Vitamin E is a generic term used for
a group of chemically-similar compounds sharing the
tocopherol and tocotrienol structures, which are
lipid-soluble; hence, vitamin E is a fat-soluble
vitamin that exists in eight different forms. Each
form has its own biological activity, which is the
measure of potency or functional use in the body.
Alpha-tocopherol (α-tocopherol) is the name of the
most active form of vitamin E in humans.
Vitamin E is also a powerful biological antioxidant
which protects against lipid peroxidation (which
could contribute to cell membrane weakness).
Vitamin E in supplements is usually sold as
alpha-tocopheryl acetate, a form that protects its
ability to function as an antioxidant.
The synthetic form is labeled "D, L" while the
natural form is labeled "D". The synthetic form is
only half as active as the natural form.
The vitamin can exist as
two types of structures:
The tocopherol and
tocotrienol structures. Both structures are similar
except the tocotrienol structure has double bonds on
the isoprenoid units. There are many derivatives of
these structures due to the different subsistent
possible on the aromatic ring at positions 5, 6, 7,
Notice that there are three chiral centers, at
positions 2', 4', and 8', in the phytyl tail. There
is thus the possibility of eight stereoisomer's. The
most abundant of the naturally-occurring forms is
the R,R,R form.
The tocotrienols share the same ring
structure, but have an unsaturated tail.
methyl groups on aromatic ring
(Vitamin E) are equipped to perform a unique
function. They can interrupt free radical chain
reactions by capturing the free radical; this
imparts to them their antioxidant properties. The
free hydroxyl group on the aromatic ring is
responsible for the antioxidant properties. The
hydrogen from this group is donated to the free
radical, resulting in a relatively stable free
radical form of the vitamin.
Physiological Aspects of Vitamin E: Absorption:
The process of absorption is passive and does not
require the use of a protein carrier to bring in the
vitamin. Absorption occurs in the small intestine
and the vitamin can only be absorbed if it has been
cleaved by esterase located in the stomach lining.
The vitamin is then packaged into very low-density
lipoproteins (v LDL) by the addition of lipid-like
substances. vLDLs enter the lymphatic cells and are
eventually released into the bloodstream. The
alpha-tocotrienol appears to be better absorbed than
the other tocopherol forms.
Vitamin E does not have a specific carrier protein
in the bloodstream but it is transferred by hepatic
and lymphatic mechanisms. When it is first absorbed
into the hepatic portal vein it is contained inside
a lipid like structure called a "chylomicron". This
structure is then converted hepatically to three
distinct lipoprotein structures: high-density lipoprotein)(HDL),
low-density lipoprotein (LDL),
The alpha-tocotrienol is the form most rapidly
secreted into the plasma after uptake.
The transport process is an important aspect in the
delivery of vitamin E. Without the protective
barrier of the lipoproteins (HDL, LDL, and VLDL),
the vitamin would be exposed to the oxidative
radicals circulating through the blood.
Additionally, vitamin E will prevent the oxidation
of LDL; oxidized LDL is thought to be a factor in
atherogenesis, so adequate amounts of vitamin E will
help protect against hardening of the arteries,
heart attacks, and stroke.
Tissue uptake occurs by one of two ways: by lipases
digesting the lipoprotein constituents or by
"receptor mediated uptake" by binding of the
lipoprotein to a specific tissue receptor site. This
allows for the vitamin to enter the tissue.
Vitamin E enters a variety of different tissue
types, with adipose and the adrenal gland having the
highest levels. It is found primarily in
mitochondria. It is thought to play a role there in
either stabilizing ubiquinone, or in helping
ubiquinone transfer electrons.
The body's capacity for storage of the vitamin is
worth noting. The vitamin can be stored in tissue
for long periods of time (years) due to its
exceedingly slow turnover rate. Interestingly, the
natural R,R,R-alpha-tocopherol form of the vitamin
is stored up to six times longer than synthetic
Vitamin E is considered to be metabolized after it
has performed its antioxidant function (see Vitamin
E Chemistry). It is converted from a tocopherol to a
tocopherylquinone. The elimination of this end
product is primarly through the feces but a small
fraction is removed by urine (less than 1 percent).
In order for tocopherylquinone to be excreted, it
first has to be converted to tocopherylhydroquinone,
a partially reduced form. This form can then
combined with glucuronic acid so that it can mix
with bile. Bile is removed from the body through
If you don't want to take vitamin E supplements then
consider wheat germ and wheat germ oil . These are
great sources of natural vitamin E. Because they are
rich in vitamin E you can raise your intake without
taking in too much fat.
Wheat germ oil Extracted from the germ of wheat,
Wheat germ oil has been used since the '20s as a
vitamin E supplement. It offers a good combination
of tocopherols plus tocotrienols.
Foods containing wheat are a good source of vitamin
E. These foods vary in their content of vitamin E
based on the particular source and processing
involved. Wheat germ oil is the richest source of
natural vitamin E. If the wheat product is processed
to make other foods such as margarine, the content
of vitamin E is reduced due to the methods involved
in formulation and exposure to chemicals (acids and
bases) that can destroy vitamin E.