Biopotency of vitamin E in barley

Abstract

1. Investigations were carried out to establish the total biopotency of the natural vitamin E isomers in barley compared with that of DL-α-tocopheryl acetate.2. The chick was used as an experimental animal. Prevention of nutritional encephalomalacia (NE) and chick liver-storage and plasma-storage assays of vitamin E were the methods used in the study. The individual tocopherols and tocotrienols, both in the tissue samples and in the grain and barley oil, were analysed using high-pressure liquid chromatography (HPLC) with fluorescence detection. The diagnosis of NE was based on careful clinical and histopathological observations.3. It can be concluded from the results that full protection against NE in the chicks was obtained with a supplementation level of 7.5 mg DL-α-tocopheryl acetate/kg diet (i.e. a total vitamin E content of 11.20 mg/kg diet) or with a supplement of 8.7 g barley oil/kg diet (i.e. a total vitamin E content of 22.99 mg from barley oil/kg diet). This gave a biopotency factor of 0.49 for barley for prevention of NE of the chicks, as compared to that of DL-α-tocopheryl acetate.4. Using regression analysis a statistically linear relationship could be observed between the total dietary vitamin E level and the response, as measured by the total vitamin E content in the liver and plasma, both in the groups supplemented with DL-a-tocopheryl acetate and in the groups supplemented with corresponding amounts of vitamin E in barley oil. The liver and plasma responses to the total vitamin E in the barley-oil diet compared with those of the DL-a-tocopheryl acetate reference diet gave identical values for the regression coefficients, i.e. in both liver-storage and plasma-storage assays the value for slopes of dose-response lines was 0.37. This means that the biopotency of the total vitamin E in barley was 37% of that of dietary DL-a-tocopheryl acetate. Thus, barley is not as rich a source of vitamin E as could be supposed on the basis of the chemical determination of its total vitamin E content.5. It was possible to verify this experimentally established biopotency of 0.37 for the total vitamin E in barley by converting the chemically determined amounts of the vitamin E isomers in barley into DL-α-tocopheryl acetate equivalents by multiplying them with internationally accepted potency factors for the individual natural isomers (DL-α-tocopheryl acetate 1.00, D-α-tocopherol 1.49, D-β-tocopherol 0.60, D-gamma;-tocopherol 0.1 5, D-α-tocotrienol 0.37).6. In spite of the high proportion of α- and β-tocotrienols in the barley-oil diets (about 60% of the total vitamin E content), only traces of these isomers could be detected in the plasma and none could be detected in the liver. On the other hand, calculation of the individual hiopotencies for the different isomers in the barley-oil diet by comparing the dose responses, diet: liver, separately for each isomer with those of DL-α-tocopheryl acetate, resulted in biopotency values for α- and β-tocopherol which were twice as high as the internationally accepted conversion factors. These results of the present study tempted the authors to draw the conclusion that there may have been a chemical reduction of the α- and β-tocotrienols to the corresponding tocopherols before entering the liver.