Although
beta-carotene has been considered to be a key
cancer preventive agent in green and yellow vegetables, other types of
carotenoids, such as
alpha-carotene, may also contribute to anticarcinogenic action, since these
carotenoids usually coexist with
beta-carotene and are detectable in human blood and tissues. In this study, we compared the inhibitory effect of natural
alpha-carotene, obtained from
palm oil, with that of
beta-carotene on spontaneous liver
carcinogenesis in C3H/He male mice. The mean number of
hepatomas per mouse was significantly decreased by
alpha-carotene supplementation (per os administration in
drinking water at a concentration of 0.05%, ad libitum) as compared with that in the control group (P < 0.001, Student's t test). On the other hand,
beta-carotene, at the same dose as
alpha-carotene, did not show any such significant difference from the control group. Furthermore, we also compared the antitumor-promoting activity of
alpha-carotene with that of
beta-carotene against two-stage mouse lung
carcinogenesis (initiator,
4-nitroquinoline 1-oxide; promoter,
glycerol).
alpha-Carotene, but not
beta-carotene, reduced the number of lung
tumors per mouse to about 30% of that in the control group (P < 0.001, Student's t test). The higher potency of the antitumor-promoting action of
alpha-carotene compared to
beta-carotene was confirmed in other experimental systems; e.g.,
alpha-carotene was also found to have a stronger effect than
beta-carotene in suppressing the promoting activity of 12-O-tetradecanoylphorbol-13-acetate on skin
carcinogenesis in 7,12-dimethylbenz[a]
anthracene-initiated mice. These results suggest that not only
beta-carotene, but also other types of
carotenoids, such as
alpha-carotene, may play an important role in
cancer prevention.