Magnesium is one of the few minerals many adults fall short of from food alone, and one of the most confusing to buy. The same bottle size can hold very different amounts of the mineral, depending on what it is bound to. Here is how to read the label.
How much you need
The US Recommended Dietary Allowance is 400–420 mg a day for adult men and 310–320 mg for adult women, counting food and supplements together. Green leafy vegetables, legumes, nuts, seeds and whole grains are the richest food sources.
One number matters for supplements: the upper limit for magnesium from supplements and medicines is 350 mg a day for adults. It does not include the magnesium in food. Above it, the most common effect is loose stools.
Elemental magnesium: the number to compare
Magnesium in a capsule is always bound to something else. A label that says "500 mg magnesium glycinate" may describe the whole compound, of which only a fraction is magnesium. Look for the line that says elemental magnesium per serving. That is the number to compare against the allowance and the limit.
The common forms
- Glycinate (bisglycinate). Magnesium bound to the amino acid glycine. It carries relatively little magnesium by weight, so capsules are larger or more numerous, but it is generally easy on the stomach. A sensible default for daily use.
- Citrate. Dissolves well and is well absorbed. At higher doses it draws water into the bowel, which is why it is also sold as a laxative. Fine for daily use at modest doses.
- Oxide. Packs the most magnesium per tablet, so it is cheap per milligram, but less of it is absorbed and it is the form most likely to loosen stools.
- L-threonate. A newer, patented form studied mainly for whether it raises magnesium in the brain. Most of that evidence comes from animal studies and small human trials. It carries very little magnesium per capsule, so it is an expensive way to meet your daily need.
Studies comparing forms generally find that forms that dissolve well (citrate, lactate, chloride, aspartate) are absorbed better than oxide and sulfate. For most people the practical choice is between glycinate (gentle) and citrate (efficient), taken with food.
What about sleep?
Magnesium is often sold for sleep. The research is limited and mixed: a few small trials in older adults found modest effects, and many studies were too small or too short to be sure. If you try it, try it on its own for a few weeks so you can tell whether it makes a difference for you.
Who should check first
- People with kidney disease, because the kidneys clear excess magnesium.
- Anyone taking tetracycline or quinolone antibiotics, or bisphosphonates for bone. Magnesium can reduce their absorption, so doses are usually separated by at least two hours.
- People on long-term acid-reducing medicine (proton pump inhibitors) or certain diuretics, which can push magnesium levels in either direction.
Sources
- NIH Office of Dietary Supplements, Magnesium: Fact Sheet for Health Professionals.
- Institute of Medicine, Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride.
This guide is general information, not medical advice. Talk to your doctor or pharmacist before starting a supplement if you are pregnant or nursing, take medication, or have a medical condition.