Agriculture Reference
In-Depth Information
dietary iodine allowances range from 90 µg per day for 1- to 3-year-old children to
290 µg per day for lactating women.
f
L u o r i D e
Fluorine (a gas) bound to inorganic or organic compounds is designated fluoride (F
−
),
the form that predominates in nature. Inclusion of fluoride among the essential nutri-
ents resides with its effectiveness in reducing the incidence of dental caries and its
possible role in maintaining skeletal integrity. During formation of apatite crystals in
bone and tooth enamel, fluoride can substitute for some of the hydroxyl ions, produc-
ing a fluorohydroxyapatite that is quite acid resistant. Mineralized tissues account for
about 99% of fluoride in the body.
Fluoride absorption appears to occur by passive diffusion in the stomach, where it
exists primarily as hydrogen fluoride (HF). Both ionic and nonionic fluoride is found
in the blood, with some of the latter bound to plasma proteins.
Foods are generally low in fluoride, although marine fish consumed with bones
(such as anchovies) and tea (which concentrates fluoride in leaves) may supply con-
siderable amounts. In the United States, many drinking water sources are fluoridated
by addition of sodium fluorosilicate. Some may enter the body from toothpaste con-
taining added sodium fluoride or monofluorophosphate. Estimated adequate fluoride
intakes range from 0.7 mg per day for 1- to 3-year-old children to 3.0 mg per day for
pregnant or lactating women and 4.0 mg per day for adult men.
m
o L y b D e n u m
Molybdenum is found primarily as Mo
4+
or Mo
6+
and is generally bound in biologi-
cal systems to sulfur or oxygen. It functions as a component of three enzymes that
catalyze oxidation-reduction reactions, xanthine dehydrogenase/oxidase, aldehyde
oxidase, and sulfite oxidase.
Absorption has not been well studied but is thought to be by passive diffusion.
Transport in the blood may be as molybdate (MoO
4
2−
), or molybdenum may be bound
to albumin or α-2 macroglobulin.
Molybdenum is found in many foods, but concentrations tend to vary regionally,
coincident with soil levels. Legumes, meat, poultry, and fish are good sources. Fruits
and dairy products are low in molybdenum. Recommended dietary molybdenum
allowances range from 17 µg per day for 1- to 3-year-old children to 50 µg per day
for pregnant or lactating women.
s
e L e n i u m
About 15 mg of selenium is found in the adult human body. Its chemical character-
istics are similar to those of sulfur, and it can replace sulfur in amino acids such as
cysteine, cystine, or methionine. Selenium is an essential cofactor for glutathione
peroxidases that accept reducing equivalents from glutathione to convert hydro-
gen peroxide to water or lipid peroxides to hydroxy lipids, thus protecting cellular
membranes against damage. Selenium-containing deiodinases catalyze removal of
