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 esterases located in the stomach lining. It is then packaged
into very low-density lipoproteins (VLDL) by the addition of lipid-like substances.
VLDLs enter the lymphatics and are eventually released into the bloodstream.
The alpha-tocotrienol appears to be better absorbed than the other tocopherol
forms.
Transport
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, and VLDLs.
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
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 versions.
Metabolism
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 feces.