolate... Supplementation makes sense in specific scenarios we will see shortly.

Magnesium is one of the few supplements with several uses backed by solid evidence, and at the same time one of those that carries the most inflated expectations: where it really works and where the effect is modest or nil.

The evidence is solid on four fronts: prevention of chronic migraines, modest reduction of blood pressure in mild hypertension, a mild laxative effect, and irreplaceable hospital uses (eclampsia, certain arrhythmias, severe asthma) given by injection under supervision.

Special mention goes to sleep, probably the no. 1 reason many people come to magnesium. It helps older people with insomnia or with a deficiency, but it is not a sedative: do not expect to fall asleep better from a one-off dose.

The most rigorous studies suggest that supplementing magnesium without a prior deficiency does not improve aerobic performance, and may even slightly worsen it. For hypertrophy or injury recovery there is no solid evidence. The headline “more magnesium = better athlete” does not hold.

Magnesium in the form of L-threonate has been studied for deep sleep and for mild memory complaints, with positive results that have not yet been confirmed by independent teams.

It is worth asking whether our diet covers the minimum intake before supplementing. The early symptoms of deficiency are nonspecific and shared with many other situations: fatigue, muscle weakness, sleep disturbances, irritability and headache. On their own they diagnose nothing, but in people with poor diets or chronic medication that predisposes to losing magnesium, they are signals not to ignore.

Supplementation makes sense above all in four groups where deficiency is likely or the indication is backed by clinical trials: older people (they eat less and absorb worse); patients who take omeprazole or other stomach protectors chronically (the FDA issued an alert about this in 2011); people with type 2 diabetes (because of urinary losses); and those with specific indications such as chronic migraine or cramps during pregnancy. For a healthy person who eats well, the benefit of supplementing is marginal.

The Recommended Dietary Allowance (RDA) is about 400 mg in adult men and about 320 mg in women, counting everything you ingest from food, water and supplements. A varied diet with seeds, nuts, green leaves and legumes usually covers those figures effortlessly.

When supplementing is needed, the usual useful dose is between 200 and 400 mg a day of elemental magnesium — not of magnesium salt. In a supplement of “500 mg of magnesium oxide” you are not taking 500 mg of magnesium, but a much smaller fraction. Serious labeling states the elemental magnesium separately.

Three practical rules:

Higher doses — such as the 600 mg a day used in some migraine prophylaxis trials — are backed by the evidence, but must be prescribed by a professional because they exceed that limit.

The short answer is no. There are more than a dozen forms of magnesium on the market and, although the differences between them are smaller than marketing suggests, they do matter in two things: how much magnesium really enters the body and how well the stomach tolerates it.

For most people who want to cover their day-to-day magnesium, magnesium citrate is the option with the best cost-absorption-tolerance balance. If your stomach is sensitive or you take it at night and do not want any laxative effect, bisglycinate is the “gentle” form par excellence. It has a very salty, bitter taste, hard to disguise. The powder form may be cheaper, but it is worth considering taking it in capsules. Magnesium oxide, paradoxically the best-selling in pharmacies, is the worst absorbed — perhaps useful as an occasional laxative or antacid, but not the best for replenishing magnesium.

There is no simple test to know whether you are deficient: the blood magnesium test is done but reflects the body's real status very poorly. It is not clear how much magnesium each person needs: the optimal dose depends on diet, age, kidney function and medications. And it is not clear whether supplementing someone without a deficiency provides any benefit: the FDA itself issued a statement in 2022 rating the general evidence as “inconsistent and inconclusive”.

Magnesium from food poses no problems in healthy people: the kidney excretes the surplus. The precautions apply to supplementation, above all in four situations that must be discussed with a professional:

If you take chronic medication, the prudent thing is to discuss it with your doctor or pharmacist before starting to supplement.