CHAPTER 16. Essentiality, Bioavailability, and Health Benefits of α-Tocopherol Stereoisomers

Vitamin E is the term that describes eight structurally-related vitamers, specifically four tocopherols (α-, β-, γ-, and δ-) and four tocotrienols (α-, β-, γ-, and δ-). Although each vitamer has antioxidant activity that prevents the cyclic progression of lipid peroxidation, α-tocopherol is the only form that is essential for humans because it prevents and reverses clinical deficiency of vitamin E. These vitamers also differ substantially in their bioavailability, which results in α-tocopherol accumulating to the greatest extent in the circulation and tissues in humans. In nature, α-tocopherol exists as a single stereoisomer (2R, 4′R, 8′R- or RRR-α-tocopherol) whereas most dietary supplements and fortified foods contain an all racemic mixture of eight α-tocopherol stereoisomers. Importantly, only 50% of these stereoisomers are appreciably bioavailable, and therefore capable of contributing to dietary α-tocopherol requirements in humans. This chapter will therefore focus on the bioavailability, metabolism, and trafficking of vitamin E forms along the gut–liver axis. It will also discuss their structure–function aspects concerning their ability to contribute to dietary vitamin E requirements and associated health benefits.