Introduction to Human Nutrition

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206 Introduction to Human Nutrition


absorbed mainly in the duodenum by an active process
that transports iron from the gut lumen into the
mucosal cell. When required by the body for meta-
bolic processes, iron passes directly through the
mucosal cell into the bloodstream, where it is trans-
ported by transferrin, together with the iron released
from old blood cells (i.e., the effi cient iron recycling
system, Figure 9.5), to the bone marrow (80%) and
other tissues (20%). If iron is not required by the body,
iron in the mucosal cell is stored as ferritin and is
excreted in feces when the mucosal cell is exfoliated.
Any absorbed iron in excess of needs is stored as ferri-
tin or hemosiderin in the liver, spleen, or bone marrow.
Iron can be released from these iron stores for utiliza-
tion in times of high need, such as during pregnancy.


Heme iron is absorbed by a different mechanism
from nonheme iron. The heme molecule is absorbed
intact into the mucosal cell, where iron is released by
the enzyme heme oxygenase. Its absorption is little
infl uenced by the composition of the meal, and varies
from 15% to 35% depending on the iron status of the
consumer. Although heme iron represents only 10–
15% of dietary iron intake in populations with a high
meat intake, it could contribute 40% or more of the
total absorbed iron (Figure 9.6). Many poorer regions
of the world consume little animal tissue and rely
entirely on nonheme iron. The absorption of nonheme
iron is strongly infl uenced by dietary components,
which bind iron in the intestinal lumen. The com-
plexes formed can be either insoluble or so tightly
bound that the iron is prevented from being absorbed.
Alternatively, the complexes can be soluble and iron
absorption is facilitated. Under experimental condi-
tions, nonheme iron absorption can vary widely from
less than 1% to more than 90%, but under more
typical dietary conditions it is usually in the region of
1–20%. The main inhibitory substances and enhanc-
ers of iron absorption are shown in Table 9.9.

Metabolic function and essentiality
Iron acts as a catalytic center for a broad spectrum of
metabolic functions. As present in hemoglobin, iron
is required for the transport of oxygen, critical for cell
respiration. As myoglobin, iron is required for oxygen
storage in muscle. Iron is also a component of various
tissue enzymes, such as the cytochromes, that are
critical for energy production, and enzymes necessary
for immune system functioning. Therefore, these
iron-containing molecules ensure that body fuels,
such as carbohydrate, fat, and protein are oxidized to

Absorption of iron

Iron stores

Circulating RBCs

Plasma:
Transferrin iron

Bone marrow:
Red blood cell
precursors

Spleen:
Reticulo-endothelial
macrophages

Iron losses from body

Tissue iron

Figure 9.5 Metabolism of iron. There is a main internal loop with a
continuous reutilization of iron and an external loop represented by
iron losses from the body and absorption from the diet. Adapted from
Hallberg et al. (1993) with permission of Elsevier.


(a)

Nonheme
60%

Heme
40%

(b)

Nonheme
100%

(c)

Nonheme
90%

Heme
10%

Figure 9.6 Heme and nonheme iron in foods: (a) foods of animal origin; (b) foods of plant origin; (c) dietary iron intake from all foods, daily
average.

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