Ht. Godoy et Db. Rodriguezamaya, OCCURRENCE OF CIS ISOMERS OF PROVITAMINS-A IN BRAZILIAN VEGETABLES, Journal of agricultural and food chemistry, 46(8), 1998, pp. 3081-3086
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.