Magnesium Bisglycinate

Magnesium bisglycinate — also sold as magnesium glycinate or magnesium diglycinate — is magnesium bound to two molecules of glycine, an amino acid. "Bis" means two.

That binding does two things. It changes how the compound behaves in the gut, which is the part every supplement brand talks about. And it accounts for roughly six-sevenths of the compound's weight, which is the part almost nobody explains — and the part that decides how much magnesium you actually get.

A mineral supplement is sold as a compound. The mineral itself is only part of what's in the capsule.

Magnesium bisglycinate, in its pure anhydrous form, is about 14% magnesium by mass. In that ideal anhydrous compound, the remaining ~86% is glycine.

So a bottle whose front panel reads "Magnesium Bisglycinate 500 mg" is describing the compound. The amount of magnesium in that dose is closer to 70 mg. Both numbers are true. Only one of them is the dose.

Two checks, ten seconds each:

1. Read the Supplement Facts panel. US labeling practice is to declare minerals as the element. The line reading Magnesium … mg … % DV is elemental magnesium — the number that counts toward your intake.
2. If the front panel gives you a compound weight, do the division. Elemental figure ÷ compound figure. For fully reacted bisglycinate, you should land near 14%.

This is the most useful thing on this page, and it's chemistry rather than opinion.

Magnesium bisglycinate has a fixed formula: one magnesium atom, two glycinate groups. Run the atomic masses and magnesium works out to about 14.1% of the molecule. That is the theoretical maximum for a pure, fully reacted, anhydrous material — not a target, a ceiling.

What to do with that number: if a material is presented as pure, fully reacted bisglycinate but assays materially above the theoretical value, that's a question worth asking. There are legitimate answers — assays are sometimes reported on a dried basis rather than as-is, and hydration state moves the figure — so the right response is to ask the supplier how the number was derived, not to assume the worst. But the question is worth asking, because one possible answer is that some of the magnesium is coming from a cheaper inorganic source, either blended in or left over from an incomplete reaction. The trade calls the complete version fully reacted; the blended version is often sold as buffered.

We ran this check while sourcing our first batch. Four candidate materials came back:

Material D would have been the easiest way to hit our label number. We didn't chase it. That isn't a finding about that supplier — we didn't ask how the assay was run, and there may be a perfectly good answer. It's a description of how we chose: when a number sits outside what the chemistry allows for the pure compound, we go elsewhere rather than build a product on an explanation we'd have to take on trust.

Before any raw material was bought, we wrote a floor of 14.0% elemental into the purchase agreement with our manufacturer, and required certificates of analysis to carry measured values rather than the word "conforms."

Here we have to be careful, because this is where the category stops being honest — and where we found something that doesn't flatter us either.

The claim you've read everywhere. Supplement pages routinely state that magnesium glycinate absorbs at "80–90%" while oxide manages "4%," implying a fourfold to twentyfold difference. Those two figures come from different studies, in different populations, measured by different methods. Dividing one by the other is not a finding; it's arithmetic performed on unrelated numbers.

What we found when we went looking. We read the studies most commonly cited for this claim. Four of them, described honestly:

1994 — the head-to-head trial. Published in the Journal of Parenteral and Enteral Nutrition: a double-blind randomized crossover study using isotope-labeled magnesium at a 100 mg dose, in twelve patients who had undergone ileal resection. Across the group, absorption was low and statistically indistinguishable between the two forms — 23.5% for the chelate against 22.8% for the oxide.

Its more interesting findings were these. In the four patients whose absorption on oxide was most impaired, the chelate did substantially better (23.5% against 11.8%) — a subgroup of four, in a specific clinical population. Peak absorption came earlier with the chelate, by around three hours on average. Every patient in the trial tolerated the chelate better. And the authors concluded that their data support the suggestion that part of the dose is absorbed intact, probably through a dipeptide transport route rather than as a free mineral ion.

