Effect of adding alpha-tocopherol on the oxidation advance during in vitro gastrointestinal digestion of sunflower and flaxseed oils

Few in vitro studies have tackled the effect of alpha-tocopherol on lipid oxidation during digestion, and discrepant results have been reported, As a result, the aim of this study was to elucidate whether the addition of alpha-tocopherol enhances or slows down the advance of oxidation that occurs during in vitro gastrointestinal digestion of polyunsaturated lipids. For this purpose, commercial sunflower and flaxseed oils (as models of omega-6 and omega-3 rich lipid systems, respectively) were in vitro digested in the absence or in the presence of this tocol at different concentrations (0.02%, 0.2% and 2%). Proton Nuclear Magnetic Resonance (H-1 NMR) and Solid Phase Microextraction followed by Gas Chromatography/Mass Spectrometry (SPME-GC/MS) were used to investigate in detail potential differences among the digests regarding lipolysis and oxidation level. Alpha-tocopherol addition did not affect the advance of lipolysis, whereas lipid oxidation was enhanced in a dose-dependent manner. In this regard, the increased degradation of polyunsaturated lipids and greater generation of primary and secondary oxidation products observed at higher concentrations of alpha-tocopherol confirmed this observation. Among the formed oxidation products, hydroperoxy-, hydroxy- and keto-dienes, as well as oxygenated alpha, beta-unsaturated aldehydes are worth mentioning. The in vitro bioaccessibility of added tocopherol was estimated to be very low, suggesting a notable transformation under the assayed conditions. Further in vivo studies are necessary to confirm this prooxidant activity of alpha-tocopherol during gastrointestinal digestion.