Threonic acid.

The threonate is doing the job of the anion, the same structural role sulfate or citrate plays in other magnesium salts. Because threonate is a light molecule, a gram of the salt carries a relatively small amount of elemental magnesium, so label doses of the salt and of the mineral are easy to confuse. Anyone comparing this to other magnesium forms should compare elemental milligrams, not salt milligrams.

This is settled ascorbate chemistry, and it means measurable threonate in a person is partly a signature of vitamin C turnover rather than of anything supplemental. It also means commercial threonate is usually made from ascorbic acid rather than isolated from a natural source. The relationship runs one way: ascorbate makes threonate, and threonate does not make ascorbate.

Threonate salts on the market are made from ascorbate feedstock through controlled oxidation, then neutralised with the desired cation. That is why the L stereochemistry is specified. It is a manufacturing relationship, not a nutritional one, and it should not be read as vitamin C and threonate working together in the body.

Threonate serves the same counter-ion role for calcium as it does for magnesium, and calcium threonate has its own use history as a soluble calcium source. Solubility is the practical property being bought. Claims about the threonate portion adding something of its own beyond solubility need their own evidence.

Calcitriol drives the transcellular calcium transport machinery in the small intestine, so calcium arriving without adequate vitamin D status is handled less efficiently. This applies to a threonate salt exactly as it does to carbonate or citrate. The counter-ion is not the variable here.

B6 and magnesium are combined by convention, with some support for the pairing in older work on magnesium status. The rationale is not specific to threonate as the counter-ion. Read it as formulation practice carried over from other magnesium salts.

Metabolomic work on probiotic supplementation lists threonic acid among the metabolites that move, which places it in the gut microbial and host ascorbate turnover picture. A metabolite that changes is a marker, not an outcome, and a shift of this kind says nothing about whether anyone felt different. It is included because it is where the molecule shows up in human data at all.

The counter-ion approach generalises across divalent minerals, and zinc threonate is sold on that basis. Evidence specific to the zinc salt is thin compared to the magnesium version. The soluble-salt rationale is chemistry, and the rest is extrapolation.