Magnesium citrate has better absorption data than magnesium oxide. Magnesium glycinate is a well-tolerated organic form with less head-to-head research behind it. Oxide fits the most magnesium into the smallest pill but dissolves poorly. For most people, the useful questions are how much elemental magnesium a product delivers and how well your digestion handles the form.
Below: what "elemental" means, what the research shows, how much adults need, and who should be careful.
Key takeaways
- Labels list elemental magnesium, meaning the magnesium itself rather than the weight of the whole compound. That is the number to compare.
- Oxide is about 60% magnesium by weight. Citrate is about 16% and glycinate about 14%, so these two need much more powder for the same dose.
- Small human studies favor citrate over oxide for absorption. Direct comparisons that include glycinate are scarce, so be skeptical of precise absorption rankings.
- Oxide and citrate are the forms more likely to loosen stools. Glycinate is often chosen by people who want to avoid that.
- The adult RDA is 400–420 mg for men and 310–320 mg for women. The NIH upper limit for magnesium from supplements is 350 mg/day.
Elemental magnesium vs. compound weight
Magnesium in a supplement is always bound to something else: oxygen in oxide, citric acid in citrate, the amino acid glycine in glycinate. The partner molecule adds weight but no magnesium. That is why the NIH Office of Dietary Supplements notes that the Supplement Facts panel declares elemental magnesium, not the weight of the compound.
The difference is large. In one trial described by the NIH, 500 mg of magnesium oxide supplied 300 mg of elemental magnesium. Glycine and citrate are much heavier partners than oxygen. The approximate figures below are calculated from each compound's molecular weight:
| Form | Elemental magnesium by weight | Compound needed for 400 mg elemental | Solubility and absorption evidence | Digestive notes |
|---|---|---|---|---|
| Magnesium oxide | About 60% | About 0.66 g | Nearly insoluble in water. One small study estimated fractional absorption at 4%. | The NIH lists oxide among the forms most often linked to diarrhea. |
| Magnesium citrate | About 16% (anhydrous; hydrated grades are lower) | About 2.5 g | More soluble than oxide. Small studies found higher urinary magnesium than with oxide. | Also sold at higher doses as an over-the-counter saline laxative. |
| Magnesium glycinate | About 14% (unbuffered) | About 2.8 g | An organic, amino acid–bound form. Little published human data compares it directly with citrate or oxide. | Not on the NIH's list of forms most often linked to diarrhea. |
Two practical points follow. First, "1,000 mg magnesium glycinate" on a front label is not 1,000 mg of magnesium, so check the Supplement Facts panel. Second, some glycinate products are blended ("buffered") with oxide to raise the magnesium per capsule. That is not wrong, but the label should make clear what you are getting.
What the absorption research actually shows
The NIH summarizes the evidence this way: forms that dissolve well in liquid tend to be absorbed more completely. Aspartate, citrate, lactate and chloride have shown better bioavailability than oxide and sulfate. The NIH adds that these findings come from small studies.
A few of those studies:
- Lindberg et al. (1990) found magnesium oxide virtually insoluble in water. Even at simulated peak stomach acid, only 43% dissolved, while 55% of citrate dissolved in plain water. In volunteers, a citrate dose raised urinary magnesium (a marker of absorption) significantly more than oxide.
- Firoz and Graber (2001) compared four commercial preparations in healthy volunteers. Oxide had a fractional absorption of 4%. Chloride, lactate and aspartate were significantly better absorbed and roughly equal to each other.
- Blancquaert et al. (2019) tested 15 products in a laboratory gut model and then gave some to 30 people. Organic forms generally did better, but two organic products did not. The authors concluded that salt type is not the only factor and that solubility mattered more than how much magnesium a tablet held. They also noted that earlier comparisons of citrate and oxide had not all agreed.
What this means: oxide's poor solubility is consistent across the research. The gap between the better-absorbed forms is much less certain. Glycinate is rarely tested head-to-head, so a single "most absorbable form" ranking overstates what is known. Taking magnesium consistently, at a dose your gut tolerates, probably matters more than small differences between the well-absorbed forms.
