Occurrence of Cis Isomers of Provitamins A in Brazilian Vegetables

Because cis isomerization lowers provitamin A activity, concern has been raised over the necessity of separating the provitamin A isomers in determining the vitamin A values of foods. Sixteen different raw vegetables and eight cooked vegetables (total of 123 samples) were analyzed to appraise the occurrence of cis isomers. The raw vegetables analyzed, except tomato and carrot, had cis isomers (trace to 3.4 μg/g 13-cis-β-carotene and trace to 4.1 μg/g 9-cis-β-carotene). The trans-β-carotene range was 1.1−38.4 μg/g. Appreciable amounts of α-carotene were found only in carrot (16.5−19.0 μg/g trans-α-carotene) and in a cultivar of squash (0.3 μg/g 13-cis-α-carotene, 17.1 μg/g trans-α-carotene, and 0.1 μg/g 9-cis-α-carotene). The vitamin A values were calculated with and without isomer separation, using currently accepted biological activities, showing overestimations of 10−22% when the isomers were not separated. Cis isomers were found in all samples of cooked vegetables analyzed; the overestimations ranged from 5 to 20%. Isomer separation may be necessary for some vegetables, but the biopotency of the isomers has to be reevaluated. Keywords: Provitamin A determination; carotenoid geometric isomers; vegetables