Magnesium forms: which one and at what time

What it is

Behind "magnesium" there are a dozen different salts, and the difference that matters at the counter is twofold: how much pure magnesium each gram of salt carries, and how it sits in the gut. A note on vocabulary: "oxalate" is not a supplement form of magnesium (oxalate is a substance in kidney stones); what is usually meant is **oxide**.

The EU only authorises for magnesium, whatever the salt, that it contributes to a reduction of tiredness and fatigue, to electrolyte balance, to normal muscle and nervous system function, to energy metabolism and to the maintenance of bones and teeth. No salt has its own claim: the "brain", "heart" or "sleep" talk of certain forms is not authorised. For supplements, EFSA sets a tolerable upper level of 250 mg a day of supplemental magnesium.

The forms, one by one

Approximate percentage of pure magnesium and what each is used for:

• Oxide (~60 %): a lot of magnesium but poorly absorbed; the rest draws water into the gut and loosens stools. Used more as a mild laxative or antacid than as a supply.
• Citrate (~11-16 %): very soluble with acceptable absorption. The usual choice to supply magnesium; loosens stools at high doses.
• Bisglycinate (~10-14 %): bound to glycine, the one that least often upsets the stomach. Useful if other salts caused diarrhoea.
• Chloride (~12 %): soluble, in sachets or drops; may also loosen stools at high doses.
• Malate (~15 %): well tolerated; sold for tiredness, and only the general magnesium claim applies, not one of its own.
• Lactate (~12 %): well tolerated and rarely laxative; common in slow-release tablets.
• Threonate (~7 %) and taurate (~9 %): promoted for brain and heart; evidence in people is scarce and no claim is authorised.
• Sulfate and hydroxide (milk of magnesia): these are laxatives, not supplements to supply magnesium.

At the counter

When to recommend it

• Supplying magnesium when the diet falls short: citrate or bisglycinate, in one or two doses
• Taking it with a meal: it is better tolerated and the time of day does not change what it does
• Spacing it from other medicines: at least 2 h from quinolones, tetracyclines and bisphosphonates, and 4 h from levothyroxine
• If a one-off laxative effect is wanted, oxide or sulfate, making clear it is not a magnesium supply

When not to

• Kidney disease or low filtration: magnesium can build up
• Going over the daily dose on the pack thinking more does more: diarrhoea appears and, in diseased kidneys, toxicity
• Choosing the form because of a brain, heart or sleep promise that is not authorised

Warnings

• Important INTERACTIONS: magnesium reduces absorption of quinolones, tetracyclines, bisphosphonates and levothyroxine. Space at least 2 h (4 h for levothyroxine) and warn anyone on several medicines.
• Important KIDNEY: in renal failure magnesium builds up (drowsiness, low blood pressure, arrhythmias). Do not offer it without knowing kidney function.
• Caution No health claim of its own for each salt: "better absorbed", "for the brain" or "for sleep" cannot be promised.
• Caution The tolerable upper level of supplemental magnesium is 250 mg a day; some packs exceed it with two doses.
• Worth knowing Excess shows in the gut: diarrhoea and abdominal pain are the signal to lower the dose.