Absorption and function of dietary carotenoids.

Carotenoids are highly hydrophobic pigments with yellow to red color and their major dietary sources are fruits and vegetables. They have an essential physiological function as a vitamin A precursor and also have antioxidant, anticancer, immune enhancement and antiobesity activities related to prevention of degenerative diseases. The release of carotenoids from food matrix, their dispersion within the digestive tract, and their solubilization in mixed micelles are important steps for carotenoid bioaccessibility. Solubilized carotenoids are taken up by epithelial cells of the small intestine by simple diffusion and/or transporter-mediated processes and then secreted to lymph as chylomicron. Carotenoids accumulated in tissues are thought to be metabolized to small molecules by enzymatic cleavage and/or chemical oxidation with active oxygen species at conjugated double bonds. The hydroxyl group of xanthophylls can be oxidatively metabolized to carbonyl group. Carotenoids with long chain of conjugated double bonds physically quench singlet oxygen and scavenge oxygen radicals, particularly under low oxygen pressure, and thereby they have been thought to work as lipophilic antioxidants for human health. In addition to antioxidant activities, each carotenoid has characteristic functions such as cell cycle inhibition, induction of cell differentiation and apoptosis, and enhancement of gap-junctional communication. However, the detailed mechanisms of these biological actions have not been fully revealed yet and deserve future studies.