carotenoids or their metabolic products may occur in vivo because isomers have been found upon extraction of carotenoids from human tissues (Clinton et al., 1996). Although little attention has been given to the study of carotenoid excretion pathways, epoxides and carotenoid metabolic products with less than 15 carbon chain lengths would presumably have no vitamin A activity. It is assumed that bile and urine would be excretion routes for metabolites (Olson, 1999).
The carotenoids are transported in blood exclusively by lipoproteins. The carotenoid content of individual lipoprotein classes is not homogeneous. In the fasted state, the hydrocarbon carotenoids such as α-carotene, β-carotene, and lycopene are carried predominantly by low-density lipoprotein. The remaining carotenoids, including the more polar xanthophylls such as lutein and zeaxanthin, are carried by high-density lipoprotein (HDL) and, to a lesser extent, by very low-density lipoprotein (Johnson and Russell, 1992; Parker, 1996; Traber et al., 1994). It is thought that β-carotene and other hydrocarbon carotenoids reside in the hydrophobic core of the particles, whereas the more polar xanthophylls reside closer to the surface (Parker, 1996).
β-Carotene is the most studied carotenoid in terms of metabolism and its potential effects on health. Lycopene, lutein, zeaxanthin, and α-carotene have received increasing attention in recent years. Much remains to be learned, however, about the relative metabolic effects of these carotenoids.
Recently, 34 carotenoids were identified in human serum and milk (Khachik et al., 1997b). Of these, 13 were geometrical isomers of their all-trans parent structures and 8 were metabolites. This finding is in contrast to the up to 50 carotenoids that have been identified in the U.S. diet and the more than 600 found in nature. The most prevalent carotenoids in human serum (Khachik et al., 1997b) are the same as those most commonly found in the diet: β-carotene, lycopene, and lutein (Nebeling et al., 1997). Cis-isomers of lycopene are commonly found in the serum and in fact have been shown to constitute more than 50 percent of the total serum lycopene (Stahl et al., 1992). In contrast, cis-isomers of β-carotene are considerably less common in serum with the trans-isomers being more common. In addition to these forms of α-carotene, β-carotene, lycopene, and zeaxanthin are also major serum carotenoids. The concentrations of various carotenoids in human serum and