In brief. Magnesium is always bound to a carrier (a salt or an amino acid), and that carrier determines the percentage of elemental magnesium, solubility, absorption and tolerability, not what magnesium does in the body. In human studies, readily soluble organic forms (citrate, gluconate, bisglycinate, malate) are generally absorbed more effectively than poorly soluble magnesium oxide, much of which remains in the intestine. A blend containing several forms, such as our Magnesium 7-in-1, brings together different carriers of the same mineral in one product. This article is for information only and does not constitute medical advice.
Why the form of magnesium matters
Magnesium is an essential mineral that the body cannot produce itself. It is the fourth most abundant cation in the body and a cofactor in more than 300 enzymatic reactions, including all reactions involving ATP, energy production, protein synthesis, nerve transmission and muscle contraction. In supplements, magnesium is never present as a pure metal but is always bound to a carrier. This carrier, such as citric acid, glycine or oxide, determines its chemical form.
The choice of form is not a minor detail. Two things vary considerably between forms. The first is the percentage of elemental magnesium: the amount of pure magnesium within the total weight of the compound. Magnesium oxide contains approximately 60% elemental magnesium, magnesium citrate approximately 16% and magnesium gluconate only approximately 5%. The second is absorption: how much of that magnesium actually enters the bloodstream. Importantly, a higher percentage of elemental magnesium does not automatically mean that more is absorbed. In fact, the water solubility of the salt matters more for absorption than its elemental content.
What magnesium does
The following effects are officially authorised EFSA claims for the mineral magnesium itself. They apply to magnesium regardless of its form, provided that the product is at least a source of magnesium:
- Magnesium contributes to normal muscle function
- Magnesium contributes to the normal functioning of the nervous system
- Magnesium contributes to normal energy-yielding metabolism
- Magnesium contributes to the reduction of tiredness and fatigue
- Magnesium contributes to normal psychological function
- Magnesium contributes to electrolyte balance
- Magnesium contributes to normal protein synthesis
- Magnesium contributes to the maintenance of normal bones
- Magnesium contributes to the maintenance of normal teeth
- Magnesium has a role in the process of cell division
These claims are generic to the mineral and are not specific to any form. No form may be presented as having a greater effect on a particular aspect of health, and magnesium must not be presented as a remedy for disease. The differences in absorption between the forms below are factual, substantiated product information and are separate from these authorised claims.
Comparison table: forms of magnesium
The overview below is for information and is based on published research. The elemental percentages are approximations for commonly used (hydrated) salt forms and vary according to the degree of hydration.
| Form | Elemental Mg | Absorption | Suitable for | Notes |
|---|---|---|---|---|
| Magnesium oxide (MgO) | ~60% | Low | High elemental content; has a laxative effect at higher doses | Inorganic, poorly soluble; remains largely in the intestine |
| Magnesium citrate | ~16% | High (compared with oxide) | Common general-purpose form | Organic, readily soluble; higher doses have a mild laxative effect |
| Magnesium gluconate | ~5% | High (organic salt) | Gentle, well-tolerated supplementation | Lowest elemental content; low intestinal burden |
| Magnesium bisglycinate (glycinate) | ~14% | High (chelate) | Sensitive stomach | Amino acid chelate, gentle on the intestine; buffered grades have a higher content |
| Magnesium malate | ~12-15% | Good | Often taken during the day | Malate is an intermediate in the citric acid cycle |
| Magnesium L-threonate (Magtein) | ~7-8% | Comparable to other organic salts | Limited research; no form-specific EU claim | Low elemental content; a large dose in grams is required |
| Magnesium taurate | ~8-9% | Good (chelate, limited research) | Limited research; no form-specific EU claim | Amino acid chelate (taurine); no human head-to-head data |
| Magnesium lactate | ~12% | Good | Daily supplementation; often used in extended-release tablets | Soluble organic salt; low osmotic load |
| Magnesium phosphate (E343) | ~14-24% | Low to moderate | Mineral fortification of foods; additive | Inorganic, poorly soluble; also provides phosphorus |
| Magnesium ascorbate | ~6% | Fair (soluble salt) | Combines vitamin C and magnesium in one salt | Low elemental content; less acidic than ascorbic acid |
| Magnesium orotate | ~7% | Moderate | Limited research; no form-specific EU claim | Dissociates less readily than citrate; more material per dose |
| Magnesium sulphate (Epsom salt) | ~10% | Low when taken orally | No form-specific EU claim | Readily soluble but poorly absorbed; not a form for daily supplementation |
| Magnesium chloride | ~12% | Good (soluble) | General supplementation; topical magnesium oil | Dissociates readily; may have a mild laxative effect and taste salty |
A brief explanation of the forms
Magnesium oxide (MgO) has the highest percentage of elemental magnesium but is inorganic and very poorly soluble. In a 60-day randomised, double-blind, placebo-controlled study, oxide did not differ from placebo in serum and urinary magnesium, while citrate increased both [Walker 2003]. Because much of the dose remains in the intestine, oxide acts osmotically and can have a laxative effect at higher doses. The latter is scientific context, not a claim about managing a medical condition.
