Introduction to Human Nutrition

Minerals and Trace Elements 217

appreciable concentrations of copper. Particularly
rich food sources of copper include offal, seafood,
nuts, seeds, legumes, wholegrain cereals, and choco-
late. Milk and dairy products are very low in copper
and infants are at risk of copper defi ciency if they are
fed exclusively on cow’s milk.

Micronutrient interactions

The major micronutrient interactions with copper
are those involving zinc and iron, high intakes of
which can restrict copper utilization in infants and
adults. The mechanism by which zinc appears to exert
on antagonistic effect on copper status is through
the induction of metallothionein synthesis by zinc in
mucosal cells in the intestine. Metallothionein has a
particularly strong affi nity for copper. Metallothionein-
bound copper is not available for transport into the
circulation and is eventually lost in the feces when the
mucosal cells are sloughed off. Molybdenum also has
a strong interaction with copper and thiomolybdates
are potent systemic copper antagonists. Although
both cadmium and lead can inhibit copper utiliza-
tion, this inhibition only occurs at dietary intakes of
these heavy metals above those normally consumed
by humans. Vitamin E, selenium, and manganese
have metabolic interactions with copper as antioxi-
dants, but data on benefi cial interactions of these on
symptoms of copper defi ciency are largely confi ned
to animal studies. Copper defi ciency exerts an effect
on iodine metabolism resulting in hypothyroidism, at
least in animal models.

9.10 Selenium

Selenium is a nonmetallic element that has similar
chemical properties to sulfur and has four natural
oxidation states (0, −2, +4, +6). It combines with other
elements to form inorganic selenides [sodium
selenide (−2) Na 2 Se], selenites [sodium selenite
(+4) Na 2 SeO 3 ] and selenates [sodium selenate (+6)
Na 2 SeO 4 ], and with oxygen to form oxides [selenium
(+4) dioxide SeO 2 ] and oxyacids [selenic (+6) acid
H 2 SeO 4 ]. Selenium replaces sulfur to form a large
number of organic selenium compounds, parti-
cularly as selenocysteine, the twenty-fi rst amino acid.
Selenium is a component of selenoproteins, where it
also occurs as selenides on the side-chains of seleno-
cysteine at physiological pH. Selenium also displaces
sulfur to form the amino acid selenomethionine.

Elemental selenium is stable and has three allotropic

forms, deep red crystals, red amorphous powder, and

the black vitreous form.

Selenium has many industrial uses, e.g., in elec-

tronics, glass, ceramics, pigments, as alloys in steel, as

catalysts in pharmaceutical production, in rubber

vulcanization and in agriculture, as feed supplements

and fertilizers. Because of its increasing use, selenium

has become a potential health and environmental

hazard. The primary pathway of exposure to selenium

for the general population is food, followed by water

(predominantly inorganic selenate and selenite), and

air (mainly as elemental particulate selenium from

combustion of fossil fuels and from volcanic gas).

Absorption, transport and

tissue distribution

Absorption of dietary selenium takes place mainly in

the small intestine, where some 50–80% is absorbed.

Organic forms of selenium are more readily absorbed

than inorganic forms and selenium compounds from

plants are generally more bioavailable than those

from animals, and particularly from fi sh. Some natu-

rally occurring inorganic and organic compounds of

selenium are given in Table 9.17.

The bioavailability of selenium from water (mainly

inorganic selenates) and supplements is lower than

from food. The overall bioavailability of selenium

from the diet depends on a number of factors, includ-

ing selenium status, lipid composition, and metals.

Inorganic forms of selenium are passively trans-

ported across the intestinal brush border, whereas

organic forms (selenomethionine and probably sele-

nocysteine) are actively transported. On reaching the

bloodstream, selenium is transported largely bound

to protein (mainly very low-density β-lipoprotein

with a small amount bound to albumin) for

Table 9.17 Some naturally occurring inorganic and organic com-

pounds of selenium

Selenite {SeO 32 −]

Selenate {SeO 42 −]

Methylselenol (CH 3 SH)

Dimethylselenide (CH 3 -Se-CH 3 )

Trimethyselenonium ion [(CH 3 ) 3 -Se+]

Selenocysteine

Selenomethionine

Se-Methyl-selenocysteine