Digestive tolerance and who tends to choose which
Unabsorbed magnesium pulls water into the bowel. That is why the NIH lists diarrhea, nausea and abdominal cramping as common effects of high supplemental doses. Carbonate, chloride, gluconate and oxide are the forms it names as most often responsible. Citrate's effect is well known too: it is sold in larger doses as an over-the-counter saline laxative.
- Oxide is chosen for low cost and a small pill, by people who are not bothered by its digestive effects.
- Citrate is chosen by people who want a soluble form with more absorption data than oxide and who tolerate a looser stool.
- Glycinate is often chosen by people who are sensitive to laxative effects and want to take a full daily amount without that trade-off. The cost is more capsules per serving, because the compound is only about 14% magnesium.
How much you need, the upper limit, and who should be careful
Magnesium is a cofactor in more than 300 enzyme systems, including those involved in protein synthesis, muscle and nerve function, and energy production. The adult RDA from all sources is 400 mg (men 19–30) or 420 mg (men 31 and older), and 310 mg (women 19–30) or 320 mg (women 31 and older). During pregnancy it is 350–360 mg, depending on age.
The tolerable upper intake level (UL) for adults is 350 mg/day from supplements and medications. It does not count magnesium from food, which is why it can be lower than the RDA. The NIH notes that too much magnesium from food is not a health risk for healthy people. High doses from supplements, by contrast, often cause diarrhea.
- Kidney function: healthy kidneys excrete excess magnesium. With reduced kidney function, the risk of magnesium building up rises. Anyone with kidney disease should take magnesium only under medical supervision.
- Medications: magnesium can reduce absorption of bisphosphonates (separate them by at least 2 hours). It can also bind tetracycline and quinolone antibiotics: take the antibiotic at least 2 hours before or 4–6 hours after magnesium. Some diuretics and long-term proton pump inhibitor use also affect magnesium status.
- Pregnancy, nursing or chronic conditions: talk with your healthcare professional before starting.
Where MAXIS Magnesium Glycinate fits
MAXIS Magnesium Glycinate provides 400 mg of elemental magnesium from magnesium glycinate per serving of 3 vegan capsules (95% DV). The label states the elemental amount, so there is no compound-weight math to do. Magnesium supports normal muscle and nerve function and is required for energy production and protein synthesis.†
Take it as directed. Because 400 mg is above the NIH's 350 mg/day upper limit for supplemental magnesium, check with a healthcare professional before taking a full serving every day. Ask whether fewer capsules makes more sense for you, especially if you already get magnesium from other supplements. The research above concerns magnesium forms in general, not the finished MAXIS product. Our testing and manufacturing standards are on our standard page.
FAQ
If a label says "400 mg magnesium," is that the compound or the magnesium?
On the Supplement Facts panel, it is the elemental magnesium. Front-of-pack phrases like "1,000 mg magnesium glycinate" usually refer to the whole compound, which contains far less magnesium.
Is magnesium glycinate better absorbed than citrate?
It has not been clearly shown either way. Both are organic forms that dissolve better than oxide, but direct human comparisons involving glycinate are limited. The clearer difference is digestive: citrate is more likely to loosen stools.
Is magnesium oxide useless?
No. It delivers a lot of magnesium per pill, and some of it is absorbed. Its fractional absorption in small studies was low, though, and it is among the forms most likely to cause loose stools.
Can I take magnesium with other supplements?
Generally yes. Very high zinc intakes (142 mg/day in one study) interfered with magnesium absorption, and timing matters for certain medications listed above. Count magnesium from every product, including electrolyte or pre-workout mixes, toward the 350 mg/day supplemental limit.
References
- National Institutes of Health, Office of Dietary Supplements (2026). Magnesium: Fact Sheet for Health Professionals. ods.od.nih.gov
- Lindberg JS, Zobitz MM, Poindexter JR, Pak CY (1990). Magnesium bioavailability from magnesium citrate and magnesium oxide. Journal of the American College of Nutrition, 9(1):48–55. PubMed
- Firoz M, Graber M (2001). Bioavailability of US commercial magnesium preparations. Magnesium Research, 14(4):257–262. PubMed
- Blancquaert L, Vervaet C, Derave W (2019). Predicting and testing bioavailability of magnesium supplements. Nutrients, 11(7):1663. DOI
- MedlinePlus (U.S. National Library of Medicine). Magnesium citrate. medlineplus.gov
†These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
