Agriculture Reference
In-Depth Information
iodine from T
4
to yield T
3
, a major active hormone form. Thioredoxin reductase
contains selenocysteine and transfers reducing equivalents from NADPH to produce
NADP
+
. Selenophosphate synthetase (containing selenocysteine) catalyzes synthesis
of selenophosphate from selenide (H
2
Se), a compound needed for synthesis of the
mentioned selenium-activated or selenium-containing enzymes. Other selenium-
containing enzymes that play antioxidant roles also have been discovered.
Selenium absorption appears to occur mainly in the duodenum, with less in the
jejunum and ileum. Organic forms, such as selenocysteine, selenocystine, or sele-
nomethionine, enter the enterocyte via an amino acid transporter, whereas inorganic
forms appear to cross the luminal membrane as selenite (H
2
SeO
3
) or as selenate
(H
2
SeO
4
). On entering the portal blood, selenium binds to sulfhydryl groups in lipo-
proteins or to selenoprotein P for transport in plasma.
Selenium levels in plant foods vary regionally with available selenium concentra-
tions in soil, and geographical areas of deficiency and toxicity have been identified.
Animal food products tend to be less variable because of the need to supplement
animal diets to prevent regional deficiencies. Seafoods may be among the better
sources due to selenium supplies in the ocean, although elevated mercury levels in
some saltwater fish may reduce selenium bioavailability. Forms in food include the
selenium analogues of sulfur-containing amino acids and inorganic selenites and
selenates. Recommended dietary selenium allowances range from 20 µg per day for
1- to 3-year-old children to 70 µg per day for lactating women.
C
h r of m i u m
The chromium content of the adult human body has been estimated to be about 4 to
6 mg. In nature, chromium exists in several valence states, but trivalent chromium
(Cr
3+
) is the most stable and is thought to be the most important functional form. It
potentiates the action of insulin by mechanisms still under investigation but which
may involve Cr
4
-chromodulin (four chromium atoms bound to an oligopeptide com-
posed of glycine, cysteine, aspartate, and glutamate).
Absorption of chromium may require formation of complexes with low molecular
weight ligands (e.g., methionine or histidine) in the acid environment of the stom-
ach (in which Cr
3+
is soluble), followed by movement into the enterocyte by either
diffusion or a carrier-mediated transporter. Absorption is enhanced by picolinate
and ascorbate and inhibited by phytate and antacids. Inorganic chromium (Cr
3+
) is
transported in the blood by transferrin and possibly by albumin or globulins. Some
may circulate unbound.
Good food sources include liver, beef, pork, poultry, whole grains, cheese, mush-
rooms, some spices, tea, and wine. An organic complex of chromium (glucose tolerance
factor) is found in brewer's yeast. Estimated adequate chromium intakes range from
11 µg per day for 1- to 3-year-old children to 45 µg per day for lactating women.
conclusIons And study toPIcs
Diseases of excess and deficiency of vitamins and mineral elements are addressed
in Chapters 9 and 10, respectively. The provision of essential vitamins and minerals
