Tocopherols (Vitamin E)
Introduction
The term “Vitamin E” refers to a class of lipid-soluble
antioxidants consisting of four tocopherol isomers and four tocotrienol isomers.
Vitamin E is present in all cell membranes, plasma lipoproteins and red
blood cells. As the major lipid-soluble chain-breaking antioxidant in
humans, it functions to protect proteins, DNA, low density lipoproteins
(LDL) and polyunsaturated fatty acids (PUFAs) from free radical-induced
oxidation. Vitamin E also quenches singlet oxygen.47 In this regard, d-alpha-tocopherol is
the most biologically active isomer.47
Absorption, Metabolism and Distribution in Humans
Along with dietary fat, vitamin E is absorbed in the intestine and incorporated
into chylomicrons and transported via the lymph system where it is associated
with erythrocytes, very low, low and high density lipoproteins (VLDL,
LDL, HDL). Half of serum alpha-tocopherol resides in
the HDL sub-fraction.55 Therefore, vitamin E levels are highly correlated
with total serum lipids. Approximately 40% of ingested vitamin E gets
absorbed, although the percentage is affected by the presence of PUFAs,
which decrease absorption efficiency. Long chain dietary triglycerides
enhance absorption. The different isomers are also absorbed with different
efficiency.55
Iron supplements destroy vitamin E. High intakes of vitamin A reduce the uptake of vitamin E, whereas high doses of vitamin E can impair the absorption of vitamin K.56 Higher levels of delta- and gamma-tocopherols have been detected in serum having lower levels of alpha-tocopherol, suggesting that absorption and/or release from body pools is influenced by the presence of other isomers.
Effect of Food Processing, Cooking and Storage
Due to oxidation, the vitamin E content of food decreases upon exposure
to oxygen. This process is accelerated by heat and light. Therefore,
various amounts of vitamin E are lost from food during cooking, processing
and storage at room temperature. Although minimal, loss of vitamin E
also occurs during freezing.55
Dietary Sources
Dietary sources include vegetable oils such as safflower, peanut and
sunflower oils. Other sources include mayonnaise, margarine, broccoli,
asparagus, apples, brown rice, bananas, strawberries, spinach, peas,
broccoli, baked potatoes, whole milk, liver, shrimp, chicken, steak,
egg yolk, fish, nuts, oatmeal, cornflakes and wheat germ.55
Antioxidant Defense: Cancer and Disease Prevention
Inverse
correlations between serum levels of vitamin E and the incidence of diseases
such as arthritis, cancer, cataracts and age-related macular degeneration
have been established. The risks of developing coronary diseases such
as ischemic heart disease,19 atherosclerosis52 and angina pectoris53
were shown to be dramatically reduced in individuals with a high vitamin
E status. Moreover, the ability of vitamin E and its derivatives to inhibit
mutagenesis and chromosomal damage caused by radiation and chemical damage
in in vitro studies has been demonstrated.43 Additionally, Vitamin
E can moderate increases in lipid peroxidation resulting from heavy exercise.54
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