Dietary Sources

The composition and content of the different toco-pherol components in plant tissue vary considerably, ranging from extremely low levels found in potato tubers to high levels found in oil seeds. a-Tocopherol is the predominant form in photosynthetic tissues and is mainly localized in plastids. The particular enrichment in the chloroplast membranes is probably related to the ability of tocopherols to quench or to scavenge reactive oxygen species and lipid peroxy radicals by physical or chemical means. In this way, the photosynthetic apparatus can be protected from oxygen toxicity and lipid peroxidation. In nonphoto-synthetic tissues, 7-tocopherol frequently predominates and can be involved in the prevention of autoxidation of polyunsaturated fatty acids.

Most of the tocopherol content of wheat germ, sunflower, safflower, and canola and olive oils is in the form of a-tocopherol, and these oils contain approximately 1700, 500, 350, 200, and 120 mg a-TEkg-1, respectively. Vegetable oils (e.g., corn, cottonseed, palm, soybean, and sesame) and nuts (e.g., Brazil nuts, pecans, and peanuts) are rich sources of 7-tocopherol. Corn and soybean oils contain 5-10 times as much 7-tocopherol as a-tocopherol-rich sources of 7-tocopherol, and each contains approximately 200 mg a-TEkg-1. Because of the widespread use of these plant products, 7-tocopherol is considered to represent ^70% of the vitamin E consumed in the typical US diet. The level of vitamin E in nuts ranges from 7mg a-TEkg-1 in coconuts to 450 mg a-TEkg-1 in almonds. Cereals are moderate sources of vitamin E, providing between 6 (barley) and 23 mg a-TEkg-1 (rye). Fresh fruit and vegetables generally contain approximately 1-10 mg a-TEkg-1. The concentration of vitamin E (a-tocopherol is the predominant form) in animal products is usually low, but these may be significant dietary sources because of their high consumption.

Mean dietary intakes of 6.3-13.0 mg a-TE per day have been reported in various European and US population studies. Data from the Third National Health and Nutrition Examination Survey

Compound

R1

R2

R3

«-Tocopherol

CH3

CH3

CH

^-Tocopherol

CH3

H

CH

7-Tocopherol

H

CH3

CH

¿-Tocopherol

H

H

CH

Figure 1 The four major forms of vitamin E (a-, ,3-, 7-, and ¿-tocopherols) differ by the number and positions of methyl groups on the chromonol ring. In a-tocopherol, the most biologically active form, the chromonol ring is fully methylated. In ,3- and 7-tocopherols, the ring contains two methyl groups, whereas ¿-tocopherol is methylated in one position. The corresponding tocotrienols have the same structural arrangement except for the presence of double bonds on the isoprenoid side chain of C31, C71, and C111.

Figure 1 The four major forms of vitamin E (a-, ,3-, 7-, and ¿-tocopherols) differ by the number and positions of methyl groups on the chromonol ring. In a-tocopherol, the most biologically active form, the chromonol ring is fully methylated. In ,3- and 7-tocopherols, the ring contains two methyl groups, whereas ¿-tocopherol is methylated in one position. The corresponding tocotrienols have the same structural arrangement except for the presence of double bonds on the isoprenoid side chain of C31, C71, and C111.

(NHANES III) (1988-1994) in the United States indicate a median total intake (including supplements) of a-TE of 12.9 mg day-1 and a median intake from food only of 11.7 mg day-1 in men aged 31-50 years. In women in this age range, the median total intake (including supplements) of a-TE was 9.1 mg day-1 and the median intake from food only was 8.0 mg day-1. In the United States, fats and oils used in spreads, etc. contribute 20.2% of the total vitamin E intake; vegetables, 15.1%; meat, poultry, and fish, 12.6%; desserts, 9.9%; breakfast cereals, 9.3%; fruit, 5.3%; bread and grain products, 5.3%; dairy products, 4.5%; and mixed main dishes, 4.0%.

The North/South Ireland Food Consumption Survey, published in 2001, reported that the median daily intake of vitamin E from all sources was 6.3mg in men and 6.0mg in women aged 1864 years. The largest contributors of vitamin E to the diet were vegetables and vegetable dishes (18.9%) and potatoes and potato products (12.4%), most likely as a result of the oils used in composite dishes. Nutritional supplements contributed 5.5% of the vitamin E intake in men and 11.9% in women overall. In the subgroup that regularly consumed nutritional supplements (23% of total), vitamin E was the nutrient most frequently obtained in supplemental form in men (78%) and women (73%). In these people, supplements made a larger contribution to total vitamin E intakes than did food.

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Dieting Dilemma and Skinny Solutions

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