Magnesium citrate is a readily soluble organic salt and was consistently absorbed more effectively in comparative human studies. In a single-dose cross-over study, citrate resulted in higher urinary magnesium excretion and higher serum levels than oxide [Kappeler 2017]. At high doses, citrate has a mild osmotic effect.
Magnesium gluconate has the lowest elemental content of the commonly used salts but high relative bioavailability. In a controlled stable-isotope comparison of ten salts, gluconate showed the greatest absorption, although this was in an animal model [Coudray 2005]. It is generally well tolerated and has little laxative effect.
Magnesium bisglycinate (glycinate) is an amino acid chelate that is well absorbed in the small intestine and gentle on the intestine. The study did not show a consistent effect on stress or mood. Pure (unbuffered) bisglycinate contains approximately 14% elemental magnesium; buffered grades have a higher content because oxide has been added.
Magnesium malate is a soluble organic acid salt that is well tolerated. The malate anion is an intermediate in the citric acid cycle, which explains its positioning in relation to energy and muscles.
Magnesium L-threonate (Magtein) has a few dedicated human studies, mostly measuring cognitive test scores. A small number of human studies have been conducted on this form. The studies are small, short and largely funded by the manufacturer; there is no form-specific EU claim.
Magnesium taurate combines magnesium with the amino acid taurine. There is almost no direct evidence from human RCTs specifically on taurate. As a chelate, good tolerability is a reasonable expectation, not an established ranking.
Magnesium lactate is a readily soluble organic salt. A controlled study of an extended-release caplet measured an absolute bioavailability of 20.26% when fasting and 12.49% with food [Ekwaru 2018]; food reduced absorption. It is gentle on the intestine.
Magnesium chloride is readily soluble for an inorganic salt. Firoz and Graber found that the soluble salts were absorbed approximately three to four times more effectively than poorly soluble oxide [Firoz 2001]. Magnesium sulphate (Epsom salt) is poorly absorbed when taken orally and therefore acts as an osmotic laxative [Bashir 1993]. Magnesium orotate provides little elemental magnesium per dose. Magnesium phosphate (E343) and magnesium ascorbate are mainly used as fortifying or buffering sources and provide little elemental magnesium per gram.
Why we choose a combination of forms
Each form involves trade-offs. Inorganic oxide has a high elemental content but is poorly soluble and has a laxative effect. Organic salts such as citrate, bisglycinate and malate are more soluble, better tolerated and increase serum and urinary magnesium more reliably. Specialised forms are studied for specific contexts of use. No single form scores highly on every measure.
That is why we choose a combination. Our Magnesium 7-in-1 brings together seven forms: malate, phosphate, citrate, bisglycinate, gluconate, lactate and ascorbate, providing a total of 251 mg of elemental magnesium per daily serving of two capsules. Importantly, a blend does not change what magnesium does in the body, and the %NRV on the label is always calculated using the total elemental magnesium, not the salt weight. The EFSA claims remain generic to the mineral.
How to choose a form
- Looking for a readily soluble form? In human studies, readily soluble organic forms such as citrate, gluconate, bisglycinate and malate are absorbed more effectively than oxide.
- Do you have a sensitive stomach? Chelated forms such as bisglycinate and gentle salts such as gluconate are milder on the intestine than oxide or high doses of citrate.
- Look at the elemental magnesium per serving, not just the salt weight, and choose a label that lists every form and the amount of elemental magnesium. Our Magnesium 7 in 1 provides 251 mg of elemental magnesium per daily serving from seven forms.
Dosage and safety
The EU nutrient reference value (NRV) for magnesium is 375 mg. A product must provide at least 15% of the NRV (56.25 mg per serving) to be called a source of magnesium and to carry a health claim. According to the EU, the tolerable upper intake level for supplemental magnesium (from supplements and fortified foods) is 250 mg per day, as established by the Scientific Committee on Food (SCF, 2001) based on a mild laxative effect. This 250 mg is in addition to magnesium from ordinary food and water.
