Magnesium glycinate vs citrate vs oxide: what the studies show

If you want the form with the best-documented absorption, choose citrate. If citrate or oxide upsets your stomach, glycinate is a reasonable thing to try, but the evidence that it absorbs better is one small study with a mixed result. If you want magnesium for constipation, oxide is the form a gastroenterology guideline names. We found no trial showing one form beats another on a health outcome a reader would feel.

The NIH Office of Dietary Supplements' magnesium fact sheet, updated January 6, 2026, makes three points about forms. Forms that dissolve well in liquid are more completely absorbed in the gut than less soluble forms. Small studies have found magnesium in the aspartate, citrate, lactate and chloride forms absorbed more completely than magnesium oxide and sulfate. And the forms most commonly reported to cause diarrhea are carbonate, chloride, gluconate and oxide.

The words glycinate, bisglycinate, threonate and malate do not appear in that fact sheet. Federal guidance has no position on them.

Read together, these point one way for citrate versus oxide: both studies that compared them directly over time favored citrate, and Firoz found oxide absorbed poorly against three other soluble forms. For glycinate the picture is thin. The only two-way comparison with oxide, in 12 people after bowel surgery, found no overall difference, and the advantage appeared in a subgroup of four. The newest study found no significant blood rise from bisglycinate at all, though it measured only six hours after a single dose and was funded by the company that makes the product it was promoting.

None of these studies measured anything you would feel. A rise in urine or blood magnesium after a dose is a reasonable sign of absorption, but no trial has compared glycinate with citrate on sleep, cramps, blood pressure or any other outcome.

Each form is a compound, and the magnesium is a fraction of its weight. By our calculation from the molecular formulas, magnesium oxide is about 60% magnesium, magnesium citrate about 12 to 16% depending on the hydrate, and magnesium bisglycinate about 14%. NIH says the Supplement Facts panel must declare the elemental magnesium, and that is the only number to compare.

A real example from our magnesium buyer's guide: a product sold as "Magnesium Glycinate 400 mg" declares 60 mg of magnesium per capsule on its NIH label record. You would need about four of those capsules to match one 250 mg oxide tablet.

A classic study fed volunteers a standard meal with increasing amounts of magnesium. Absorption rose with each step, but the fraction absorbed fell from 65% at the lowest intake to 11% at the highest. The same study found magnesium from almonds as bioavailable as a soluble magnesium salt, and enteric-coated magnesium chloride much less so. Bigger doses mostly mean more unabsorbed magnesium in the gut, which is what causes diarrhea.

Citrate: the best-documented absorption among common forms, per NIH and the studies above. A reasonable default.

Oxide: the most magnesium per tablet and the lowest cost per milligram, poorer absorption, and more diarrhea. The 2023 joint guideline of the American Gastroenterological Association and American College of Gastroenterology makes a conditional recommendation for magnesium oxide in chronic constipation, where the laxative effect is the point.

Glycinate and bisglycinate: often marketed as gentler on the stomach. Schuette's patients tolerated the glycinate better, which is one small observation. The best sleep trial of any magnesium used bisglycinate, 250 mg of magnesium a day, and found a modest benefit; it also gave about 1.5 g of glycine a day, so the result is not purely about magnesium. Our magnesium evidence guide reads that trial and the others.

Threonate: about 7% of the label weight is magnesium in the product we checked (144 mg from 2 g), so it is the most expensive way to buy magnesium.

The upper limit for magnesium from supplements and medicines is 350 mg a day for adults, per NIH; food magnesium does not count. People with reduced kidney function should ask a clinician before supplementing. NIH says to keep magnesium at least 2 hours away from oral bisphosphonates, and to take tetracycline or quinolone antibiotics at least 2 hours before or 4 to 6 hours after it. Spacing details are in the best time to take magnesium.