ABSTRACT
Release from the food matrix and dissolution in the lipid phase are the important initial steps in the absorption process. b-Carotene is thought to be absorbed by the small intestinal mucosa via a passive, diffusion process. It is taken up by the mucosa of the small intestine and packaged into triacylglycerol-rich chylomicrons, and is partly converted to vitamin A by a specific enzyme, b-carotene 15,150-oxygenase, in the intestinal mucosa. Both b-carotene and vitamin A (primarily as retinyl esters) are incorporated into chylomicrons and secreted into lymph for transport to the liver. Additional random cleavage at several double bonds in the polyene chain of b-carotene can occur when there is not an adequate supply of antioxidants, e.g., vitamin E. However, enzymatic central cleavage plays the major role in b-carotene breakdown under normal conditions. In conditions of oxidative stress (e.g., smoking or diseases associated with oxidative stress) or when high blood or tissue concentrations of b-carotene are present, both central and random cleavage may occur (Fig. 1).[1]
The delivery of b-carotene to extrahepatic tissue is accomplished through the interaction of lipoprotein particles with receptors and the degradation of lipoproteins by extrahepatic enzymes such as lipoprotein lipase. b-Carotene is present in a number of human tissues, including adipose, liver, kidney, adrenal gland, and testes, and is one of the major carotenoids in human diet, serum, and tissues.
