Mineral Supplement Forms: Oxides, Carbonates, Citrates, Gluconates and Chelates Explained
What oxides, carbonates, citrates, gluconates and chelates are, how much real mineral each holds, and what absorption studies did and did not show, without picking a winner.
Content last updated . This page has not been medically reviewed.
If a child may have swallowed iron or other mineral tablets, call Poison Help at 1-800-222-1222 (US) right away, or 911 if the child collapses or has trouble breathing. See Poisoning: What to Do First for more.
A mineral supplement "form" is the compound that carries the mineral, such as an oxide, carbonate, citrate, gluconate or amino acid chelate. The form changes how much actual mineral is in each tablet, and in some studies it changed how much was absorbed. This page explains the labels and what studies measured. It does not crown a winner: the head-to-head pages for magnesium, calcium, iron and zinc go deeper.
Labels used here: absorption study means a small trial measuring how much mineral gets into the body, usually in healthy adults. Mixed means studies disagree. Proper evidence not found means we searched and found nothing usable.
Elemental amount versus compound weight
A tablet's big number may be the weight of the whole compound. What matters for intake is the elemental mineral, which the Supplement Facts panel should show. One consensus statement puts it plainly: calcium recommendations refer to the milligrams of elemental calcium in the supplement. [1] The share differs a lot by form, as the table shows.| Mineral | Form | Elemental share by weight | Source and year |
|---|---|---|---|
| Calcium | Carbonate | 40% | Endocrine Society of India consensus, 2026 [1] |
| Calcium | Citrate | 21% | Same [1] |
| Calcium | Gluconate or lactate | 13% | Same [1] |
| Calcium | Acetate | 25% | Same [1] |
| Zinc | Oxide | 80% | Narrative review, Nutrients 2024 [2] |
| Zinc | Citrate | 31% | Same [2] |
| Zinc | Sulfate | 23% | Same [2] |
| Zinc | Gluconate | 14% | Same [2] |
Elemental percentages for magnesium and iron compounds could not be verified from the sources we read, so use the Supplement Facts panel for those. For unit questions see Mineral Units Explained.
What absorption studies found
| Question | What the source found | Strength |
|---|---|---|
| Calcium carbonate vs citrate | A meta-analysis of 15 studies (184 people) found citrate was absorbed about 22% to 27% better, on an empty stomach or with meals [3] | Absorption only, not bone outcomes |
| Does stomach acid matter for calcium? | A 1984 single-meal study found that blocking stomach acid did not change calcium absorption, including from calcium carbonate [4] | Small physiological study, mixed with the above |
| Magnesium oxide | In healthy volunteers, fractional absorption of magnesium oxide was about 4%; chloride, lactate and aspartate were higher and similar to each other [5] | Absorption study |
| Magnesium citrate vs oxide | In 46 healthy adults given 300 mg elemental magnesium daily for 60 days, citrate and an amino acid chelate showed greater absorption than oxide, which did not differ from placebo [6]. A single-dose trial in 14 men also favored citrate [7] | Absorption studies, small |
| Zinc citrate, gluconate, oxide | In 15 adults given 10 mg zinc without food, absorption was about 61% for citrate, 61% for gluconate and 50% for oxide; three people absorbed little or none from oxide [8] | Absorption study |
Is "organic" always better?
A 2026 review says organic forms such as citrate and glycinate are better tolerated and absorbed than inorganic salts such as oxide and chloride. [9] But a 2001 study found magnesium chloride absorbed about as well as organic salts, and its authors concluded that inorganic salts may sometimes equal organic ones. [5] A systematic review found magnesium chloride absorption ranged from 9.0% to 105.5% across studies, with differing designs. [10] So the organic-versus-inorganic rule is a rough pattern, not a guarantee. Magnesium oxide and similar poorly absorbed salts also act as osmotic laxatives at higher amounts. [9]What these studies cannot tell you
Better absorption is not the same as a better health outcome. The sources we read did not show that a better-absorbed form leads to better results for any health condition. Most trials are short and small. A systematic review of magnesium and potassium salts noted its studies included only young, healthy adults, which limits transfer to children, older adults, and people who are pregnant or breastfeeding. [10] Note also that the authors of the zinc review work for a supplement company. [2]Frequently asked questions
Which form of calcium is absorbed best?
