Soy protein, phytate, and iron absorption
Search for nutritional confounding factors in the relationship
between iron deficiency and brain function
Hallberg L
Am J Clin Nutr 1989 Sep 50:3 Suppl 598-604; discussion
604-6
Abstract
This paper on confounding factors in the relationship between iron
deficiency and brain function is mainly limited to nutritional factors,
primarily factors that can contribute to the development of iron
deficiency and that may have an independent direct action on brain
function.
Three theoretically possible confounders were found in a systematic
search for dietary factors: 1) low intake of ascorbic acid, 2) excess
of phytates, and 3) increased absorption of lead.
Ascorbic acid has a marked effect on the bioavailability of dietary
iron and is also known to directly influence various metabolic
processes in the brain. Phytates inhibit the absorption not only of
iron but also of zinc. An iron deficiency may thus be accompanied by a
zinc deficiency which may affect mental performance.
A state of iron deficiency may increase the absorption of lead from
the diet, which in turn may affect brain function.
Soy protein, phytate, and iron absorption in humans.
Hurrell RF, Juillerat MA, Reddy MB, Lynch SR, Dassenko SA, Cook
JD
Am J Clin Nutr 1992 Sep 56:3 573-8
Abstract
The effect of reducing the phytate in soy-protein isolates on
nonheme-iron absorption was examined in 32 human subjects.
Iron absorption was measured by using an extrinsic radioiron label
in liquid-formula meals containing hydrolyzed corn starch, corn oil,
and either egg white or one of a series of soy-protein isolates with
different phytate contents.
Iron absorption increased four- to fivefold when phytic acid was
reduced from its native amount of 4.9-8.4 to less than 0.01 mg/g of
isolate.
Even relatively small quantities of residual phytate were strongly
inhibitory and phytic acid had to be reduced to less than 0.3 mg/g of
isolate (corresponding to less than 10 mg phytic acid/meal) before a
meaningful increase in iron absorption was observed.
However, even after removal of virtually all the phytic acid, iron
absorption from the soy-protein meal was still only half that of the
egg white control.
It is concluded that phytic acid is a major inhibitory factor of
iron absorption in soy-protein isolates but that other factors
contribute to the poor bioavailability of iron from these products.
Iron bioavailability studied in infants: the influence of phytic
acid and ascorbic acid in infant formulas based on soy isolate.
Davidsson L, Galan P, Kastenmayer P, Cherouvrier F, Juillerat MA,
Hercberg S, Hurrell RF
Pediatr Res 1994 Dec 36:6 816-22
Abstract
The influence of phytic acid and ascorbic acid content of soy
formula on iron (Fe) bioavailability was investigated in infants by
analysis of the incorporation of stable isotopes of Fe into red blood
cells 14 d after administration using a double stable isotope
technique.
Paired comparisons were made with each infant acting as his or her
own control.
The geometric mean fractional Fe incorporation into red blood cells
increased from 5.5 to 6.8% (p < 0.05) when soy formula with the
native content of phytic acid was compared with a 83% dephytinized
formula. A more pronounced effect was shown with soy formula containing
no phytic acid; the mean fractional Fe incorporation increased from 3.9
(native phytic acid) to 8.7% (zero phytic acid; p < 0.001).
A significant (p < 0.01) effect was also demonstrated when the
Fe:ascorbic acid molar ratio in the native phytate-containing formula
was increased from 1:2.1 to 1:4.2; mean fractional Fe incorporation
increased from 5.9 to 9.6%.
These results demonstrate that the Fe bioavailability from soy-based
infant formulas can be similarly increased by either removing phytic
acid or increasing the ascorbic acid content.
A comparison of iron absorption in adults and infants consuming
identical infant formulas.
Hurrell RF, Davidsson L, Reddy M, Kastenmayer P, Cook JD
Br J Nutr 1998 Jan 79:1 31-6
Abstract
Fe absorption was estimated in adults and infants from the
erythrocyte incorporation of Fe isotopes added to infant formula.
Fe absorption was measured in adults using radioisotopes, and in
infants with a stable-isotope technique.
In adults, the geometric mean Fe absorption from a ready-to-feed
soya formula with its native phytic acid content was 2.4%. This
increased to 6.0% (P < 0.05) after almost complete dephytinization.
In infants, mean Fe absorption values were 3.9 and 8.7% respectively
from the same products (P < 0.05). In adults, mean Fe absorption
from a spray-dried soya formula containing 110 mg ascorbic acid/l was
4.1%, increasing to 5.3% (P < 0.05) when ascorbic acid was doubled
to 220 mg/l. In infants, mean Fe absorption values were 5.7 and 9.5% (P
< 0.05) from the same products.
Mean Fe absorption from a milk-based formula was 6.5% in adults
compared with 6.7% in infants. All meals in the adult and infant
studies were fed using an identical meal size of 217 g. Increasing the
meal size threefold in adults did not change fractional Fe absorption.
Mean Fe absorption values for each meal were lower in adults than in
infants but the relative inhibitory effect of phytic acid and the
enhancing effect of ascorbic acid were similar.
We conclude that Fe absorption studies in adults can be used to
assess the influence of enhancers and inhibitors of Fe absorption in
infant formulas fed to infants. Further studies, however, are required
to extend these findings to weaning foods and complete meals.
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