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DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
that, because vitamin C is water soluble, it does not partition into the LDL particle. Also, it must be pointed out that in one of the 13 studies summarized in Table 5-1, there was an increase in plasma thiobarbituric acid reactive substances (TBARS), an indicator of oxidative stress, with a 500-mg dose of ascorbic acid (Nyyssonen et al., 1997b).
Adhesion of mononuclear cells to endothelium is an early event in atherogenesis and may be triggered by oxidative stress. Smokers have low levels of vitamin C and increased oxidative stress. A recent study showed that monocytes of smokers display greater adhesion to endothelial cells than those of nonsmokers (Weber et al., 1996). When supplemented with 2,000 mg/day of vitamin C, the plasma ascorbate level of smokers increased, and adhesion of their monocytes to endothelium decreased to that seen in nonsmokers.
Impaired vascular function is crucial to the clinical manifestation of atherosclerosis. As depicted in Table 5-3, numerous investigators have reported a beneficial effect of high dose vitamin C administration, either orally or intraarterially, on vasodilation. This beneficial effect of vitamin C is most likely related to its antioxidant effect. Endothelium-derived relaxing factor, nitric oxide (NO), promotes vasodilation but is rapidly inactivated by superoxide. Vitamin C improves endothelial function and vasodilation, possibly by scavenging superoxide radicals, conserving intracellular glutathione, or potentiating intracellular NO synthesis. In human endothelial cells in culture, extracellular vitamin C at physiological concentrations increased cellular NO synthesis up to threefold, and the increase in NO synthesis followed a time course similar to ascorbate uptake into the cells (Heller et al., 1999).
Antioxidant Functions in Leukocytes
The content of vitamin C in leukocytes is especially important because the ROS generated during phagocytosis and neutrophil activation are associated with infectious and inflammatory stresses (Jariwalla and Harakeh, 1996; Levine et al., 1994). Along with pituitary and adrenal glands and eye lens, leukocytes contain the highest vitamin C concentrations of all body tissues (Moser, 1987). Studies with guinea pigs and monkeys show that the concentration of ascorbate in the leukocytes more accurately reflects liver and body pool ascorbate than does the concentration in plasma or erythrocytes (Omaye et al., 1987). The vitamin is transported into leukocytes by an energy-dependent transport system that concentrates the vitamin some twenty-five-, forty-, and eightyfold over plasma levels in neutr-