Cs. You et al., EVIDENCE OF CIS-TRANS ISOMERIZATION OF 9-CIS-BETA-CAROTENE DURING ABSORPTION IN HUMANS, The American journal of clinical nutrition, 64(2), 1996, pp. 177-183
Absorption and metabolism of [C-13]9-cis-beta-carotene ([C-13]9c beta
C) was studied in three subjects after a single oral dose. Subjects gi
ven 1.0 mg [C-13]beta-carotene (mean: 99.4% 9-cis-beta-carotene, 0.6%
all-trans-beta-carotene; dose A) had substantial concentrations of [C-
13]all-trans-beta-carotene ([C-13]tr beta C) and [C-13]all-trans retin
ol ([C-13]retinol) but very low concentrations of [C-13]cis-beta-carot
ene ([C-13]cis beta C) in saponified plasma 5 h after dosing, as deter
mined by HPLC and isotope-ratio mass spectrometry. There was no eviden
ce of appreciable absorption of [C-13]9-cis retinol. To determine the
proportion of [C-13]tr beta C and [C-13]retinol derived from [C-13]9c
beta C, a second set of studies in the same subjects was performed wit
h the same isomeric composition except with C-13 labeling only in all-
trans-beta-carotene (dose B). The results indicated that > 95% of plas
ma [C-13]tr beta C and [C-13]retinol observed after dose A was derived
from [C-13]9c beta C. The concentrations of [C-13]tr beta C observed,
in excess of that derived from the trace amounts of [C-13]tr beta C i
n the dose, indicated that a significant proportion of the [C-13]9c be
ta C dose was isomerized to [C-13]tr beta C before entering the bloods
tream. Although precise quantitative estimates of the extent of isomer
ization of 9-cis-beta-carotene could not be made, it is apparent that
cis-trans isomerization of 9-cis-beta-carotene to all-trans-beta-carot
ene contributed to the near absence of postprandial plasma 9-cis-beta-
carotene after its oral administration in humans. The observation of d
ifferent ratios of beta-carotene to retinol between the two dosing pro
tocols suggests that isomerization did not occur exclusively before up
take by the intestinal mucosa. These results indicate that isomerizati
on of ingested 9-cis-beta-carotene before its secretion into the blood
stream limits the potential supply of 9-cis retinoids to tissues, and
increases the vitamin A value of 9-cis-beta-carotene.