Agriculture Reference
In-Depth Information
ferric iron solubility. Iron absorption may be enhanced by ascorbic acid, citric acid,
low molecular weight sugars, and mucin (digestive juice glycoproteins). Iron absorp-
tion may be inhibited by polyphenols in food and drinks (such as tannins in tea and
coffee), oxalic acid in spinach, phytates in soy and some cereal grains, and excessive
intakes of calcium, zinc, manganese, and nickel. Research has shown that genetic
modification that lowers polyphenol concentrations in common beans (
Phaseolus
vulgaris
) may improve both iron and zinc bioavailability. Transport of iron across the
interior of the enterocyte appears to involve association with amino acids or proteins.
Movement through the basolateral membrane appears to involve a transport pro-
tein, coupled with oxidation of any ferrous iron to ferric iron by copper-containing
ferroxidases, followed by association with transferrin, the ferric iron transporter in
blood plasma.
Iron in foods is found either in heme or nonheme forms. Heme iron is found
mainly in hemoglobin and myoglobin in meat, poultry, and fish, comprising about
50-60% of the total iron in these foods; the remainder is nonheme iron. Plant foods,
milk, cheese, and eggs contain mostly nonheme iron, although iron concentrations
are very low in dairy products. Organ meats, particularly liver, are high in iron, and
red meat, clams, oysters, molasses, nuts, legumes, green leafy vegetables, and iron-
enriched whole-grain breads and cereals are good sources. Rice is generally consid-
ered a poor source, but by selective breeding, iron concentrations in polished rice can
be increased two to four times. Iron sources approved for food fortification include
elemental iron, ferrous ascorbate, ferrous carbonate, ferrous citrate, ferrous fumar-
ate, ferrous gluconate, ferrous lactate, ferric ammonium citrate, ferric chloride, ferric
citrate, ferric pyrophosphate, and ferric sulfate. However, these substances differ in
their bioavailability and reactivity with other compounds—issues that must be con-
sidered when selecting them for use. Heme iron is highly available, and its bioavail-
ability is generally affected less by other dietary components than is nonheme iron.
Recommended dietary iron allowances range from 7 mg per day for 1- to 3-year-old
children to 27 mg per day for pregnant women.
C
o P P e r
There are about 50 to 150 mg of copper in the adult human body in either cuprous
(Cu
1+
) or cupric (Cu
2+
) valence states. Copper functions as a structural component of,
or a cofactor for, several enzymes. Ceruloplasmin is an α-2 glycoprotein that trans-
ports copper in the blood but also has a ferroxidase function as ceruloplasmin-Cu
2+
oxidizes Fe
2+
to Fe
3+
and is reduced to ceruloplasmin-Cu
1+
. Superoxide dismutase is
a copper- (and zinc-) dependent enzyme that protects cell membranes against peroxi-
dative damage by superoxide radicals generated during inflammation. Cytochrome
c oxidase is a copper-containing enzyme that participates in electron transfer such
that molecular oxygen (O
2
) is reduced to form water, and energy is released to permit
ATP production. Some other copper-dependent enzymes are involved in amino acid
metabolism and influence synthesis of elastin and collagen—and thus the integrity
of connective tissue—and melanin, thus affecting skin and hair color.
Copper absorption is prominent in the duodenum but occurs throughout the small
intestine. It appears to be absorbed by an active, carrier-mediated transport system
