The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
protein kinase C activity, which is involved in cell proliferation and differentiation, in smooth muscle cells (Boscoboinik et al., 1991; Chatelain et al., 1993; Clement et al., 1997; Stauble et al., 1994; Tasinato et al., 1995), human platelets (Freedman et al., 1996), and monocytes (Cachia et al., 1998; Devaraj et al., 1996). Protein kinase C inhibition by α-tocopherol is in part attributable to its attenuating effect on the generation of membrane-derived diacylglycerol, a lipid that facilitates protein kinase C translocation, thus increasing its activity (Kunisaki et al., 1994; Tran et al., 1994).
Vitamin E enrichment of endothelial cells downregulates the expression of intercellular cell adhesion molecule (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), thereby decreasing the adhesion of blood cell components to the endothelium (Cominacini et al., 1997). Vitamin E also upregulates the expression of cytosolic phospholipase A2 (Chan et al., 1998a; Tran et al., 1996) and cyclooxygenase-1 (Chan et al., 1998b). The enhanced expression of these two rate-limiting enzymes in the arachidonic acid cascade explains the observation that vitamin E, in a dose-dependent fashion, enhanced the release of prostacyclin, a potent vasodilator and inhibitor of platelet aggregation in humans (Szczeklik et al., 1985; Tran and Chan, 1990).
Physiology of Absorption, Metabolism, and Excretion
Absorption and Transport
Intestinal Absorption. While the efficiency of vitamin E absorption is low in humans, the precise rate of absorption is not known with certainty. In the early 1970s, vitamin E absorption was estimated to be 51 to 86 percent, measured as fecal radioactivity following ingestion of α-tocopherol (Kelleher and Losowsky, 1970; MacMahon and Neale, 1970). However, when Blomstrand and Forsgren (1968) measured vitamin E absorption in two individuals with gastric carcinoma and lymphatic leukemia, respectively, they found fractional absorption in the lymphatics to be only 21 and 29 percent of label from meals containing α-tocopherol and α-tocopheryl acetate, respectively.
Vitamin E absorption from the intestinal lumen is dependent upon biliary and pancreatic secretions, micelle formation, uptake into enterocytes, and chylomicron secretion. Defects at any step lead to impaired absorption (Gallo-Torres, 1970; Harries and Muller, 1971; Sokol, 1993; Sokol et al., 1983, 1989). Chylomicron