Micronutrient Interactions: Impact on Child Health and ... - Idpas.org

Iron–ascerbic Acid and Iron–calcium <strong>Interactions</strong> and Their Relevance in Complementary Feeding
Iron-calcium <strong>Interactions</strong>
An inhibitory effect of calcium on iron absorption
was recognized nearly 60 years ago.
In the past 10–15 years many investigators
have focused on this issue, trying to define
the practical importance of the interaction
and the mechanism by which it occurs. The
results of these studies are often conflicting
because several factors influence the interaction
between calcium and iron absorption,
including the molar ratio of calcium
to iron, the forms of calcium and iron,
whether these minerals are consumed in the
presence of food, and the iron status of the
subjects.
The mechanism by which calcium impairs
iron absorption remains controversial
(Hallberg et al. 1992a). It probably does
not involve reduction of intestinal pH or
the formation of insoluble iron salts such as
carbonates. It is not clear whether calcium
competes for low-molecular-weight ligands
that enhance iron absorption. Calcium
equally inhibits the absorption of nonheme
and heme iron (Hallberg et al. 1991), which
enter the mucosal cell by different pathways
and leave in the same form, suggesting that
calcium inhibits the intracellular transport
of iron (Hallberg et al. 1992b). Calcium may
compete for iron-binding sites in
mobilferrin, a protein in the duodenal mucosa
that may assist iron transport through
the cell (Conrad and Umbreit 1993).
Mobilferrin can bind calcium although it
has a greater affinity for iron. Calcium may
also inhibit the release of iron from mucosal
cells into the circulation (Wienk et al. 1996).
The inhibitory effect of calcium on iron
absorption is independent of the amount of
phytate in a meal (Hallberg et al. 1992b)
and does not interfere with the enhancing
effect of meat (Hallberg et al. 1992b) or
ascorbic acid (Hallberg et al. 1992a). It is
important to recognize that adding calcium
to wheat flour before leavening causes a further
substantial decrease in iron absorption
because calcium inhibits phytase activity in
yeast (Hallberg et al. 1991). As little as 40
mg calcium added to 80 g flour inhibited
phytate degradation during leavening by
50%. This means that calcium fortification
of wheat flour may reduce the beneficial effects
of fermentation on the bioavailability
of iron and other minerals bound to phytate.
Amount of Calcium
Hallberg et al. (1991) added between 40 and
600 mg of calcium, as calcium chloride or
dairy products, to wheat rolls containing
3.8 mg iron. Iron absorption fell relatively
linearly to about 40% at 300–600 mg calcium.
There was no effect of adding 40 mg
calcium (calcium-iron molar ratio 14:1),
and at the upper end adding 600 mg rather
than 300 mg had relatively little additional
inhibitory effect. However, it is not only the
calcium-iron molar ratio that is important,
because there was no effect of adding 3 mg
calcium on the absorption of a trace amount
(0.01 mg) of iron, a calcium-iron molar ratio
of 420:1 (Hallberg et al. 1992a).
Form of Calcium
The form of calcium also influences its inhibitory
effect on iron absorption. A review
by Whiting (1995) revealed that 500–600 mg
calcium added to meals typically reduced
iron absorption by about 40% when added
as cheese and milk; 40–50% when added as
calcium phosphate, carbonate, or hydroxyapatite;
and 60–70% when added as
more soluble calcium citrate and calcium
citrate malate supplements (Hallberg et al.
1992b, Cook et al. 1991b, Deehr et al. 1990,
Dawson-Hughes et al. 1986). Calcium carbonate
had no effect on iron absorption in
at least one report (Seligman et al. 1983).
Although these observations may indicate
that more soluble forms of calcium have a
greater effect on iron absorption because the
inhibition occurs within the mucosal cell,
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