Magnesium Bisglycinate vs Magnesium Glycinate (UK)

You are standing in Boots holding two tubs. One says magnesium glycinate. The other says magnesium bisglycinate, costs four pounds more, and promises "superior absorption" on the front. Neither label explains the difference, and the shelf label is no help either.

So let's settle it. Magnesium bisglycinate and magnesium glycinate are the same compound. Same chemistry, same absorption, same effects. One name is the chemically precise one and the other is the shorter one marketing departments prefer. If you have been agonising over which tub to buy, you can stop.

Magnesium bisglycinate vs magnesium glycinate: what actually differs

Nothing differs. That is the honest answer, and it deserves a moment of your attention because a great deal of supplement marketing depends on you believing otherwise.

Glycine is an amino acid. Magnesium carries a double positive charge, so it needs two glycine molecules to balance it out. The resulting molecule is one magnesium atom held between two glycines, a structure chemists call a chelate. Because "bis" is the Latin prefix for two, the strictly correct name is magnesium bisglycinate.

"Magnesium glycinate" is that same molecule with a shorter name. Easier to say, easier to fit on a label, easier to search for. That is the whole story.

You will find pages online claiming that glycinate has one glycine and bisglycinate has two, and that this makes bisglycinate better absorbed. A magnesium ion bound to a single glycine would carry a leftover positive charge and would not be a stable, fully reacted chelate. Manufacturers are not selling two different molecules under two different names at two different prices. They are selling one molecule under whichever name their marketing team chose.

Does bisglycinate really absorb better?

The claims get slippery here, so look at the evidence rather than the label copy.

The study almost everyone points to when arguing that glycinate chelates are superior is a 1994 trial by Schuette and colleagues, published in the Journal of Parenteral and Enteral Nutrition. It gets cited constantly. It is also routinely misrepresented.

The trial was a double-blind crossover study in twelve patients who had undergone ileal resection, meaning people missing part of their small intestine who absorb magnesium poorly by definition. Each received a 100mg dose of labelled magnesium diglycinate or magnesium oxide. Across the group as a whole, absorption was 23.5% for the chelate and 22.8% for the oxide. That is not a meaningful difference. The chelate only pulled ahead in the four patients whose absorption of magnesium oxide was worst, where it reached 23.5% against 11.8%.

So the headline finding of the most-cited chelate study is that in most of the participants, it made no difference at all. It helped a small subgroup who had significant bowel surgery, and it was better tolerated by everyone. Useful, but a long way from "superior absorption" printed on a tub in Boots.

There is no published head-to-head trial showing bisglycinate outperforming glycinate, because there could not be one. They are the same substance.

How bisglycinate compares with the other forms

The bisglycinate versus glycinate question is a distraction. The comparison that changes your experience is bisglycinate against the other magnesium salts on the shelf, because those differ enormously in how much elemental magnesium they carry and how they treat your bowels.

Magnesium oxide is the interesting one. It carries by far the most magnesium per gram, which is why it appears in so many cheap products, yet very little of it crosses into your bloodstream. Most of it stays in the gut and pulls water in behind it. That is the mechanism, not a side effect.

Citrate sits in the middle. It absorbs reasonably well and it moves your bowels, which makes it a decent choice if constipation is the reason you are shopping and a poor one if it isn't.

Bisglycinate absorbs well and does almost nothing to your bowels. That combination is why it has become the default recommendation for people who want to raise their magnesium intake and carry on with their day.

The number that actually matters: elemental magnesium

Most people buying magnesium compare the wrong figure. The large number on the front of the tub is usually the weight of the whole compound, and magnesium is only a small fraction of it. The rest is glycine.

Magnesium bisglycinate is roughly 10 to 14% elemental magnesium by weight. The arithmetic looks like this:

That last row is the one to look for. A well-labelled product states the elemental magnesium separately, usually as "provides 100mg elemental magnesium" or "Magnesium (as magnesium bisglycinate) 100mg". If a label only gives you the compound weight, you are being asked to do maths the manufacturer could have done for you.

This is also why two products can look wildly different in strength and deliver almost the same dose. A 1,000mg bisglycinate capsule and a 130mg elemental magnesium capsule may well be the same thing described two ways.

Buffered blends: the difference that does cost you

Some products sold as "magnesium glycinate" are what the industry calls buffered. The manufacturer blends genuine magnesium bisglycinate with cheap magnesium oxide to raise the elemental magnesium figure without raising the production cost. The tub still says glycinate. You may be getting a substantial share of your dose as oxide, which barely absorbs and is far more likely to loosen your bowels.

