Is Magnesium Water Soluble? 6 Forms, a 60,000-Fold Range

“Is magnesium water soluble?” sounds like a yes-or-no question. It has two problems.

The first is the category. Water-soluble and fat-soluble are vitamin classes. MedlinePlus lists the fat-soluble vitamins as A, D, E and K, and the water-soluble ones as vitamin C and the B vitamins. Magnesium is not on either list, because it is a mineral.

The second is that “magnesium” in a supplement is always magnesium bonded to something, and that partner decides whether it dissolves.

Six forms, measured

From PubChem’s solubility records at room temperature:

Form	Dissolves in water	PubChem figure
Chloride	Very well	54.6 g per 100 g, 20 °C
Sulfate (Epsom salt)	Well	about 30 to 36 g per 100 mL, 20 °C
Citrate	Partially	”partially soluble in cold water”
Carbonate	Practically not	0.0106 g per 100 mL
Oxide	Practically not	86 mg per litre, 30 °C
Hydroxide	Practically not	0.0009 g per 100 mL, 18 °C

From chloride to hydroxide is a range of roughly 60,000-fold. Chloride dissolves more than 6,000 times better than oxide.

Glycinate, one of the best-selling forms, has no water-solubility record in PubChem, so we are not giving it a number.

Why it matters: dissolving comes first

The NIH Office of Dietary Supplements puts it plainly:

Forms of magnesium that dissolve well in liquid are more completely absorbed in the gut than less soluble forms.

The studies bear that out for the cheapest form on the shelf, magnesium oxide:

• Lindberg, 1990: in 300 mL of water, oxide was “virtually insoluble” and only 43% dissolved even in fluid mimicking peak stomach acid. Citrate dissolved 55% in plain water. After a dose, urinary magnesium rose 0.22 vs 0.006 mg per mg creatinine, citrate vs oxide.
• Firoz and Graber, 2001: oxide’s fractional absorption was 4%, significantly lower than chloride, lactate and aspartate.
• Walker, 2003: 46 adults took 300 mg of elemental magnesium daily for 60 days. Citrate and an amino-acid chelate absorbed better than oxide, and oxide was no different from placebo.

The exception: soluble is not the same as absorbed

The same NIH page lists the more bioavailable forms as aspartate, citrate, lactate and chloride, and groups magnesium sulfate with oxide as the less bioavailable ones.

Sulfate dissolves at about 30 g per 100 mL. Oxide barely dissolves at all. Both land on the same side of the NIH’s line. So solubility is a strong signal about absorption, not a guarantee of it.

Organic or inorganic does not decide it

Marketing often sells “organic” magnesium salts as superior. In the 2001 comparison, magnesium chloride, an inorganic salt, was absorbed as well as organic lactate and aspartate. The authors’ conclusion:

Inorganic magnesium salts, depending on the preparation, may have bioavailability equivalent to organic magnesium salts.

The dividing line in these studies was not organic against inorganic. It was oxide against the rest.

What this means when you buy

• Read the elemental magnesium figure on the label, not the weight of the compound.
• If the label says oxide, expect little of it to be absorbed: 4% in one study, no different from placebo in another.
• Citrate and chloride have the strongest solubility and absorption data of the common forms.
• Keep supplemental magnesium at or under 350 mg a day, the NIH upper limit, unless a doctor says otherwise. Magnesium from food does not count toward it.

For choosing between the two most popular forms, see magnesium glycinate vs citrate, and for glycinate on its own, magnesium glycinate benefits and side effects.