OCCURRENCE OF CIS ISOMERS OF PROVITAMINS-A IN BRAZILIAN VEGETABLES

Because cis isomerization lowers provitamin A activity, concern has be en raised over the necessity of separating the provitamin A isomers in determining the vitamin A values of foods. Sixteen different raw vege tables and eight cooked vegetables (total of 123 samples) were analyze d to appraise the occurrence of cis isomers. The raw vegetables analyz ed, except tomato and carrot, had cis isomers (trace to 3.4 mu g/g 13- cis-beta-carotene and trace to 4.1 mu g/g 9-cis-beta-carotene). The tr ans-beta-carotene range was 1.1-38.4 mu g/g. Appreciable amounts of al pha-carotene were found only in carrot (16.5-19.0 mu g/g trans-alpha d -carotene) and in a cultivar of squash (0.3 mu g/g 13-cis-alpha-carote ne, 17.1 mu g/g trans-alpha-carotene, and 0.1 mu g/g g-cis-ol-carotene ). The vitamin A values were calculated with and without isomer separa tion, using currently accepted biological activities, showing overesti mations of 10-22% when the isomers were not separated. Cis isomers wer e found in all samples of cooked vegetables analyzed; the overestimati ons ranged from 5 to 20%. Isomer separation may be necessary for some vegetables, but the biopotency of the isomers has to be reevaluated.