The Short Version
Both magnesium glycinate and magnesium L-threonate are well-absorbed chelated forms. They are not interchangeable. Glycinate is the form with decades of data for sleep, anxiety, muscle relaxation, and correcting a plain magnesium deficiency. L-threonate is a newer, more expensive form with one specific claim to fame: it is the only form shown to meaningfully raise magnesium levels in the brain, which is why every trial testing it is a cognitive trial, not a sleep trial.
If you want to fix a deficiency, sleep better, or calm an overactive nervous system, buy glycinate. If you are specifically targeting memory, focus, or age-related cognitive decline and you are willing to pay 2-3x the price, L-threonate is the only form with a mechanism and clinical data behind that claim. Most people should start with glycinate.
What Each Form Actually Is
Magnesium on its own is a reactive metal. In supplements it is always bonded to something - a salt, an acid, or an amino acid - which changes how it absorbs and what it does. The bonded compound is not filler; it travels with the magnesium through the gut and, in some cases, affects where the mineral ends up in the body.
Magnesium glycinate
Magnesium glycinate (also called magnesium bisglycinate) is magnesium bonded to two molecules of glycine, an amino acid with calming, inhibitory effects on the nervous system. The chelated structure is small, stable, and absorbs well in the small intestine. Because it does not rely on stomach acid to dissociate, it tends to cause less GI distress than older forms like oxide or citrate. A typical 200 mg elemental magnesium serving delivers roughly 1.4 grams of glycine alongside it. That glycine is not a throwaway - a 2012 study in J Pharmacol Sci showed 3 grams of glycine at bedtime improved subjective sleep quality, and glycinate supplements deliver roughly a third to a half of that dose as a bonus.
Magnesium L-threonate
Magnesium L-threonate is magnesium bonded to L-threonic acid, a vitamin C metabolite. It was developed at MIT in the late 2000s specifically to solve a pharmacokinetic problem: most forms of magnesium raise serum magnesium but do a poor job of raising magnesium in the cerebrospinal fluid and brain. L-threonate was engineered as a delivery vehicle for the central nervous system.
A 2010 study in Neuron by Slutsky and colleagues showed that magnesium L-threonate elevated magnesium levels in the brain, enhanced synaptic plasticity, and improved learning and memory in rats. This is the paper that launched the product category. Nearly every L-threonate product on the market uses the branded "Magtein" form, which is the ingredient originally patented off that research. Most supplements in this class list Magtein explicitly on the label.
What the Research Supports
Glycinate: sleep, anxiety, muscle, deficiency
The evidence base for magnesium supplementation in general is decades deep, but glycinate specifically shines in three applications.
Sleep. A 2012 double-blind RCT in older adults with insomnia showed that 500 mg of magnesium daily improved sleep onset, sleep time, and serum melatonin compared to placebo. A 2022 systematic review in BMC Complementary Medicine and Therapies found magnesium supplementation improved subjective insomnia outcomes, though the authors rated evidence quality as moderate. Most of those trials used mixed forms, but glycinate has become the clinical favorite for sleep because of the glycine bonus.
Anxiety and stress. A 2017 systematic review in Nutrients looked at 18 studies on magnesium status and subjective anxiety and found that supplementation reduced self-reported anxiety in mildly anxious individuals and people with premenstrual symptoms. The mechanism is plausible: magnesium modulates NMDA and GABA receptors, both of which are relevant to anxious arousal.
Muscle cramps and restless legs. The evidence here is mixed but not negative. Small trials have shown modest benefit in nocturnal leg cramps, especially in people with low magnesium status. If you get cramps and the fix is cheap, glycinate is a reasonable first thing to try.
Fixing a deficiency. About half of Americans fall short of the estimated average requirement for magnesium. If you simply want to top up your levels and not give yourself diarrhea doing it, glycinate is the default recommendation because it is well-absorbed and GI-friendly at the clinically relevant doses.
L-threonate: cognition, memory, age-related decline
Every meaningful human trial on magnesium L-threonate has been a cognitive trial. There are not many, but the ones that exist point in the same direction.
