Vitamin E is found naturally in some foods, added to others, and available as a dietary supplement. “Vitamin E” is the collective name for a group of fat-soluble compounds with distinctive antioxidant activities .
Naturally occurring vitamin E exists in eight chemical forms (alpha-, beta-, gamma-, and delta-tocopherol and alpha-, beta-, gamma-, and delta-tocotrienol) that have varying levels of biological activity . Alpha- (or α-) tocopherol is the only form that is recognized to meet human requirements.
Serum concentrations of vitamin E (alpha-tocopherol) depend on the liver, which takes up the nutrient after the various forms are absorbed from the small intestine. The liver preferentially resecretes only alpha-tocopherol via the hepatic alpha-tocopherol transfer protein ; the liver metabolizes and excretes the other vitamin E forms . As a result, blood and cellular concentrations of other forms of vitamin E are lower than those of alpha-tocopherol and have been the subjects of less research [3,4].
Antioxidants protect cells from the damaging effects of free radicals, which are molecules that contain an unshared electron. Free radicals damage cells and might contribute to the development of cardiovascular disease and cancer . Unshared electrons are highly energetic and react rapidly with oxygen to form reactive oxygen species (ROS). The body forms ROS endogenously when it converts food to energy, and antioxidants might protect cells from the damaging effects of ROS. The body is also exposed to free radicals from environmental exposures, such as cigarette smoke, air pollution, and ultraviolet radiation from the sun. ROS are part of signaling mechanisms among cells.
Vitamin E is a fat-soluble antioxidant that stops the production of ROS formed when fat undergoes oxidation. Scientists are investigating whether, by limiting free-radical production and possibly through other mechanisms, vitamin E might help prevent or delay the chronic diseases associated with free radicals.
In addition to its activities as an antioxidant, vitamin E is involved in immune function and, as shown primarily by in vitro studies of cells, cell signaling, regulation of gene expression, and other metabolic processes . Alpha-tocopherol inhibits the activity of protein kinase C, an enzyme involved in cell proliferation and differentiation in smooth muscle cells, platelets, and monocytes . Vitamin-E–replete endothelial cells lining the interior surface of blood vessels are better able to resist blood-cell components adhering to this surface. Vitamin E also increases the expression of two enzymes that suppress arachidonic acid metabolism, thereby increasing the release of prostacyclin from the endothelium, which, in turn, dilates blood vessels and inhibits platelet aggregation .
Sources of Vitamin E
Numerous foods provide vitamin E. Nuts, seeds, and vegetable oils are among the best sources of alpha-tocopherol, and significant amounts are available in green leafy vegetables and fortified cereals (see Table 2 for a more detailed list) . Most vitamin E in American diets is in the form of gamma-tocopherol from soybean, canola, corn, and other vegetable oils and food products .
|Table 2: Selected Food Sources of Vitamin E (Alpha-Tocopherol) |
|Wheat germ oil, 1 tablespoon||20.3||100|
|Sunflower seeds, dry roasted, 1 ounce||7.4||37|
|Almonds, dry roasted, 1 ounce||6.8||34|
|Sunflower oil, 1 tablespoon||5.6||28|
|Safflower oil, 1 tablespoon||4.6||25|
|Hazelnuts, dry roasted, 1 ounce||4.3||22|
|Peanut butter, 2 tablespoons||2.9||15|
|Peanuts, dry roasted, 1 ounce||2.2||11|
|Corn oil, 1 tablespoon||1.9||10|
|Spinach, boiled, ½ cup||1.9||10|
|Broccoli, chopped, boiled, ½ cup||1.2||6|
|Soybean oil, 1 tablespoon||1.1||6|
|Kiwifruit, 1 medium||1.1||6|
|Mango, sliced, ½ cup||0.7||4|
|Tomato, raw, 1 medium||0.7||4|
|Spinach, raw, 1 cup||0.6||3|
*DV = Daily Value. DVs were developed by the FDA to help consumers compare the nutrient content of different foods within the context of a total diet. The DV for vitamin E is 30 IU (approximately 20 mg of natural alpha-tocopherol) for adults and children age 4 and older. However, the FDA does not require food labels to list vitamin E content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.
The U.S. Department of Agriculture’s (USDA’s) Nutrient Database Web site lists the nutrient content of many foods, including, in some cases, the amounts of alpha-, beta-, gamma-, and delta-tocopherol. The USDA also provides a comprehensive list of foods containing vitamin E arranged by nutrient content and by food name.