Which type of magnesium is best?

There is no single answer. On absorption, citrate and bisglycinate sit at the top end (~25–30%). On GI tolerability, bisglycinate has the cleanest profile in the available trial data. On trial breadth, citrate has the longest comparator history. On elemental dose per gram, oxide is densest but absorbs poorly. 'Best' depends on whether the buyer prioritises absorption, tolerability, cost per elemental milligram, or trial coverage. The label should disclose elemental magnesium per serving — that number is the only fair cross-product metric.

Why does cheap magnesium oxide give me diarrhoea?

Magnesium oxide has a low absorption fraction — roughly 4–12% on average — which means the majority of the dose remains unabsorbed in the small bowel and colon. Magnesium ions in the gut lumen pull water by osmosis, producing a laxative effect proportional to the unabsorbed dose. This is the same mechanism that makes magnesium hydroxide and magnesium sulfate licensed osmotic laxatives. The effect is dose-dependent and varies between individuals. None of this is medical advice; it is the pharmacology brands rely on when formulating laxative versus general-mineral products.

Is magnesium threonate worth the premium?

Magnesium L-threonate, marketed as Magtein, is typically priced 5–10x per elemental milligram of magnesium versus citrate or bisglycinate. The clinical evidence base rests primarily on one well-known 2015 trial in older adults plus a small number of follow-on studies, several of which are manufacturer-funded. Independent replication on hard cognitive endpoints in healthy adults is limited as of 2026. The chemistry rationale (better blood-brain-barrier crossing) is real; the human outcome evidence is thin. Whether the premium is justified is a personal evidence-vs-cost decision, not a question with a generic answer.

Do I need to take magnesium with food?

Absorption pharmacology suggests organic chelates and salts (bisglycinate, citrate, malate) are absorbed similarly with or without food, while magnesium oxide is somewhat improved by food because gastric acid solubilises the otherwise-poorly-soluble oxide. Splitting larger doses across the day rather than taking a single bolus generally improves absorption fraction, because magnesium absorption has a saturable active-transport component that is more efficient at lower local concentrations. None of this is a dosing recommendation — it is the absorption pharmacology described in published pharmacokinetic studies.

Magnesium chloride vs sulfate — what's the difference?

Magnesium chloride is roughly 12% elemental magnesium and is found in topical sprays and oral liquids; oral chloride has a noticeably salty taste. Magnesium sulfate (Epsom salt) is around 10% elemental and is licensed as an oral and rectal laxative as well as the magnesium source in IV magnesium for emergency use. Topical or transdermal absorption claims for both forms are not well supported in human trials — published data on systemic uptake from a magnesium-chloride spray applied to intact skin is sparse and inconsistent. Both are inorganic salts with relatively low oral absorption fractions compared with citrate or chelated forms.

Can I tell elemental Mg from the label?

Yes — under EU and UK food regulations the supplement-facts panel must declare 'Magnesium' followed by the milligram amount and the % nutrient reference value. That number is elemental magnesium, not compound weight. The compound weight (e.g. '500 mg magnesium bisglycinate') is separately disclosed in the magnesium-source line. If a label emphasises the compound weight on the front and buries the elemental amount, it is usually because the elemental dose is small relative to the compound weight — common with low-percentage forms like L-threonate and malate.

Magnesium Types Compared: Oxide, Citrate, Glycinate, Malate, Threonate (2026)

Nutripedia Research Team8 April 2026

Magnesium is not one thing. The five forms on UK shelves differ in elemental percentage, absorption, GI tolerability, and the trial evidence behind them. We walk through the chemistry and what the research actually shows — without recommending anyone take a supplement.

Not medical advice

Nutripedia summarises published peer-reviewed research. This content is for informational purposes only and is not a substitute for professional medical advice. Product mentions are not endorsements.

Before You Read This

Nutripedia is a research librarian, not a doctor. Nothing on this page is medical advice. This article describes the chemistry and trial evidence behind different magnesium compound forms — it does not recommend that any reader take magnesium and does not interpret symptoms. Consult a UK GP, NHS pharmacist, or registered dietitian before starting any supplement, especially if you are pregnant, breastfeeding, on medication, or managing a chronic condition. The European Food Safety Authority sets a tolerable upper intake level for supplemental magnesium of 250 mg/day in adults, separate from dietary intake. The NHS reference nutrient intake for total magnesium is 270 mg/day for women and 300 mg/day for men aged 19–64.

Our research is based on 59 peer-reviewed studies. View the full evidence database

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Nutripedia is an educational resource. Content is sourced from peer-reviewed studies and does not constitute medical advice. Product mentions are not endorsements. Consult a healthcare professional before starting any supplement.

Reviewed by

Archie Roberts

Founder, Nutripedia — ALDR Ltd

This page summarises published research from PubMed, NHS, EFSA, and SACN. It does not constitute medical advice; consult a qualified healthcare professional before changing any supplement regimen.

Last reviewed: 08 Apr 2026Methodology