Barbara:

>>PHYTIC ACID is a well-known ligand, present in cereals, which form insoluble
>>complexes with metals such as Zn.
>
>>OXALIC ACID binds with Ca (calcium), and an excess of oxalic acid (which is
>>present in spinach) can create a calcium deficiency.
>
>Do these effects only occur when the mineral is present in the same meal as
>the ligand, or can the ligand bind with minerals that have already been
>processed by the body, or indeed 'hang around' and interfere with
>absorption of minerals eaten at later meals?

From what I have read so far (but there are a few articles I should read to make
a conclusive statement; I'll revert to the list if I find some new stuff),
phytic acid interferes with the "absorption", or "bioavailability" of minerals:
mainly iron and calcium, but also with iron and magnesium. Given the
formulation, I think that phytates are not absorbed by the intestinal wall (I'll
have to check).

However, I came across an abstract (1), according to which phytic acid increases
the excretion of endogenous zinc. In other words, rats fed a zinc-deficient diet
and phytate lose minerals more quickly than rats fed a zinc-deficient diet with
no phytates. The reason is that some zinc from the body ("endogenous zinc") is
released in the intestine; that zinc is normally reabsorbed, but with the
presence of phytates, it forms a complex and is not reabsorbed.

Now, I don't know what would happen with humans, which are fed seeds containing
phytates (instead of a simple sodium phytate).


>While on this topic - is it better to eat hulled or unhulled sesame seeds?
>I understand that they contain a lot more calcium when the hulls are left
>on, but that the hulls also contain phytic acid, which is problematic.

My book says that the percentages of phytic acid are:
sesame seeds, dehulled:  5.3
sesame seed:             5.18
sesame meal, dehulled:   3.6

It seems from the above that all parts of sesame contain phytates.
[but 87% of the wheat phytates are in the aleurone]

Now, in general, processings (soaking, sprouting, fermenting and heating)
destroy phytates, at least partly (phytates are released in the soaking water,
and other processings induce chemical transformations). Provided you soak with
enough water, soaking extracts 99.6, 97, 69, 86 and 13% of the phytates in beans
(Phaseolus vulgaris), soybean fllakes, soybean meaal, corn germ and sesame meal
respectively. So, with sesame, it's not so good.

My opinion is that, except possibly with sesame, processings (like soaking and
prouting) remove phytates efficiently. Now, if you don't eat sesame at each
meal, I don't think you risk mineral deficiencies (usually, no one eats huge
amounts of sesame).

Best wishes,

Jean-Louis
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1. Flanagan PR., A model to produce pure zinc deficiency in rats and its use to
demonstrate that dietary phytate increases the excretion of endogenous zinc, J
Nutr. 1984 Mar; 114(3): 493-502.