A 2016 trial in J Alzheimer's Disease gave 1.5-2 g/day of magnesium L-threonate to older adults with subjective cognitive decline for 12 weeks. Treated subjects showed improvement in executive function, working memory, attention, and episodic memory compared to placebo. The reported gain was roughly equivalent to reversing 9 years of cognitive aging on the executive function measure. That is a striking claim from a small trial, and it has not been replicated at scale, but it is consistent with the mechanism.
A 2023 review summarizing the preclinical and clinical data on L-threonate concluded the form has unique CNS penetrance and a plausible role in supporting age-related cognitive function, while noting that the human evidence base is still small.
What L-threonate does not have good evidence for: sleep, anxiety, cramps, basic deficiency correction, or athletic performance. Those are not what it was designed for and the trials supporting it have not tested those endpoints.
Bioavailability: Both Are Good, Differently
"Bioavailability" gets thrown around loosely. What matters is: does the magnesium show up in serum, and does it reach the tissue you care about?
For serum magnesium, glycinate and L-threonate are both well absorbed, and both clearly outperform the cheap forms like oxide (which is only about 4% absorbed and is effectively a laxative). Direct head-to-head absorption studies are scarce, but both are categorized as high-bioavailability chelates.
For brain magnesium, this is where the difference shows up. Most chelated forms raise serum levels fine but barely move cerebrospinal fluid magnesium. L-threonate was designed specifically to fix that - preclinical work has shown it produces measurably higher brain magnesium concentrations than other forms at equivalent doses. This is the whole reason the category exists.
If your target tissue is the central nervous system, that difference matters. If your target is skeletal muscle, nerves outside the brain, bone, or general systemic status, both forms work fine and glycinate is better value.
Dosing
Glycinate
Target 200-400 mg of elemental magnesium per day, typically taken 30-60 minutes before bed. Read the label carefully: some brands list elemental magnesium, others list total magnesium glycinate (which is about 4-5x the elemental number because glycine is heavy). "2,000 mg of magnesium bisglycinate" is roughly 400 mg of elemental magnesium. If the label does not specify elemental, assume the worst and compare products accordingly.
L-threonate
The clinically studied dose is roughly 1.5-2 grams of Magtein (magnesium L-threonate), which delivers about 144-200 mg of elemental magnesium. Note that L-threonate delivers less elemental magnesium per gram of compound than glycinate - this is a feature, not a bug, because the value is in how the carrier delivers it, not total load. Doses are almost always split, with some taken in the morning and some before bed. The clinical trial used three capsules daily, split.
One practical note: L-threonate is not a good form for correcting a plain magnesium deficiency, because the elemental load per dollar is low. If you are deficient, you will not close the gap efficiently with L-threonate alone.
Price: The Real Gap
This is where the choice actually pinches for most buyers.
Magnesium glycinate: $0.10 to $0.25 per 200 mg elemental magnesium daily dose. A high-quality third-party-tested bottle runs $20-$30 for a 2-3 month supply. See the magnesium glycinate scorecard for head-to-head product comparisons.
Magnesium L-threonate: $0.80 to $1.50 per clinically studied daily dose. Branded Magtein products run $35-$60 per month. The price premium is roughly 4-8x per day of use, not a small difference.
This matters because magnesium is almost always a long-running supplement, not a 2-week trial. An extra $25-$40 a month for 12 months compounds into real money, and most people buying L-threonate do not need the specific mechanism it provides.
Side Effects and Tolerability
Both forms are well tolerated. Glycinate is among the gentlest forms for GI tolerance - it rarely causes the loose stools that citrate and oxide can cause at equivalent elemental doses. L-threonate is also GI-friendly, partly because the elemental magnesium load is lower per capsule.
A small number of people report mild morning grogginess with glycinate at higher doses (400+ mg), probably from the glycine component. A small number report mild headaches or transient anxiety on their first few days of L-threonate, which typically resolves. Neither form has meaningful serious-adverse-event risk in healthy adults at standard doses. People with impaired kidney function should not supplement any form of magnesium without medical supervision.
Who Should Buy Which
Buy magnesium glycinate if you...
- Have trouble falling asleep or staying asleep
- Feel wired, anxious, or stressed and want something calming
- Get muscle cramps, twitches, or restless legs
- Want to correct a probable dietary deficiency
- Take magnesium alongside other evening supplements for general wellness
- Want the best cost-to-benefit ratio on the market
Buy magnesium L-threonate if you...
