Magnesium oxide, citrate or glycinate? How the forms compare

Spend a few minutes in the supplement aisle and magnesium starts to look like a chemistry quiz. Oxide, citrate, glycinate, chloride, malate, threonate. Some labels promise calm, others promise regularity, and prices range from a few dollars to quite a lot more.

The form does matter, though not always in the way the marketing suggests. Here's what research shows about how the common forms differ in absorption and in how they affect your gut.

Why the form matters at all

Magnesium in a supplement is always bound to another compound, and that partner affects how well it dissolves and how much your body takes up. According to the NIH Office of Dietary Supplements, forms that dissolve well in liquid are absorbed more completely. Small studies have found better absorption from magnesium aspartate, citrate, lactate and chloride than from magnesium oxide or sulfate (NIH Office of Dietary Supplements).

The form also affects what happens in your intestines. Unabsorbed magnesium salts draw water into the bowel, which is why high supplement doses often cause diarrhea, sometimes with nausea and cramping (NIH Office of Dietary Supplements). That can be useful if you're constipated and unwelcome if you aren't.

Magnesium oxide: concentrated, poorly absorbed

Magnesium oxide packs a lot of magnesium into a small, inexpensive tablet, which is why it's so common. But in a study of US commercial preparations, only about 4% of the magnesium from oxide was absorbed. Chloride, lactate and aspartate were absorbed significantly better (Firoz and Graber, 2001).

What isn't absorbed stays in the bowel and acts as a laxative. In a Japanese trial of 34 women with mild to moderate chronic constipation, 1.5 grams of magnesium oxide a day for 28 days led to overall improvement in 70.6% of women, compared with 25.0% on placebo (Mori et al., 2019). That's helpful for constipation, but less ideal if you want magnesium without extra bathroom trips.

Magnesium citrate: well absorbed, can loosen stools

Citrate is one of the better-studied forms. In a 60-day randomized trial in 46 healthy adults taking 300 mg of elemental magnesium a day, citrate produced the highest blood magnesium levels, both after the first dose and after 60 days. Magnesium oxide performed no differently from placebo (Walker et al., 2003).

Magnesium citrate is also sold as an over-the-counter laxative in larger doses. If you're prone to loose stools or urgency, keep that in mind before choosing it.

Magnesium glycinate: often chosen for a sensitive gut

Glycinate, also sold as bisglycinate or diglycinate, binds magnesium to the amino acid glycine. It belongs to a group called amino acid chelates. In the same 2003 trial, an amino acid chelate was absorbed better than oxide at 60 days, based on magnesium measured in urine (Walker et al., 2003).

Glycinate-specific research is small. In a crossover study of 12 patients who had part of their small intestine removed, overall absorption from glycinate and oxide was similar. But glycinate was absorbed much better in the four patients who absorbed oxide most poorly, and it was better tolerated by all of them (Schuette et al., 1994). That's a very specific group, so the results are suggestive rather than definitive for healthy women.

Side by side

• Oxide: lots of magnesium per tablet, low absorption, commonly linked with diarrhea. Studied as a constipation option.
• Citrate: well absorbed in trials, and can loosen stools at higher doses.
• Glycinate or bisglycinate: an amino acid chelate that was well tolerated in a small study, with limited head-to-head research in healthy adults.
• Chloride, lactate and aspartate: absorbed better than oxide in small studies, though chloride is among the forms most often reported to cause diarrhea (NIH Office of Dietary Supplements).
• Sulfate: less well absorbed than the soluble forms above (NIH Office of Dietary Supplements), and best known as Epsom salt.

How to choose

• Start with your goal. If constipation is the main issue, a form that stays in the bowel may be the point. If you want magnesium without digestive side effects, a better-absorbed, gentler form makes more sense.
• Compare elemental magnesium. The Supplement Facts panel lists how much magnesium each serving provides. Compare that number, not the weight of the whole compound.
• Respect the upper limit. For adults, the upper limit for magnesium from supplements and medications is 350 mg a day. It doesn't apply to magnesium from food (NIH Office of Dietary Supplements).
• Start low if your gut is sensitive, and increase only if you tolerate it well.
• Check for interactions. Magnesium can interact with certain antibiotics, osteoporosis drugs called bisphosphonates, diuretics and long-term acid-reducing medication (NIH Office of Dietary Supplements).

For reference, Night Magnesium uses magnesium bisglycinate and provides 300 mg of elemental magnesium per two capsules, with no magnesium oxide.

When to see a doctor

If you have kidney disease, don't take magnesium supplements without medical advice, because the risk of magnesium building up rises when the kidneys can't clear it (NIH Office of Dietary Supplements). Also talk to your doctor if diarrhea continues after you stop a supplement, or if you're taking magnesium for constipation that isn't improving, especially alongside blood in your stool, unexplained weight loss or a lasting change in bowel habits.

Sources

1. National Institutes of Health, Office of Dietary Supplements. Magnesium: fact sheet for health professionals. ods.od.nih.gov
2. Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001;14(4):257-62. PMID 11794633
3. Mori S, Tomita T, Fujimura K, et al. A randomized double-blind placebo-controlled trial on the effect of magnesium oxide in patients with chronic constipation. J Neurogastroenterol Motil. 2019;25(4):563-575. PMID 31587548
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. Check with your healthcare provider before starting magnesium, especially if you have kidney disease or take prescription medication.