Agriculture Reference
In-Depth Information
zinc acetate seems to be the best tolerated. Recommended dietary zinc allowances
range from 3 mg per day for 1- to 3-year-old children to 12-14 mg per day for lactat-
ing women.
m
a n g a n e s e
About 10 to 20 mg of manganese is found in the body of a 70-kg adult male human,
typically as Mn
2+
or Mn
3+
. Manganese functions as a constituent of metalloenzymes
or as an enzyme cofactor. Some of the transferases that are activated by manganese
can also be activated by magnesium. However, several enzymes concerned with con-
nective tissue metabolism, inhibition of lipid peroxidation, and regulation of cal-
cium-dependent processes specifically require manganese.
Mechanisms of manganese absorption are poorly understood. A quickly satu-
rable, active transport system has been demonstrated in rats, and at high intakes,
absorption efficiency declines, possibly to protect against manganese toxicity. Low
molecular weight ligands, such as citrate or histidine, may enhance absorption,
whereas phytate, oxalate, nonsoluble fiber, and excess iron or copper may inhibit it.
Manganese entering the portal blood as Mn
2+
may remain free or be bound to α-2
macroglobulin. After traversing the liver, Mn
2+
may be transported to other tissues
in free form, bound to α-2 macroglobulin or albumin, or oxidized to Mn
3+
by ceru-
loplasmin and then complexed with transferrin.
Foods that are relatively high in manganese include whole-grain breads and cere-
als, nuts, leafy vegetables, and dried fruits. Milk and cheeses are low. Estimated
adequate manganese intakes range from 1.2 mg per day for 1- to 3-year-old children
to 2.6 mg per day for lactating women.
i
of D i n e
Iodine is typically found as iodide (I
−
) in the body. Its primary function resides in its
role as a constituent of the thyroid hormones thyroxine (T
4
) and triiodothyronine (T
3
)
and their regulation of metabolic rate.
Iodine in the diet may exist free as I
−
or iodate (IO
3−
) or bound to amino acids.
During digestion, dietary IO
3−
is reduced to I
−
by glutathione. Iodide is readily
absorbed from the stomach as well as from the intestinal tract. Iodinated amino
acids and T
4
and T
3
may be absorbed intact, thus allowing oral administration of T
4
medication. Iodine in the portal blood may be present as I
−
or as T
4
and T
3
.
Terrestrial plant food sources are variable in their iodine concentration, reflecting
regional differences in the concentration of iodine in soil. Likewise, iodine in drink-
ing water is a reflection of the origin of rock strata from which water is withdrawn.
Animals raised for food receive diets supplemented with salts of iodine, and iodized
NaCl is commonly used for salting food at the table in the United States. Iodate also
may be added as an oxidizing agent to bread dough to improve the physical cross-
linking of gluten. Since oceans contain considerable iodine, edible portions of salt-
water fish may have 300 to 3000 µg of iodine per kilogram compared to 20 to 40 µg
per kilogram in freshwater fish. Iodophors used for cleansing cattle udders and milk-
ing equipment may increase iodine concentrations in dairy products. Recommended
