Agriculture Reference
In-Depth Information
beef, eggs, and fortified milk (400 IU/quart) in the United States. Some other foods,
such as cereals, bread, and margarine, also may be fortified. Estimated adequate
dietary intakes range from 200 IU per day for children to 600 IU per day for adults
over the age of 70. It has been estimated that 10 minutes of summer sun exposure of
the hands and face will supply about 400 IU, but latitude and time of day (as well as
season) may greatly affect this estimate.
v
i t a m i in
e
The term
vitamin E
may be applied to eight related natural compounds: α-, β-, γ-, and
δ-tocopherol and α-, β-, γ-, and δ-tocotrienol. Unlike most vitamins that have specific
metabolic roles or that function as cofactors, vitamin E serves as a chain-breaking
antioxidant to protect unsaturated fatty acids in cell membrane phospholipids from
oxidative damage by scavenging peroxyl radicals that originate from metabolic reac-
tions. Most effective in humans is
RRR
-α-tocopherol (the natural form, also known
as d-α-tocopherol), which has a stereoisomeric
R
-coniguration at 2, 4′, and 8′ posi-
tions in the tocopherol molecule. Synthetic α-tocopherol (
all racemic
or dl) has eight
potential stereoisomers, with less biological activity than the natural form and with
activity dependent on the
R
-coniguration in position 2. Interaction of α-tocopherol
with peroxyl radicals results in its oxidation, but it can be restored to its reduced,
and functional, form by vitamin C or glutathione. Selenium-dependent glutathione
peroxidase also has antioxidant functions in the body, so the quantitative supply of
selenium influences the quantitative need for vitamin E and vice versa.
Steps in the digestion and absorption of vitamin E compounds differ somewhat,
depending on chemical form. Tocopherols are found as free alcohols in food, but
tocotrienols are commonly esterified. Thus, ester bonds must be hydrolyzed by
pancreatic esterase or duodenal mucosal esterase before absorption can take place.
This is also true for synthetic ester forms of tocopherols, such as
all rac-
or dl-α-
tocopheryl acetate.
Absorption of vitamin E alcohols from micelles in the intestinal lumen occurs
primarily in the jejunum by passive diffusion. Absorbed vitamin E alcohols are
incorporated into chylomicrons within the enterocyte and reach the liver via the
lymph and general circulation. Equilibration with or transfer to plasma lipoproteins
may occur during transport. Delivery of vitamin E to extrahepatic tissues appears
restricted to
RRR-
α-tocopherol, which is incorporated into very low density lipo-
proteins (VLDLs) and bound to a very specific protein made in the liver called
α-tocopherol transfer protein (αTPP). Other vitamin E forms are poorly recognized
by this transfer protein. A genetic defect in the ability of the liver to synthesize αTPP
may result in vitamin E deficiency.
Plants, particularly green leaves and seed oils, are good sources of vitamin E,
with α-tocopherol predominating in green leaves and in canola, cottonseed, olive,
safflower, and sunflower oils. Corn and soybean oils contain some α-tocopherol, but
γ-tocopherol predominates. Vitamin E levels in animal products tend to be low and
are concentrated in fatty tissues, with most as α-tocopherol. Recommended dietary
allowances range from 6 mg per day for 1- to 3-year-old children to 19 mg per day
for lactating women.
