Magnesium glycinate vs citrate, oxide and marine: which form, and when

Walk down the supplement aisle in any New Zealand pharmacy and you'll find half a dozen magnesium products that look interchangeable and aren't. Glycinate, bisglycinate, citrate, oxide, marine, chelate, "complex" — same mineral, different chemistry, quite different prices.

This is a plain comparison of the forms you'll actually encounter here, what the research says about how well each is absorbed, and — the part most articles skip — how much the form really matters for a generally healthy adult.

The short version

Organic forms such as glycinate and citrate are better absorbed than inorganic ones such as oxide. But for a healthy adult who isn't already running low, the evidence suggests most forms will do the job of maintaining normal levels. The form matters more if you're older, unwell, or already short — and it matters a lot for how your gut feels about it.

In other words: don't pay a large premium for a form you don't need, and don't buy the cheapest one if it sends you running to the bathroom.

First, the bit nobody explains: elemental magnesium

Magnesium is never sold on its own. It's always bound to something else — glycine, citric acid, oxygen — and that compound is what's in the capsule. The elemental magnesium figure is how much actual magnesium you get once you discount the carrier.

This is where labels get slippery. "1,000mg magnesium glycinate" is not 1,000mg of magnesium; it might be nearer 140mg elemental. A label showing "magnesium (as glycinate) 100mg" is telling you the useful number. If a product only advertises the big compound figure, it isn't necessarily dishonest, but you're not being told what you need to know.

Always compare products on elemental magnesium, not compound weight.

The forms, compared

Magnesium glycinate (and bisglycinate — the same thing)

Magnesium bound to glycine, an amino acid. Well absorbed and generally the gentlest on digestion, which is why it dominates the sleep and evening-supplement category.

On the naming: bisglycinate and glycinate are the same compound. "Bis" simply means two glycine molecules are attached to one magnesium. Some brands use the more technical name, some don't. Do not pay more for "bisglycinate" on the assumption it's a superior form — it isn't a different ingredient.

Downside: because glycine is a relatively heavy carrier, the elemental magnesium percentage is low, so capsules tend to be large or you need several. It's also usually the most expensive per unit of elemental magnesium.

Magnesium citrate

Magnesium bound to citric acid. Well absorbed and considerably cheaper than glycinate. Its well-known catch is a laxative effect at higher doses — which is precisely why citrate is also sold as a constipation remedy. Fine at modest doses for many people; a poor choice for a large dose late at night.

Magnesium oxide

The cheap, high-street default, and the one to be most careful with. It carries a high percentage of elemental magnesium by weight, which lets labels show an impressive number — but it is poorly absorbed, so a good deal of that passes straight through. It's also the form most likely to cause digestive upset. If a product is unusually cheap and the dose looks unusually high, check whether it's oxide.

Marine or sea-sourced magnesium

Extracted from seawater, and typically a mix of magnesium compounds rather than a single one. Common in New Zealand formulas, often paired with botanicals. Absorption sits in a reasonable middle range; the appeal is usually the blend and the source story rather than the chemistry of the magnesium itself.

Magnesium chloride, malate, taurate, threonate

All exist, all are organic forms with decent absorption, all are far less common on New Zealand shelves. Threonate in particular is heavily marketed on the basis of brain-related research that is, at this stage, thin in humans. Treat confident claims about it with the scepticism you'd apply to any early-stage science sold at a premium.

What the research actually shows

A 2021 systematic review of magnesium supplement bioavailability, published in the journal Nutrition, screened 433 studies and analysed 14 that directly compared forms. Its findings were refreshingly plain:

• Inorganic formulations (oxide) appear less bioavailable than organic ones (citrate, glycinate and similar).
• The percentage absorbed is dose-dependent — you absorb a smaller proportion of a large dose than a small one, which is an argument for splitting rather than stacking.
• All the supplement forms studied could maintain normal magnesium levels in healthy people without a pre-existing shortfall — but the reviewers were explicit that this could not be assumed for older people, or for those with illness or already-low levels.

That last point is the one worth carrying into the shop. If you're generally well and simply topping up, the form is a smaller decision than the marketing suggests. If you're older, unwell, or genuinely short, choosing a better-absorbed, gentler form is a more reasonable investment. Pardo et al., Nutrition, 2021 (via PubMed).

