Vitamin E is a term used to describe eight isomers (α-, β-, γ- and δ-tocopherol and α-, β-, γ- and δ-tocotrienol), which all have the biological activity of α-tocopherol. The isomers may have slightly different effects in the body. For example, γ-tocopherol may be more effective that α-tocopherol at inhibiting some cancer cell types, at protecting DNA, decreasing free radicals and at decreasing inflammation; and δ‑tocopherol may have stronger anti-proliferative effects on cancer cells than α- and γ‑tocopherol. Vitamin E isomers vary in their concentrations in foods, with α-tocopherols being the main form of vitamin E in wheat germ, almond and sunflower oil, but γ‑tocopherol being the main form of vitamin in corn and soybean oil. The main form of vitamin E in plasma is α-tocopherol, but the typical US diet may possess as much as 70 % of the vitamin E as γ-tocopherol.
Supplements generally contain α-tocopherol, and this is invariably the form used in large scale studies. However, research suggests that high intakes of α-tocopherol decreases the levels of other isomers of vitamin E in humans. For example, research published in the Journal of Nutrition in 20031 investigated the effect of supplemental α-tocopherol on the serum concentrations of γ- and δ-tocopherol. The researchers found that supplementing 184 adult non-smokers with 400 IU per day of α-tocopherol decreased γ-tocopherol concentration by 58 % and reduced the number of individuals with detectable levels of δ‑tocopherol. The researchers suggest that the inconsistent results detected in large scale trials using vitamin E may result from the deleterious effect of α-tocopherol on other isomers of vitamin E. Because of the known benefits of γ- and δ-tocopherol, supplementation with mixed tocopherols is recommended.
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