A meta-analysis found citrate was absorbed about 22% to 27% better than carbonate in absorption studies [3], while a separate single-meal study found stomach acid did not affect calcium absorption. [4] Evidence is mixed. See the calcium carbonate vs citrate page.
Is magnesium oxide useless?
In absorption studies it was absorbed poorly, about 4% in one. [5] Poorly absorbed magnesium salts can act as osmotic laxatives. [9] Whether that suits a person is a question for a clinician or pharmacist.
Are chelated minerals better?
Results depend on the mineral and form. See Chelated Minerals and "Bisglycinate" Marketing Checked.
Why do two "500 mg" products hold different amounts of mineral?
One may list the weight of the compound and the other the elemental mineral. Calcium carbonate is 40% elemental calcium by weight while citrate is 21%. [1] Check the Supplement Facts panel.
Do natural sources like coral or oyster shell matter?
One consensus statement lists coral calcium as absorbed like carbonate, with no added benefit and a risk of contamination. [1] See Calcium Sources in Supplements.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety and quality
- Every form carries the mineral's own risks. The NHS notes that high doses of zinc reduce copper absorption [11], high doses of iron cause constipation, nausea and stomach pain, and very high iron doses can be fatal, particularly in children. [12]
- Quality varies. A review found calcium products made from dolomite, bone meal or natural carbonate historically had more lead, though the gap has narrowed, and that without third-party testing it is hard to know which outlier products to avoid. [13] See Quality Testing and Certifications.
- Forms differ in medicine interactions. See Medication Interactions and ask a pharmacist.
- Talk to a clinician or pharmacist before using mineral supplements if you are pregnant or breastfeeding, giving them to a child, have kidney or liver disease, take regular medicines, or have surgery planned. See Minerals, Supplements and Kidney Disease.
References
- Bhattacharya S, et al; Endocrine Society of India Skeletal Health Taskforce. Calcium Supplementation in the Indian Context: A Consensus Statement. Indian J Endocrinol Metab 2026. Read 2026-10-23.
- Devarshi PP, et al. Comparative Absorption and Bioavailability of Various Chemical Forms of Zinc in Humans: A Narrative Review. Nutrients 2024;16(24):4269 (authors are employees of a supplement company). Read 2026-10-23.
- Sakhaee K, et al. Meta-analysis of calcium bioavailability: a comparison of calcium citrate with calcium carbonate. Am J Ther 1999;6(6):313-321. Read 2026-10-23.
- Bo-Linn GW, et al. An evaluation of the importance of gastric acid secretion in the absorption of dietary calcium. J Clin Invest 1984;73(3):640-647. Read 2026-10-23.
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res 2001;14(4):257-262. Read 2026-10-23.
- Walker AF, et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res 2003;16(3):183-191. Read 2026-10-23.
- Werner T, et al. Assessment of bioavailability of Mg from Mg citrate and Mg oxide by measuring urinary excretion in Mg-saturated subjects. Magnes Res 2019;32(3):63-71. Read 2026-10-23.
- Wegmüller R, et al. Zinc absorption by young adults from supplemental zinc citrate is comparable with that from zinc gluconate and higher than from zinc oxide. J Nutr 2014;144(2):132-136. Read 2026-10-23.
- Sankararaman S, et al. Magnesium: Hype and Health Benefits. Curr Nutr Rep 2026. Read 2026-10-23.
- Merschmann R, et al. Bioavailability of Magnesium and Potassium Salts Used as Potential Substitutes for Sodium Chloride in Human Nutrition: A Review. Mol Nutr Food Res 2025. Read 2026-10-23.
- NHS (UK). Vitamins and minerals: Others (magnesium, zinc, sodium chloride and others). Page last reviewed 3 August 2020. Read 2026-10-23.
- NHS (UK). Vitamins and minerals: Iron. Page last reviewed 3 August 2020. Read 2026-10-23.
- White CM. Lead in Mineral or Multivitamin-Multimineral Products. Ann Pharmacother 2022;56(3):339-345. Read 2026-10-23.