Spotting it takes ten seconds. Read the full ingredients list rather than the front of the pack. If magnesium oxide appears anywhere in it, or you see the phrase "magnesium bisglycinate buffered", the product is a blend. Products that say "fully reacted" or "fully chelated" and list magnesium bisglycinate on its own are what they claim to be.

Choosing between "glycinate" and "bisglycinate" on the front of two tubs is a decision with no consequences. Choosing between a fully reacted chelate and an oxide blend is a decision with several.

How much magnesium do you actually need?

The NHS sets the UK requirement at 300mg a day for men and 270mg a day for women aged 19 to 64. Most of that should come from food. Magnesium turns up in spinach, nuts, wholemeal bread, pulses and seeds, and a varied diet usually covers it.

For supplements, the NHS guidance is straightforward: 400mg or less a day from supplements is unlikely to cause harm, and doses above 400mg taken over a short period can cause diarrhoea. That upper figure refers to supplemental magnesium, not the magnesium in your dinner.

In practice, most people who supplement sensibly land somewhere between 100mg and 300mg of elemental magnesium a day. Splitting it across two doses tends to sit better than taking it all at once.

There is no case for taking more. Magnesium is not a nutrient where extra buys you extra benefit, and the main consequence of overdoing it is spending your morning in the bathroom.

What about magnesium for sleep?

This is the reason a lot of people start taking it, so it deserves a straight answer. Glycine has mild sedative properties of its own, independent of the magnesium it is carrying, which is the usual argument for taking bisglycinate in the evening rather than any other form.

The evidence in people who are not magnesium deficient is thin. Small trials in older adults with insomnia have shown modest improvements, but they were small, and correcting a deficiency is not the same as improving sleep in someone whose levels are already fine. If you want to try it, 100 to 200mg of elemental magnesium an hour or so before bed is a reasonable starting point, and you will know within a fortnight whether it does anything for you. If it doesn't, the answer is not a higher dose.

Magnesium when you are eating a lot less

This is the situation we see most often at heySlim, and it is absent from the supplement-brand articles that dominate this search.

If you are taking a GLP-1 medication like Mounjaro or Wegovy, your food intake may have dropped substantially. Smaller portions mean less of everything, minerals included. Appetite loss also makes people selective, and the foods that tend to go first are the bulky plant foods carrying most of the magnesium.

Three things usually bring people to magnesium in this situation: constipation, night-time muscle cramps, and disrupted sleep in the early weeks of treatment.

Magnesium can help with some of that, but be clear about what it will and will not do. For constipation on a GLP-1, the NHS advises starting with fibre, fluids and movement, which we cover in more detail in our guide to constipation on Mounjaro. Magnesium bisglycinate is a poor choice as a laxative anyway, precisely because it is gentle on the bowel. Citrate is the form that shifts things.

Cramps and sleep are more plausible territory for a glycinate chelate. If you are eating very little and cramping regularly, raise it with your prescriber rather than self-treating indefinitely, because cramps can also point to dehydration or low potassium. Our notes on hydration while taking Mounjaro and on protecting muscle during weight loss cover the ground around it.

Who should be careful with magnesium supplements

Magnesium is cleared by the kidneys, which shapes most of the cautions around it.

If your kidney function is reduced, magnesium can build up to dangerous levels. Anyone with chronic kidney disease should not take magnesium supplements without their kidney team's agreement. The same applies with heart block or myasthenia gravis, where raised magnesium can worsen the underlying condition.

Magnesium also binds to several medicines in the gut and stops them being absorbed properly. The interactions to know about:

Pregnancy and breastfeeding call for a conversation with your midwife or GP rather than a decision made in a shop aisle. And if you develop persistent diarrhoea, unusual muscle weakness, drowsiness or a slow heartbeat after starting magnesium, stop and get medical advice. Suspected side effects from any supplement or medicine can be reported through the MHRA Yellow Card scheme.

So which one should you buy?

Ignore the glycinate versus bisglycinate framing. It is a naming convention, not a product difference, and any brand charging a premium for the longer word is charging you for a word.

Check four things instead. The elemental magnesium per capsule, stated plainly on the label. Whether the ingredients list contains magnesium oxide, which tells you it is a buffered blend rather than a fully reacted chelate. Whether the dose gets you to a sensible daily total once you have worked out how many capsules you would actually swallow. And whether the product carries third-party testing, which is not a legal requirement for food supplements in the UK and therefore tells you something about the manufacturer.

A fully reacted magnesium bisglycinate delivering 100mg of elemental magnesium per capsule, from a brand that publishes its testing, is a good product regardless of which of the two names it prints on the front.

For a broader view of which supplements earn their place during weight loss and which don't, we have written about that in our guide to weight loss supplements and the evidence behind them.

This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional before starting any treatment.