How much is too much magnesium?

The two overflow valves, which are the reason a healthy adult cannot overload on magnesium

How much is too much magnesium, and the variant asking whether you can overload your body with it, are answered by two overflow valves the body already has, and understanding them is what turns a number into a picture. Valve one is the gut: magnesium is absorbed by a saturable transport system in the small intestine, so the fraction absorbed falls as the dose rises and a large dose leaves a large surplus in the bowel, where it draws water osmotically and leaves as loose stools within hours, which is the laxative effect, the body's first refusal of a surplus, and the reason the salts differ, oxide being least absorbed and most laxative, bisglycinate best absorbed and least. Valve two is the kidneys: magnesium that is absorbed above what the tissues need is filtered and excreted in the urine, serum magnesium returning to its narrow normal range within a day or two of a surplus, so that even the fraction the gut let through does not accumulate in a healthy adult. Between the two valves sits the UK expert group's supplemental guidance level of about 400mg a day, the point at which valve one usually opens for most people, so that too much magnesium in a healthy adult means a laxative amount rather than a harmful one, and the honest number is: over about 400mg a day from supplements is too much in the sense of surplus and loose stools, with harm not entering the picture while the valves work. That is why a healthy adult cannot overload on magnesium from a supplement habit, the surplus leaving by one valve or the other faster than a capsule can add it.

Where the valves fail, the units and the hidden sources, and the number filed

The valves fail in two situations, and both are where too much becomes dangerous. Kidney disease: valve two fails when excretion is impaired, so absorbed magnesium accumulates from doses a healthy adult clears, and hypermagnesaemia, lethargy, weakness, low blood pressure, slowed heart conduction, becomes possible from ordinary intake, which is why any magnesium in kidney disease, antacids and laxatives included, is a doctor's decision and why the safety pages make it the red gate. Laxative-scale intake: valve one can be overwhelmed by grams of magnesium at a time, Epsom salts, magnesium hydroxide laxatives and antacids taken in quantity delivering enough absorbable magnesium that even working kidneys lag, the source of nearly every serious case in the literature, and a picture that a supplement capsule at a few hundred milligrams never produces. The units matter to the number: too much is counted in elemental magnesium, so 500mg of magnesium glycinate at about 100mg elemental is nowhere near it and 500mg of elemental magnesium is over it, the too-much matrix's reading, and the hidden sources matter too, an evening antacid or a magnesium laxative adding elemental magnesium to a supplement's total without announcing it. The number filed: too much is over about 400mg of elemental magnesium a day from supplements in a healthy adult, meaning loose stools rather than harm, you cannot overload while the gut and the kidneys work, and the valves fail only in kidney disease or at laxative-scale intake, the worked example sitting inside both valves at 300mg elemental and 80% NRV, £19.99 for 90 days at 22p.

The verdict, too much is a laxative amount until the valves fail

How much is too much magnesium: over about 400mg of elemental magnesium a day from supplements in a healthy adult, which is a laxative amount rather than a harmful one, because the gut refuses a surplus as loose stools within hours and the kidneys excrete what was absorbed within days, so you cannot overload while both valves work, and the valves fail only in kidney disease, where any dose is a doctor's decision, or at laxative-scale intake of grams at a time from Epsom salts, magnesium laxatives or antacids. The dose inside both valves: Futuro Labs Magnesium Glycinate, bisglycinate, 300mg elemental at 80% NRV, £19.99 for 90 days at 22p a day.

What to check on the label

Questions people also ask

Can you overload your body with magnesium?

Not with working kidneys: the gut refuses a surplus as loose stools within hours and the kidneys excrete what was absorbed within a day or two, so a supplement habit cannot accumulate magnesium in a healthy adult.

How much magnesium is too much per day?

Over about 400mg of elemental magnesium a day from supplements, the UK guidance level, which for most people is the point at which the laxative effect arrives, a surplus rather than a harm.

When does too much magnesium become dangerous?

When the valves fail: in kidney disease, where impaired excretion lets ordinary doses accumulate, and at laxative-scale intake of grams at a time from Epsom salts, magnesium hydroxide laxatives or antacids, the sources in nearly every serious case.

Is 500mg of magnesium glycinate too much?

No: 500mg of the compound is about 100mg of elemental magnesium, well under the level, whereas 500mg of elemental magnesium is over it, the difference being the unit and the %NRV line on the tub telling which.

Does the laxative effect mean I've taken too much?

It means the dose exceeded what the gut absorbed, the body's first valve opening, so the dose is reduced or the salt changed to a better-absorbed one, with no harm implied in a healthy adult.

Sources

1. The magnesium absorption literature: fractional absorption of magnesium falls as the dose rises, so the same daily amount taken in divided doses is absorbed at a higher fraction and with less osmotic laxative effect than a single large dose. PubMed record
2. Expert Group on Vitamins and Minerals. Safe Upper Levels for Vitamins and Minerals. Food Standards Agency, 2003 (magnesium guidance level 400mg a day from supplements). https://cot.food.gov.uk/sites/default/files/vitmin2003.pdf
3. The hypermagnesaemia literature: the kidneys excrete surplus magnesium, so supplementation is safe for healthy adults within guidance and warrants medical advice in chronic kidney disease, where impaired excretion allows magnesium to accumulate. PubMed record
4. The hypermagnesaemia literature: raised serum magnesium is rare with normal kidney function and occurs with impaired excretion or massive intake from magnesium laxatives and antacids, presenting with nausea, lethargy, low blood pressure, muscle weakness and slowed heart conduction, and treated in hospital with intravenous calcium and, where needed, dialysis. PubMed record

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