Agriculture Reference
In-Depth Information
at low luminal copper concentrations and by passive diffusion when luminal concen-
trations are high. Absorption may be enhanced by histidine, methionine, cysteine,
and several organic acids, such as acetic, citric, gluconic, lactic, and malic acids.
Inhibitors of copper absorption include excessive dietary concentrations of zinc,
iron, calcium, phosphorus, vitamin C, and antacids. Transport of copper through
the enterocyte has not been well characterized, but because free copper ions damage
cells, it is believed to be bound to amino acids, proteins, or peptides, such as gluta-
thione. Active transport across the basolateral membrane is the current presumption.
Copper is transported in the portal blood loosely bound to albumin.
Rich food sources of copper include organ meats and shellfish. Nuts, legumes,
potatoes, whole grains, and dried fruits are good sources. Copper sulfate is a pri-
mary supplemental form. Recommended dietary copper allowances range from 340
µg per day for 1- to 3-year-old children to 1300 µg per day for lactating women.
z
i n C
About 1.5 to 2.5 g of zinc are present in the adult human body, with most found
in bone, liver, kidneys, muscle, and skin. Different valence states exist, but most
zinc in the body is in the divalent (Zn
2+
) form. At least 70 enzymes are zinc depen-
dent, requiring this element as a structural component or as a cofactor. Functions are
diverse but include cellular respiration, conversion of retinol to retinal in the visual
cycle, cleavage of amino acids during protein digestion, defense against damage by
superoxide radicals, hydrolysis of glutamate residues from dietary folate to permit
its absorption, and support of nucleic acid metabolism. As a consequence, zinc is
important for cell growth and replication, formation of bone and integrity of skin,
immunity, hormone function, glucose tolerance, and many other processes.
Zinc is released from protein and nucleic acids in food by proteases and nucleases
in the stomach and small intestine, with absorption occurring mainly in the proxi-
mal small intestine. Glutathione, some amino acids, such as histidine and cysteine,
and endogenous chelators, such as citric acid and picolinic acid, appear to enhance
zinc absorption. Phytate, oxalate, polyphenols, and some soluble fibers appear to
inhibit absorption. High levels of divalent cations, like Fe
2+
, Cu
2+
, and Ca
2+
, also
may inhibit absorption by competing for binding ligands. Transport into the entero-
cyte appears to be carrier mediated at low intakes and by passive diffusion at high
intakes. Movement through the enterocyte and across the basolateral membrane is
presumed to involve a zinc transporter. High zinc intakes tend to induce thionein
formation, which when bound with metals is termed
metallothionein
. When bound
with zinc, it may serve as a tissue zinc reserve, or when bound with other metals,
it may modulate their absorption and toxicity. In the portal blood, zinc is mainly
loosely bound to albumin.
Zinc in animal products is typically complexed with amino acids and with nucleic
acids, and organ meats, beef, oysters, and mollusks are very good food sources.
Poultry, pork, dairy products, whole grains, and leafy and root vegetables also are
good sources. Fruits and refined cereals are poor, although selected varieties of rice
that contain 20-25 mg of zinc per kilogram have been identified. Many salts of zinc
have been used as supplements, but gastric irritation is a common side effect, and
