**6. Conclusions**

Various carotenoids, in particular, xanthophylls are ingested under usual dietary habits. However, carotenoids accumulated in human tissues are limited, suggesting selectivity in the intestinal absorption and different metabolic fates of carotenoids. The responses to the feeding of highly polar xanthophylls indicated that, for humans, intestinal absorption would be strictly selective in comparison with mice. The selectivity and its differences among animal species cannot be explained by simple diffusion mechanism alone. Instead, facilitated diffusion via SR-B1 and an unknown excretion to luminal side might cause the selectivity. After intestinal absorption of xanthophylls, the enzymatic oxidation of the secondary hydroxyl group leading to keto-carotenoids would occur as a common pathway of xanthophyll metabolism in mammals. We have no knowledge about the relation of these metabolites to the biological activities of parental xanthophylls. The potential biological activities of xanthophyll metabolites and their further metabolic fates warrant future studies with respect to the beneficial effects of xanthophylls on human health. 
