Is threonate worth the price for cognition?

Independent replication of its CNS-penetration advantage is limited. Most of the supporting studies are from the manufacturer. Cheaper, well-absorbed forms at adequate dose are a more conservative bet.

Magnesium forms: which one to pick and what the bioavailability data actually shows

Magnesium is sold in many forms with confident claims about which is best. The honest answer is that they differ in absorption and GI side effects, but for most users at typical doses the difference is smaller than marketing suggests.

The UK context#

NHS reference intake for magnesium is 300 mg/day for men and 270 mg/day for women. Magnesium is widely available in green leafy vegetables, nuts, seeds, whole grains, and legumes. Subclinical deficiency is more common than overt deficiency in modern Western diets, though true deficiency is uncommon in healthy adults.

The label "elemental magnesium" is what matters. A 1000 mg magnesium oxide capsule provides about 600 mg elemental magnesium; a 1000 mg magnesium glycinate capsule provides about 140 mg elemental magnesium. Comparing forms requires comparing equivalent elemental doses.

The main forms at a glance#

Form	Elemental Mg	Absorption	GI tolerance	Best fit	Evidence base
Oxide	~60%	4–10%	Loose stools common	Cheap mineral top-up	Most older trials
Citrate	~16%	Good	Laxative effect	Constipation + supplementation	Solid
Glycinate / bisglycinate	~14%	Good	Best tolerated	Higher daily doses, sleep marketing	Indirect for sleep
Malate	~15%	Good	Well tolerated	Energy / fibromyalgia marketing	Small trials
Threonate	~8%	Good	Well tolerated	CNS-penetration marketing	Mostly manufacturer-funded

The main forms in detail#

Magnesium oxide. Cheap, widely available. High elemental content (~60%). Lower bioavailability. Most absorption studies report 4–10% absorption rate. Most likely of the common forms to cause loose stools. Most older clinical trials used this form.

Magnesium citrate. Moderate elemental content (~16%). Better-absorbed than oxide. Stronger laxative effect in higher doses (it's used pre-colonoscopy precisely for this reason). Good if you want both a magnesium supplement and a gentle bowel effect.

Magnesium glycinate / bisglycinate. Lower elemental content (~14%). Well-absorbed. Most GI-tolerable of common forms; useful at higher doses without diarrhoea. Most-marketed for sleep, though the glycine content per capsule is modest.

Magnesium malate. Moderate absorption. Often marketed for fatigue and fibromyalgia; the published evidence for those specific claims is small.

Magnesium threonate. Newer form. Has small studies (mostly funded by the manufacturer) claiming preferential CNS penetration. Independent replication is limited. Expensive per gram of elemental magnesium.

Topical magnesium (oil, baths). Skin absorption is poorly characterised. Most controlled studies suggest minimal systemic absorption through intact skin. The subjective benefits of magnesium baths may be more about warmth and bath behaviour than magnesium delivery.

What the bioavailability research shows#

Studies directly comparing absorption (e.g. Walker et al. 2003) generally show:

Organic forms (citrate, glycinate, malate) > oxide for absorption rate
Differences between organic forms are smaller than the gap between organic and oxide
Total absorbed magnesium per equivalent elemental dose is broadly similar across organic forms

For most consumer use, the practical choice is between cheap (oxide) with possible GI side effects vs more expensive (glycinate) with better tolerance.

When to pick which#

Constipation: citrate, at the higher end of normal doses.
Sleep, evening use, sensitive gut: glycinate is reasonable. Don't expect dramatic effects on sleep specifically. See the magnesium glycinate sleep page.
General mineral top-up: any well-absorbed form at 200–400 mg elemental per day is reasonable for those not meeting NHS RDA from diet.
Diabetic control, blood pressure adjuncts, leg cramps: discuss with your GP; the evidence base for these is in specific clinical contexts.

Safety#

At typical supplement doses (≤400 mg elemental/day from supplements, on top of dietary intake), magnesium is safe for most adults. Higher doses cause diarrhoea as the limiting factor in most people. Magnesium overdose is rare without renal impairment.

People with chronic kidney disease should not start supplemental magnesium without clinical input. Several common medications interact: tetracyclines, quinolones, and thyroid medication absorption can be reduced by magnesium.

The honest position#

Pick a well-absorbed form at a sensible dose. Glycinate and citrate are the two most-recommended forms for general use; the difference between them is smaller than the marketing claims. Spending double or triple on threonate or malate over glycinate is paying for narrative more than measured difference.