How much magnesium — the New Zealand numbers

The Nutrient Reference Values for Australia and New Zealand set two figures worth knowing, and they are not the same thing:

• Recommended Dietary Intake, adults 51–70: 420mg/day for men, 320mg/day for women. This is total intake, food included.
• Upper level from supplements: 350mg/day for adults. This applies to supplemental magnesium only — magnesium in food isn't capped, as it hasn't been shown to produce toxic effects at dietary levels.

Plenty of products on sale exceed that supplemental upper level in a single serve. Check the elemental figure, and remember that multi-minerals and some antacids contain magnesium too — it's easy to stack past the limit without meaning to. The first sign of too much is loose stools.

Choosing, in practice

• Sensitive stomach, or taking it in the evening: glycinate is the gentlest, and worth the premium if oxide or citrate has upset you before.
• Cost-conscious and tolerating it fine: citrate gives you well-absorbed magnesium for considerably less.
• You also want help with constipation: citrate's side effect becomes the point.
• You want a formula rather than a single ingredient: marine magnesium blends are the common New Zealand route.
• You're older, unwell, or have been told your levels are low: this is where form genuinely matters — and it's a conversation to have with your GP or pharmacist, not a decision to make from a label.

A note on what magnesium won't do

Whichever form you choose, magnesium is a nutrient, not a sedative or a treatment. It won't knock you out, and no form of it fixes sleep that's being disrupted by pain, hot flushes, alcohol or an irregular schedule. Magnesium can interact with some medicines, particularly where blood pressure or kidney function is involved — check with your GP or pharmacist if you take anything regularly.

Where Lifeguard fits

For transparency: Lifeguard Sleep uses marine magnesium, alongside tart cherry, lemon balm and saffron, in an NZ-made capsule for adults 45+. It is not a glycinate product, and if glycinate is specifically what you're after, other brands will serve you better — we'd rather tell you that than sell you the wrong thing. What Lifeguard Sleep offers is a considered evening formula with no melatonin and nothing habit-forming. See Lifeguard Sleep →

If your question is really about sleep rather than chemistry, start here instead: Best magnesium for sleep in NZ: type, timing and dose.

Frequently asked questions

Is magnesium bisglycinate the same as magnesium glycinate?

Yes. They are the same compound — "bis" just indicates two glycine molecules bound to one magnesium. Brands use the names interchangeably, so don't pay a premium for one over the other.

Which magnesium is best absorbed?

Organic forms such as glycinate and citrate are better absorbed than inorganic magnesium oxide. A 2021 systematic review in Nutrition found inorganic forms appear less bioavailable than organic ones, and that absorption is dose-dependent.

What's the difference between magnesium glycinate and citrate?

Both are well absorbed. Glycinate is gentler on digestion and usually more expensive per unit of elemental magnesium. Citrate is cheaper but has a laxative effect at higher doses — which is why it's also sold for constipation.

Is magnesium oxide any good?

It's the cheapest and carries a high percentage of elemental magnesium by weight, but it's poorly absorbed and the most likely to cause digestive upset. If a product is very cheap with a very high dose, check whether it's oxide.

How much magnesium can I take from a supplement?

The Nutrient Reference Values for Australia and New Zealand set an upper level of 350mg per day from supplements for adults. That's separate from total daily intake, which includes food — 420mg for men and 320mg for women aged 51 to 70.

What is elemental magnesium and why does it matter?

Elemental magnesium is the amount of actual magnesium in a dose, after discounting the compound it's bound to. "1,000mg magnesium glycinate" may contain far less elemental magnesium than the headline suggests. Always compare products on the elemental figure.

Is marine magnesium better than glycinate?

Not better or worse — different. Marine magnesium is drawn from seawater and is usually a mix of compounds, with absorption in a reasonable middle range. Glycinate is a single, well-absorbed, gut-gentle compound. The choice usually comes down to whether you want a single ingredient or a formula.

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Not sure where to start? Take the Healthy Ageing Quiz, or browse our healthy ageing supplements for NZ adults 45+.

This article is general information, not personal medical advice. Talk to your GP or pharmacist about what's right for you, particularly if you take other medication.