How to read the magnesium line on a supplement label

Two magnesium bottles sit side by side. One says 500 mg on the front. The other says 120 mg. The bigger number might be the better deal, or it might be a pill your gut will quickly complain about.

Magnesium labels are confusing because they mix two different numbers and a long list of forms. Here's how to sort it out.

Elemental magnesium versus compound weight

Magnesium in supplements is always bound to something else, such as oxygen, citric acid or the amino acid glycine. The amount that matters is the magnesium itself, called elemental magnesium. The Supplement Facts panel should show that number, with the form in parentheses: "Magnesium (as magnesium bisglycinate) 150 mg."

The front of the bottle sometimes shows the weight of the whole compound instead. Because the partner molecules weigh different amounts, the same compound weight delivers very different amounts of magnesium. Oxide is roughly 60% magnesium by weight, while citrate and glycinate carry closer to 15%, depending on the exact compound. So "magnesium glycinate 1,000 mg" on the front may mean about 140 mg of actual magnesium.

The main forms, compared

Magnesium oxide. Packs the most magnesium into a small pill and costs the least. It dissolves poorly, though. In one study of commercial US preparations, the fractional absorption of magnesium oxide was only 4%, while magnesium chloride, lactate and aspartate were absorbed significantly better and about equally to each other (Firoz and Graber, 2001).

What doesn't get absorbed stays in the gut and draws in water, which is why magnesium oxide is used as a laxative. In a randomized trial of 90 people with chronic constipation, 93% of them women, 1.5 g of magnesium oxide a day for 28 days led to overall improvement in 68.3% of patients, compared with 11.7% on placebo (Morishita et al., 2021). Useful if that's your goal. Less welcome if it isn't.

Magnesium citrate. Dissolves well. In a 60-day double-blind study, 46 healthy adults took 300 mg of elemental magnesium a day as citrate, an amino acid chelate or oxide. Both citrate and the chelate were absorbed better than oxide, based on urine magnesium. Citrate led to the highest blood magnesium levels, and oxide wasn't different from placebo (Walker et al., 2003).

Magnesium glycinate or bisglycinate. An amino acid chelate, magnesium bound to glycine. Research on it is limited and small. In a crossover trial in 12 patients who had part of their small intestine removed, overall absorption was similar to oxide, 23.5% versus 22.8%. But in the four patients who absorbed oxide worst, the glycinate form was absorbed about twice as well, and it was better tolerated by all patients (Schuette et al., 1994).

Others. Chloride, lactate, malate, taurate and other forms show up on labels too. Some have small absorption studies; most have little research on digestive comfort specifically.

What the NIH says about forms and side effects

According to the NIH Office of Dietary Supplements, forms of magnesium that dissolve well in liquid are more completely absorbed in the gut than less soluble forms. Forms most commonly reported to cause diarrhea include magnesium carbonate, chloride, gluconate and oxide. The upper limit for magnesium from supplements and medications is 350 mg a day for adults (NIH Office of Dietary Supplements).

Reading a magnesium label, step by step

1. Ignore the front, find the panel. Look for the elemental magnesium amount in Supplement Facts.
2. Read the form in parentheses. If several forms are listed together, you usually can't tell how much comes from each. Some "chelated" or "buffered" products also contain oxide.
3. Check the serving size. Many products need two or more capsules to reach the amount shown.
4. Add up all your sources. Multivitamins, antacids and laxatives can contain magnesium too.
5. Match the form to your goal. If you're looking for a laxative effect, oxide has trial data. If loose stools are the last thing you want, a more soluble form in a moderate dose may sit better.

As one example of how a label can look, Night Magnesium lists 300 mg of elemental magnesium from magnesium bisglycinate per 2 capsules, with no magnesium oxide.

Who should check with a doctor first

• Anyone with kidney disease, since the kidneys remove excess magnesium
• People taking antibiotics, osteoporosis medications or other prescriptions that can interact with magnesium
• Anyone whose constipation is new, persistent or comes with blood in the stool, weight loss or pain

Sources

1. National Institutes of Health, Office of Dietary Supplements. Magnesium: fact sheet for health professionals. ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/
2. Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001;14(4):257-62. PMID 11794633
3. Morishita D, Tomita T, Mori S, et al. Senna versus magnesium oxide for the treatment of chronic constipation: a randomized, placebo-controlled trial. Am J Gastroenterol. 2021;116(1):152-161. PMID 32969946
4. Walker AF, Marakis G, Christie S, Byng M. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003;16(3):183-91. PMID 14596323
5. Schuette SA, Lashner BA, Janghorbani M. Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection. JPEN J Parenter Enteral Nutr. 1994;18(5):430-5. PMID 7815675

This article is for general information and isn't medical advice. Talk to your healthcare provider or pharmacist before taking magnesium, especially if you have kidney disease or take prescription medication.