On the subject, I only found a book, "Chelates in nutrition", by F. Howard
Kratzer and Pran Vohra, CRC press 1986, about chelation and minerals, but the
book is sometimes technical and the authors are more interested in poultry
growth than in human health.

CHELATION means the formation of a complex formed with a metallic cation (i.e.
positively charged, like Fe2+, Ca2+, Zn2+, etc.) and a chelating agent. There
are many natural chelates, like some carbohydrates, lipids, proteins, vitamins
(B12, C), ATP... and many artificial chelates, like EDTA
(ethylene-diamine-tetraacetic acid).

STABILITY CONSTANTS: each pair metal-chelate has a particular stability constant
(which depends on pH). High stability constants correspond to strong bindings.
However, due to many factors, what happens in vitro often differs from what
happens in vivo, so there is usually no way to predict how efficient a chelate
will be. There is a lot of experimental data, but theoretical explanations are
not clear.

When supplements are added to a food, the chelate should have a stability
constant higher than that of the food ligand to remove it from the food, cross
the intestinal cell with or without the metal, and have a lowerw stability
constant than of those ligands in the transport system which delivers the metal
to the target tissue (otherwise, it wouldn't release the metal to the body). The
ligand should then be eliminated from the body by excretion (as such or after
being metabolized). Usually, the ligand should have a molecular weight not
higher than 1000, i.e. not be heavier than 1000 atoms of hydrogen (for EDTA:
292), otherwise it couldn't be absorbed.

PHYTIC ACID is a well-known ligand, present in cereals, which form insoluble
complexes with metals such as Zn. But when EDTA is ingested at the same time,
EDTA, having a higher constant than phytic acid, removes the Zn from the
complexe phytate-Zn, and thus Zn becomes bioavailable.

OXALIC ACID binds with Ca (calcium), and an excess of oxalic acid (which is
present in spinach) can create a calcium deficiency.

In general, a chelate can improve bioavailability of a mineral, by mechanisms
which are unclear. There is an optimum ratio ligand:metal (i.e. too much ligand
won't help). EDTA is known to increase the availability of Zn (zinc), but has a
negative effect on Fe (iron). Ascorbic acid (vitamin C) increases availability
of iron. Other dietary factors influence availability of a given metal (e.g.
ingestion of meat enhances the absorption of iron).

How effective are chelating agents? I don't know exactly. It of course depends
on the pair metal-ligand, but I think a twofold increase in absorption can be
expected.

Any complementary information would be very welcome.

Best wishes,

Jean-Louis
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