Magnesium Oxide: The Form to Avoid
Magnesium oxide has the highest elemental magnesium content (~60%) of any form, which makes it look attractive on supplement labels. This is misleading. Its extremely low solubility in gastric fluid (~0.6g/L at pH 2) means that most of it passes through the stomach undissolved and arrives in the colon intact, where it draws water osmotically—the mechanism behind milk of magnesia and MiraLax.
Bioavailability relative to magnesium chloride: approximately 4% in controlled studies. For systemic magnesium repletion—correcting a deficiency, supporting ATP synthesis, improving sleep, or reducing muscle cramps—magnesium oxide delivers almost nothing. Despite this, it remains the most commonly used form in low-cost multivitamins and budget magnesium supplements. If your supplement lists only "magnesium oxide" or "magnesium amino acid chelate" without specifying the amino acid, consider switching.
The one legitimate use for magnesium oxide is as an osmotic laxative, where its colon-reaching properties are the desired outcome. For everything else, avoid it.
Magnesium Glycinate (Bisglycinate): Best for Sleep and Anxiety
Magnesium glycinate is Mg2+ chelated to two glycine molecules. The chelation serves two purposes: it protects magnesium from forming insoluble complexes with phytates, phosphates, and oxalates in the GI tract (which would render it unabsorbable), and it delivers glycine alongside the magnesium—a compound with its own CNS activity.
Glycine is an inhibitory neurotransmitter and GABA-A receptor coagonist. It activates strychnine-sensitive glycine receptors (GlyR) on spinal and brainstem neurons, producing hyperpolarization (inhibitory neuronal activity). This has documented anxiolytic and sleep-promoting effects independent of the magnesium. The combination of well-absorbed magnesium + glycine's inhibitory CNS activity makes bisglycinate the preferred form for evening use.
Abbasi et al. 2012 (Journal of Research in Medical Sciences, n=46, elderly patients with insomnia) found 500mg/day magnesium improved sleep quality scores, reduced nocturnal cortisol, reduced serum renin, and increased melatonin compared with placebo. This trial used magnesium oxide but the sleep-promoting mechanism is more pronounced with glycinate due to the glycine component.
Relative bioavailability: ~80% vs magnesium chloride. Minimal laxative effect at doses up to 400mg elemental magnesium/day. Note: "magnesium bisglycinate" and "magnesium glycinate" are the same compound—two glycine molecules chelated to one Mg2+ ion.
Target dose: 200–400mg elemental magnesium as glycinate, taken 30–60 minutes before bed.
Look for 'magnesium bisglycinate' or 'magnesium glycinate chelate' on the label. Verify the elemental magnesium content per serving (target 200–400mg/day total). Doctors Best, NOW Foods Magnesium Bisglycinate, and Thorne Magnesium Bisglycinate are consistently well-reviewed for purity and label accuracy.
Magnesium L-Threonate (MgT / Magtein): Best for Cognition
Magnesium L-threonate was developed at MIT specifically to penetrate the blood-brain barrier. The threonate anion exploits SVCT2 (sodium-dependent vitamin C transporter 2) transporters expressed on the blood-brain barrier, facilitating magnesium entry into the central nervous system. This is the only magnesium form shown to significantly raise cerebrospinal fluid magnesium concentrations.
Slutsky et al. 2010 (Neuron) found that MgT supplementation in rats increased brain Mg concentration by 18%, increased synapse density in the prefrontal cortex and hippocampus by ~15%, and improved performance on working memory and long-term memory tasks. Human trial data: a 2014 study found cognitive improvements in older adults with MgT supplementation (Magtein brand, 1.5–2g/day).
Important caveats: the elemental magnesium content per dose of MgT is low (~144mg elemental Mg per 2g MgT). If you are also trying to correct systemic magnesium deficiency, you will need to stack MgT with a higher-absorption form like glycinate. MgT is significantly more expensive than other forms and the human evidence base, while promising, is less extensive than for glycinate or citrate in general health applications.
Target dose: 1.5–2g MgT/day (providing ~144mg elemental Mg). Standard protocol: 1 capsule morning + 2 capsules evening to align with sleep-phase synaptic consolidation.
Magtein is the branded form used in the Slutsky 2010 (Neuron) research. Look for products listing 'Magtein' or 'magnesium L-threonate' explicitly. Standard dose: 2,000mg MgT/day providing ~144mg elemental Mg. Stack with glycinate if correcting systemic deficiency simultaneously.