2007 — the trial behind the eight-hour serum claim. The crossover study often cited for bisglycinate producing the highest serum magnesium — ahead of dimagnesium malate, with oxide last — is a four-arm trial in 14 healthy volunteers, single 150 mg doses, serum sampled out to 24 hours. We know it from a European regulatory evaluation that describes it as unpublished; we have not been able to read the study itself, and we'd treat any figure attributed to it with that in mind.

2024 — a peer-reviewed trial that went the other way. Forty healthy adults were put on a low-magnesium diet for a week, then had plasma magnesium measured at 1, 4 and 6 hours after single doses of four different products. Oxide and citrate each produced a significant rise at one time point. Bisglycinate produced no significant rise at any time point. The study was designed to evaluate a microencapsulated magnesium ingredient, which is worth knowing when reading it.

The cell-culture figures. The widely quoted "3–6× better absorbed than oxide" comes from cell-culture transport assays, not human trials, and was published by the company that makes the raw material.

Read together: the most independent of these — the 1994 trial — found no absorption difference at all, and the two that report a difference each come from a party with a product in the comparison. We're not in a position to tell you which form absorbs better. Neither, on this evidence, is anyone else.

Four things we are not going to tell you, because we couldn't support them.

That bisglycinate is dramatically better absorbed. No independent human trial establishes it. The best-documented comparison found the two forms equivalent.

That bisglycinate is proven gentler in healthy adults. The 1994 trial found better tolerance — in twelve patients who had had bowel surgery. We found no peer-reviewed head-to-head tolerability comparison in healthy people. The mechanism is plausible and appears as a rationale in study protocols; that is not the same as a result.

That magnesium supplements routinely upset your stomach in the first place. This one cuts against our own sales pitch, so it's worth stating plainly. A 2023 perspective in Advances in Nutrition argued that the tolerable upper intake level — set in 1997, with diarrhea as the limiting factor — should be re-examined. Its authors reviewed intervention studies from 1997 to 2022 that used single-ingredient magnesium products and recorded diarrhea as a pre-specified adverse event, plus the FDA's adverse-event reporting database. Of ten qualifying studies — five meta-analyses and five randomized trials, at intakes from 128 to 1,200 mg per day — seven found no significant difference in diarrhea between supplement and control groups. In the FDA database, restricted to single-ingredient magnesium as the suspect product, there were forty attributable gastrointestinal cases, only about a third of which mentioned diarrhea.

Two things to hold alongside that. It's a perspective arguing a position, not a consensus statement. And its authors are affiliated with an organization dedicated to magnesium education and research — which we mention for the same reason we mentioned the funding behind the absorption studies, even though this one happens to support a point we're making.

So the familiar story — magnesium wrecks your gut, therefore buy the gentle expensive one — is weaker than the category implies. Unabsorbed magnesium does draw water into the intestine, and that laxative effect is real for some people, some forms and some doses. It is not the universal experience it's sold as.

That any of this makes magnesium a treatment. Magnesium contributes to normal muscle and nerve function and participates in a large number of enzymatic reactions.* That is a nutritional statement, not a therapeutic one.

Supplemental magnesium is commonly taken at roughly 100–400 mg of elemental magnesium per day. The tolerable upper intake level for supplemental magnesium in adults is 350 mg per day — a limit on supplements and medications, not on magnesium from food, which healthy kidneys clear without difficulty.

TAO Daily — Sleep & Calm provides 200 mg of elemental magnesium per two-capsule serving: 48% of the Daily Value, and comfortably under the upper level.

Commonly taken in the evening, with or without food.

If you're pregnant, nursing, taking medication, or have reduced kidney function, talk to your healthcare provider before supplementing magnesium.

Not for absorption. We can't evidence that, so we don't claim it.

We use fully reacted magnesium bisglycinate for three reasons we can stand behind:

It's checkable. The 14.1% ceiling gives us — and you — an arithmetic test for whether a material is plausibly what it claims to be. Very few supplement ingredients come with one. We used it while sourcing.

Glycine is a real ingredient, not a filler. The 86% of the compound that isn't magnesium is an amino acid, not a bulking agent chosen to make a capsule look full.

It fits. At 200 mg elemental, the compound weight lands inside a size 00 vegetable capsule alongside 200 mg of L-theanine.