Vitamin E is an essential nutrient that must be acquired regularly from dietary sources. It is a group of compounds consisting of tocotrienols and tocopherols. The structure of tocotrienols differs from that of tocopherols by the presence of 3 trans-double bonds in the hydrocarbon chain. In this study, a reverse-phase (RP) HPLC method was used to simultaneously measure the contents of either tocopherols or tocotrienols in brown rice, and to compare these parameters among 18 indica and 16 japonica rice (Oryza sativa L.) varieties. The results showed that the proportion of these vitamin E isomers differed substantially between indica and japonica rice varieties. The mean contents of vitamin E and total tocopherol were significantly higher (P < 0.01) in japonica rice than in indica rice, while the total tocotrienol content showed no difference between indica and japonica rice. The principal isomer composition of vitamin E was also different between the 2 subspecies. For example, γ-tocotrienol was the predominant component in indica rice, while the most abundant isomer was α-tocopherol in japonica rice. The ratio of total tocotrienols to tocopherols was also significantly higher (P < 0.01) in indica (1.61) than in japonica rice (0.95). Pearson's correlation analysis demonstrated that the content of α-tocopherol was positively correlated to that of α-tocotrienol in rice. The same relationship was observed for γ-tocopherol and γ-tocotrienol, but there was a negative correlation between α- and γ-isomers. Taken together, the results provide valuable information for studying the vitamin E metabolism or nutritional improvement in rice.
