The difference in transfer of all-rac-a-tocopherol stereo-isomers to milk from cows and the effect on its oxidative stability

Milk from cows fed high concentration of synthetic all-rac-a-tocopheryl acetate (2-3 g day-1) were shown not to have improved oxidative stability, when oxidation were initiated by light exposure or Cu(II) addition. For milk with high fraction of unsaturated lipids as obtained through feed with toasted soybeans, the high a-tocopherol content even acted prooxidative. Stereo-selective analysis of the milk showed that among the 8 stereo-isomers of all-rac-a-tocopherol, only the 2R-stereo-isomers were excreted into the milk and among these mainly the native RRR-stereo-isomer. Feeding studies of dairy cows should accordingly be reevaluated in two respect: (i) the use of high level supplementation of a-tocopherol, as it may induce lipid oxidation, (ii) the use of synthetic, none-resolved a-tocopherol as mainly the native RRR-stereo-isomer, only partly the three other 2R-stereo-isomers, and none of the three 2S-stereo-isomers are excreted into the milk.