Magnesium from food and supplements is generally well tolerated by healthy adults at normal doses. The most common side effect of higher doses is a laxative effect (loose stools or diarrhoea), which is strongest with oxide and sulphate. Absorption improves when doses are divided because fractional absorption decreases with higher single doses. People with impaired kidney function should exercise caution. Do not use magnesium above the recommended dose without advice. If you are pregnant or breastfeeding, take medication (including certain antibiotics or medication for the heart or blood pressure), or have a kidney condition, consult a doctor or pharmacist first. Supplements are not a substitute for a varied diet or medical care.
Frequently asked questions
Which form of magnesium is absorbed most effectively?
In human studies, readily soluble organic forms such as citrate, gluconate, bisglycinate and malate are generally absorbed more effectively than poorly soluble magnesium oxide. The water solubility of the salt matters more for absorption than the percentage of elemental magnesium. Absorption varies by person and dose, and there is no single form that is most suitable for everyone.
Why does magnesium oxide contain a lot of elemental magnesium but have low absorption?
Oxide contains approximately 60% elemental magnesium, the highest level among the commonly used forms, but it is inorganic and very poorly soluble. As a result, much of the dose remains in the intestine instead of entering the bloodstream. A higher elemental percentage therefore does not automatically mean more magnesium is absorbed.
Why would a product contain several forms of magnesium?
To combine different properties: elemental content, solubility, absorption and tolerability. A blend brings together different carriers of the same mineral to provide a broader profile than a single form. It does not change what magnesium does in the body.
How much magnesium can I take from supplements each day?
The EU upper intake level for supplemental magnesium from supplements and fortified foods is 250 mg per day, in addition to magnesium from ordinary food. The nutrient reference value (NRV) for total magnesium is 375 mg. Stay within the dose on the label and consult a doctor or pharmacist if you are unsure.
Can magnesium have a laxative effect?
Yes, particularly at higher doses. The effect depends on the form: it is strongest with oxide and sulphate, present with high doses of citrate, and least pronounced with gentle forms such as gluconate and bisglycinate. Dividing doses and staying within the recommended amount helps to limit this effect.
Is magnesium bisglycinate gentle on the stomach?
Bisglycinate binds magnesium to glycine and is generally well tolerated by the stomach. Magnesium contributes to the normal functioning of the nervous system and to normal psychological function. These are the authorised wordings; magnesium is not a remedy for sleep or mood disorders.
Sources
- Walker AF, et al. Mg citrate found more bioavailable than other Mg preparations in a randomised, double-blind study. Magnes Res. 2003. PubMed 14596323
- Kappeler D, et al. Higher bioavailability of magnesium citrate as compared to magnesium oxide. BMC Nutr. 2017. PubMed 28097030
- Coudray C, et al. Study of magnesium bioavailability from ten organic and inorganic Mg salts using a stable isotope. Magnes Res. 2005. PubMed 16548135
- Mori H, et al. Magnesium oxide in constipation: a randomized, placebo-controlled trial. 2019. PubMed 31587548
- Magnesium bisglycinate and sleep: randomized placebo-controlled trial. PMC12412596
- Fibromyalgia and magnesium/malic acid: review of evidence. PubMed 31150373
- Liu G, et al. MMFS-01 (magnesium L-threonate) and cognition in older adults. J Alzheimers Dis. 2016. PubMed 26519439
- Magnesium L-threonate (Magtein), cognition and sleep RCT. PubMed 41601871
- McCarty MF. Complementary vascular-protective actions of magnesium and taurine. 1996. PubMed 8692051
- Ekwaru JP, et al. Absolute bioavailability and food effect of magnesium L-lactate dihydrate. Drug Dev Ind Pharm. 2018. PubMed 29659311
- Firoz M, Graber M. Bioavailability of US commercial magnesium preparations. Magnes Res. 2001. PubMed 11794633
- Bashir Y, et al. Absorption of orally administered magnesium sulfate in man. Clin Sci. 1993. PubMed 8222512
- Stepura OB, Martynow AI. Magnesium orotate in severe congestive heart failure (MACH). Int J Cardiol. 2009. PubMed 18281113
- EU Register of Nutrition and Health Claims; Regulation (EU) No 432/2012 and Regulation (EC) No 1924/2006.
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for magnesium. EFSA Journal. 2015;13(7):4186.
- SCF. Opinion on the Tolerable Upper Intake Level of Magnesium. 2001 (supplemental UL 250 mg/day).
Written and reviewed by the NOTFORTOMORROW editorial team. How we conduct research: we base factual statements on the official EU Register of Nutrition and Health Claims and peer-reviewed sources, cite our sources and date our reviews. This article is for information only and does not constitute medical advice; it is not intended for the diagnosis or management of disease. Consult a qualified professional about your circumstances. Last reviewed: 2026-06-22.
