The IARC convened a Working Group of experts in December 1997 to evaluate the
cancer-preventive potential of
carotenoids and to compile the second volume of the IARC Handbooks of
Cancer Prevention. In observational epidemiological studies,
beta-carotene is associated with reduced risks for
cancer at many but not all sites. It is unclear, however, to what extent
beta-carotene itself is responsible for the decreased risks observed. Three large, randomized, placebo-controlled clinical trials indicate, however, that, in substantial doses, supplementation with
beta-carotene not only does not prevent
lung cancer but may actually increase the risk among individuals initially at high risk of
lung cancer. These trials do not provide clear evidence concerning
cancers at other specific sites. Thus, the Working Group considered that there is evidence suggesting a lack of
cancer-preventive activity for
beta-carotene when it is used as a supplement at high doses. At usual dietary levels of
beta-carotene, the evidence for
cancer-preventive activity was considered inadequate. However, there is sufficient evidence that
beta-carotene has
cancer-preventive activity in experimental animals, based on models of skin
carcinogenesis in mice and buccal pouch
carcinogenesis in hamsters. The observational epidemiological data on
alpha-carotene,
lycopene, and
lutein are much less extensive than those for
beta-carotene. For
canthaxanthin, there are no published data regarding associations with
cancer risk. These
carotenoids have not been studied in human trials for
cancer prevention. In animal models, there is sufficient evidence for
canthaxanthin and limited evidence for
alpha-carotene,
lycopene, and
lutein of
cancer-preventive activity. Pending further research, supplemental
beta-carotene,
canthaxanthin,
alpha-carotene,
lutein, and
lycopene should not be recommended for
cancer prevention in the general population.