the formation of bile acid micelles. The hydrocarbon backbone of the carotenoids makes them insoluble in water, and like other non-polar lipids, they must be solubilized within micelles in the gastrointestinal tract to allow for absorption. Micellar solubilization facilitates the diffusion of lipids across the unstirred water layer. The presence of fat in the small intestine stimulates the secretion of bile acids from the gall bladder and improves the absorption of carotenoids by increasing the size and stability of micelles, thus allowing more carotenoids to be solubilized. The uptake of tene by the mucosal cell is believed to occur by passive diffusion (Hollander and Ruble, 1978). Uptake by these cells, however, is not sufficient for absorption to be completed. Once inside the mucosal cell, carotenoids or their metabolic products (e.g., vitamin A) must also be incorporated into chylomicrons and released into the lymphatics. When mucosal cells are sloughed off due to cell turnover, spilling their contents into the lumen of the gastrointestinal tract, carotenoids that have been taken up by the cells but not yet incorporated into chylomicrons are lost into the lumen (Boileau et al., 1999).
Carotenoids may be either absorbed intact, or in the case of those possessing vitamin A activity, cleaved to form vitamin A prior to secretion into lymph. Portal transport of carotenoids is minimal due to the lipophilic nature of their structures. Some portal transport of more polar metabolites, such as retinoic acid, can occur (Olson, 1999).
Carotenoid cleavage is accomplished either by the intestinal mucosal enzyme β-carotene 15,15′-dioxygenase (EC 18.104.22.168) or by noncentral cleavage mechanisms (Boileau et al., 1999; Olson, 1999; Parker, 1996; Wang, 1994). The extent of conversion of a highly bioavailable source of dietary β-carotene to vitamin A in humans has been shown to be between 60 and 75 percent, with an additional 15 percent of the β-carotene absorbed intact (Goodman et al., 1966). However, absorption of most carotenoids from foods is considerably lower and can be as low as 2 percent (Rodriguez and Irwin, 1972). The effects of dietary and nondietary factors on the efficiency of carotenoid absorption are reviewed later.
Noncentral (or excentric) cleavage of carotenoids yields a wide variety of metabolic products, some of which are further metabolized. These cleavage products include aldehyde, acid, alcohol, and epoxide derivatives (Parker, 1996; Wang, 1994). Isomerization of