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

Iron-zinc-copper <strong>Interactions</strong>
Iron–ascorbic Acid and Iron-calcium
<strong>Interactions</strong> and Their Relevance in
Complementary Feeding
Lindsay H. Allen
Department of Nutrition, University of California, Davis, CA 95616
Ascorbic acid and calcium have opposite
effects on iron absorption; ascorbic acid
improves the absorption of naturally
occurring or fortificant nonheme iron
in foods whereas calcium inhibits the absorption
of both heme and nonheme iron. The
effects of ascorbic acid and calcium on iron
absorption will be considered separately,
followed by a discussion of the practical implications
of these interactions for complementary
feeding.
Iron and Ascorbic Acid
Ascorbic acid is a strong enhancer of nonheme-iron
absorption. The mechanisms for
this absorption enhancement include the
reduction of dietary ferric iron to its betterabsorbed
ferrous form and the formation
of an iron–ascorbic acid chelate in the acid
milieu of the stomach. The chelate is formed
at a lower pH than that at which iron complexes
with phytate, tannins, and other inhibitors
of iron absorption. Thus, ferrous
iron is preferentially bound to ascorbic acid
in the stomach and remains in this complex
even in the more alkaline pH of the intestine.
The overall effect is to reduce the influence
of inhibitory ligands that would otherwise
bind iron in the duodenum (Hurrell
1984). Although ascorbic acid does not enhance
iron absorption from most ferrous
iron supplements in the absence of meals, it
increases the bioavailability of ferrous iron
added as a fortificant to foods, especially
when those foods contain a large amount of
iron absorption inhibitors. This is true
whether the fortificant iron is a simple ferrous
salt or a chelate such as NaFeEDTA.
Even less-bioavailable iron salts, such as ferric
orthophosphate and ferric pyrophosphate,
are reasonably well-absorbed in the
presence of ascorbic acid.
We recently reviewed studies in which
ascorbic acid was added to meals based on
maize, wheat, and rice (Allen and Ahluwahlia
1996). Figure 1 presents, for each
study, the mean ratio of iron absorption in
the presence vs absence of ascorbic acid at
each concentration of ascorbic acid. Absorption
of the reference dose of iron in each
study was used to correct for differences in
the efficiency of iron absorption caused by
differences in iron status among subjects. It
is evident from Figure 1 that iron absorption
from meals approximately doubles
when 25 mg ascorbic acid is added and increases
three- to sixfold when 50 mg is added.
The relation between iron absorption and
ascorbic acid appears to be linear up to at
least 100 mg ascorbic acid per meal.
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