- Are specifically targeting memory, focus, or age-related cognitive decline
- Are healthy in most other respects and already have adequate magnesium intake
- Are willing to pay 2-3x the price for a specific mechanism
- Are using it alongside other cognitive health interventions (exercise, sleep, lipid management)
Can You Stack Both Forms?
Yes, and for a subset of buyers this is the most defensible approach. The stack looks like:
- Magnesium L-threonate in the morning (split dose, for cognitive support)
- Magnesium glycinate in the evening (for sleep and to top up total elemental magnesium)
The forms do not antagonize each other. The total elemental magnesium load stays within safe ranges if you keep the glycinate dose at 200 mg elemental and the L-threonate at the clinical 2 g dose. Cost is the main downside - you are running two bottles at once.
The upper tolerable intake level set by the Institute of Medicine for supplemental magnesium is 350 mg/day in adults (this does not include magnesium from food). The UL is based on GI tolerance, not systemic toxicity. Some clinicians exceed it for specific indications, but if you are stacking, it is reasonable to aim for 200-300 mg elemental glycinate plus 150-200 mg elemental from L-threonate, totaling 350-500 mg. This is above the UL but within ranges commonly used in trials.
Common Questions
Can I just take L-threonate and skip glycinate?
You can, but it is a poor deficiency-correction tool. The elemental magnesium load at the clinical L-threonate dose is around 144-200 mg, which is below the RDA. If your dietary magnesium intake is poor, L-threonate alone will underdeliver total magnesium relative to your needs.
Does L-threonate actually cross the blood-brain barrier better?
In animal models, yes - this was the original finding in the 2010 Neuron paper. In humans, we have indirect evidence: the 2016 trial showed cognitive benefits consistent with increased brain magnesium. We have not directly measured brain magnesium in large human trials, so some humility is warranted. The mechanistic case is the strongest of any form for CNS-targeted magnesium.
Is the glycine in glycinate enough to help sleep on its own?
Probably not. The glycine-for-sleep trials used 3 grams, and a standard glycinate serving delivers roughly 1-1.5 grams of glycine. It contributes but is not a full dose. If you want the glycine-for-sleep effect specifically, supplement glycine separately on top.
What about magnesium citrate, malate, taurate, and the others?
Citrate is well-absorbed and cheap but can be laxative. Malate is sometimes chosen for fatigue. Taurate has preliminary data for cardiovascular support. None of them have the specific brain-penetrance data of L-threonate or the sleep/anxiety data of glycinate. See our full comparison of magnesium forms for the broader landscape.
The Bottom Line
For 80 percent of buyers, magnesium glycinate is the right answer. It is well-absorbed, GI-friendly, inexpensive, and covers the use cases most people actually have - sleep, anxiety, muscle function, and correcting a deficiency. L-threonate is the specialist tool. It is the only form with a mechanism and trial data for crossing into the brain, and it is worth the price premium if cognitive support is specifically what you are buying magnesium for. When in doubt, start with glycinate. Add L-threonate only if your goal is unambiguously in the cognitive domain.
Sources
- Slutsky I, Abumaria N, et al. Enhancement of learning and memory by elevating brain magnesium. Neuron. 2010;65(2):165-77. PubMed
- Liu G, Weinger JG, et al. Efficacy and safety of MMFS-01, a synapse density enhancer, for treating cognitive impairment in older adults: a randomized, double-blind, placebo-controlled trial. J Alzheimers Dis. 2016;49(4):971-90. PubMed
- Abbasi B, Kimiagar M, et al. The effect of magnesium supplementation on primary insomnia in elderly. J Res Med Sci. 2012;17(12):1161-9. PubMed
- Bannai M, Kawai N. New therapeutic strategy for amino acid medicine: glycine improves the quality of sleep. J Pharmacol Sci. 2012;118(2):145-8. PubMed
- Boyle NB, Lawton C, Dye L. The effects of magnesium supplementation on subjective anxiety and stress - a systematic review. Nutrients. 2017;9(5):429. PubMed
- Mah J, Pitre T. Oral magnesium supplementation for insomnia in older adults: a systematic review and meta-analysis. BMC Complement Med Ther. 2021;21:125. PubMed
- Shrimanker I, Bhattarai S. Electrolytes and magnesium in clinical practice. StatPearls. 2023. PubMed