Magnesium Malate: Best for Energy and Fibromyalgia
Magnesium malate is Mg2+ chelated to malic acid. Malic acid is a Krebs cycle intermediate—it is directly metabolized in mitochondria as a TCA cycle substrate. This makes malate particularly relevant for energy production. Some practitioners prefer magnesium malate for morning use to leverage this energy-supporting property and avoid the sedation some users associate with glycinate, though this effect difference is not well-documented in RCTs.
Russell et al. 1995 (Journal of Rheumatology) studied magnesium malate specifically in fibromyalgia patients: 300–600mg Mg with 1,200–2,400mg malic acid reduced pain scores significantly in an open-label study. While open-label data has limitations, this is one of the few condition-specific studies for a non-glycinate magnesium form. Bioavailability is similar to glycinate (~80%).
Target dose: 200–400mg elemental magnesium as malate, morning dosing preferred for energy applications.
Magnesium Citrate: The Practical Default
Magnesium citrate is highly water-soluble and well-absorbed (~66% relative bioavailability). It is the most accessible "better-than-oxide" form found in pharmacies. The citrate anion alkalinizes urine, which may reduce kidney stone risk in calcium oxalate stone formers by increasing urinary citrate, an inhibitor of stone nucleation. Some osmotic laxative effect at higher doses but far less than oxide.
Citrate is a reasonable default when cost matters and no specific target (sleep, cognition, energy) is the priority. It is not optimized for any particular outcome vs glycinate, malate, or threonate, but it is dramatically superior to oxide and widely available.
Form Comparison Table
| Form | Relative Bioavailability | Best Application | Key Notes |
|---|---|---|---|
| Oxide | ~4% (poor) | Osmotic laxative only | Avoid for systemic Mg repletion; most multivitamins use this; essentially no systemic benefit |
| Citrate | ~66% (good) | General repletion, kidney stone prevention | Best cost-to-bioavailability ratio; mild laxative at high doses; alkalinizes urine |
| Glycinate / Bisglycinate | ~80% (excellent) | Sleep, anxiety, muscle recovery, evening | Glycine adds GlyR inhibitory activity; minimal GI effects; best overall form for most users |
| Malate | ~80% (excellent) | Energy, fibromyalgia, morning use | Malic acid is Krebs cycle substrate; morning dosing preferred; studied in fibromyalgia |
| L-Threonate (MgT) | ~80% systemic; only form raising CSF Mg | Cognitive performance, neuroprotection | Lower elemental Mg per capsule; most expensive; BBB penetration via SVCT2; stack with glycinate for deficiency repletion |
| Taurate | ~70% estimated | Cardiovascular, blood pressure | Taurine has independent cardiovascular benefits; limited standalone RCT data |
Magnesium Protocol by Goal: Evidence-Based Dosing
- Sleep and anxiety: Magnesium glycinate, 200–400mg elemental Mg, 30–60 min before bed. The glycine component activates inhibitory GlyR receptors and potentiates GABA-A activity, while Mg2+ blocks NMDA receptors. Start at 200mg and titrate up based on effect.
- Cognitive performance and aging: Magnesium L-threonate (Magtein), 1.5–2g/day (~144mg elemental Mg). Typically 1 capsule morning + 2 capsules evening. The higher evening dose exploits synergies with sleep-phase synaptic consolidation. Not justified for general Mg repletion alone—stack with glycinate if deficiency is also a concern.
- Energy and fibromyalgia: Magnesium malate, 300–400mg elemental Mg/day. Morning dosing is preferred. Malic acid co-supplementation (1,200–2,400mg) amplifies the energy-supporting effect via direct Krebs cycle substrate provision.
- Migraine prophylaxis: Any well-absorbed form at 400–600mg elemental Mg/day. Peikert 1996 used magnesium dihydrogen phosphate; meta-analyses confirm any absorbable form at adequate dose reduces migraine frequency by ~41% after 2–3 months. Split into 200mg morning + 200mg evening to reduce GI effects.
- Signs of deficiency: Nocturnal leg cramps, eyelid twitching, poor sleep quality, anxiety, constipation, low energy. Serum Mg is a poor marker for intracellular status—RBC magnesium or 24-hour urinary Mg excretion are better functional tests.
Look for products providing both magnesium and malic acid. Doctors Best High Absorption Magnesium uses a malate/glycinate blend. KAL Magnesium Malate is another common option. Target 200–400mg elemental Mg per day from malate sources.