Minerals and Trace Elements 223selenium. Conversely, selenium modifi es the toxicity
of many heavy metals. In seafoods, selenium is com-
bined with mercury or methyl mercury and this inter-
action may be one of the factors that decreases the
bioavailability of selenium in these foods. Indeed,
well-known antagonistic interactions of selenium
with both mercury and arsenic suggest that selenium
can promote detoxifi cation effects with respect to
these toxins.
9.11 Iodine
Iodine is a nonmetallic element of the halogen
group with common oxidation states of −1 (iodides),
+5 (iodates), and +7 (periodates), and less common
states of +1 (iodine monochloride) and +3 (iodine
trichloride). Elemental iodine (0) is a soft blue–black
solid, which sublimes readily to form a violet gas.
The principal industrial uses of iodine are in the
pharmaceutical industry, medical and sanitary uses
(e.g., iodized salt, water treatment, protection from
radioactive iodine, and disinfectants), as catalysts
(synthetic rubber, acetic acid synthesis), and in animal
feeds, herbicides, dyes, inks, colorants, photographic
equipment, lasers, metallurgy, conductive polymers,
and stabilizers (nylon). Naturally occurring iodine
minerals are rare and occur usually in the form of
calcium iodates. Commercial production of iodine is
largely restricted to extraction from Chilean deposits
of nitrates (saltpeter) and iodine in caliche (soluble
salts precipitated by evaporation), and from concen-
trated salt brine in Japan. Iodine is the least abundant
halogen in the Earth’s crust, at concentrations of
0.005%. The content of iodine in soils varies and much
of the original content has been leached out in areas
of high rainfall, previous glaciation, and soil erosion.
The concentration of iodine (as iodide and iodate)
in the oceans is higher, at about 0.06 mg/l. Iodine
volatilizes from the surface of the oceans and sea
spray as salt particles, iodine vapor or methyl iodide
vapor. Some iodine can then return to land in rain-
water (0.0018–0.0085 mg iodine/l). There is a large
variation of iodine content in drinking water (0.0001–
0.1 mg iodine/l).
Absorption, transport, and
tissue distribution
Iodine, usually as an iodide or iodate compound in
food and water, is rapidly absorbed in the intestine
and circulates in the blood to all tissues in the
body. The thyroid gland traps most (about 80%) of
the ingested iodine, but salivary glands, the gastric
mucosa, choroid plexus, and the lactating mammary
gland also concentrate the element by a similar active
transport mechanism. Several sulfur-containing
compounds, thiocyanate, isothiocyanate, and goitrin
inhibit this active transport by competing for uptake
with iodide, and their goitrogenic activity can be
overcome by iodine supplementation. These active
goitrogens are released by plant enzymes from thio-
glucosides or cyanogenic glucosides found in cassava,
kale, cabbage, sprouts, broccoli, kohlrabi, turnips,
swedes, rapeseed, and mustard. The most important
of these goitrogen-containing foods is cassava, which
can be detoxifi ed by soaking in water. Tobacco smoke
also contributes thiocyanate and other antithyroid
compounds to the circulation.Metabolic functions and essentiality
Iodine is an essential constituent of the thyroid
hormones, thyroxine (T 4 ) and triiodothyronine
(T 3 ), which have key modifying or permissive
roles in development and growth. Although T 4 is
quantitatively predominant, T 3 is the more active.
The mechanism of action of thyroid hormones
appears to involve binding to nuclear receptors that,
in turn, alter gene expression in the pituitary, liver,
heart, kidney, and, most crucially, brain cells. Overall,
thyroid hormones stimulate enzyme synthesis, oxygen
consumption and basal metabolic rate and, thereby,
affect the heart rate, respiratory rate, mobilization,
and metabolism of carbohydrates, lipogenesis and a
wide variety of other physiological activities. It is
probable that iodine has additional roles to those of
thyroid hormone activity, for example in antibiotic
and anticancer activity, but these are poorly
understood.
Once iodide (−1) is trapped from the circulation
and actively transported to the lumen of the thyroid
gland, it is oxidized to I 2 (0) and reacts with tyrosine
in thyroglobulin protein to form monoiodotyrosine
or diiodotyrosine. These reactions are catalyzed by
thyroid peroxidase. The iodinated compounds, in
turn, couple to form T 3 and T 4 , which are secreted
from the thyroid into the circulation.
Flavonoids, found in many plants, including pearl
millet, and phenol derivatives, released into water
from soil humus, inhibit thyroid peroxidase and the