Handbook of Vitamin C Research

Vitamin C: Daily Requirements, Dietary Sources and Adverse Effects 241reduce the oxidized form of vitamin E back to its active form leading to the protection oflipid membranes from oxidation (van den Berg et al., 1990).Vitamin C is primarily absorbed in a dose-dependent active transport process in theintestine (Rumsey & Levine, 1998). It is actively co-transported with sodium against anelectrochemical gradient into intestinal epithelial cells. Facilitated diffusion of ascorbatefollows a concentration and electrochemical gradient. Once it enters the cells, a concentrationgradient is created by both brush border absorption and intracellular reduction of dehydro-Lascorbicacid to ascorbate (Jacob 1999). The intestinal absorption of vitamin C is 80-90%efficient. However, this efficiency rate declines with increased intake. Resorption ofascorbate in the kidneys occurs by facilitated diffusion, and ascorbate and its metabolites,such as oxalate, are found in the urine only in excess. Oxalate accounts for one of the fewpotential clinical toxicities of vitamin C supplementation which may contribute to oxalic acidrenal stones (Johnston 2001). Vitamin C is proposed to participate in electron transportreactions (Szent-Gyorgyi 1960), and is required for neurotransmitter synthesis (Levine1986), carnitine synthesis (Leibovitz and Mueller, 1993), cholesterol metabolism,cytochrome P-450 activity, detoxification (Shils 1994), antioxidant activity (Sies et al.,1992), regulation of cell respiration, and adenosine triphosphate (ATP) production inosteoblasts (Komarova et al., 2000). Vitamin C regulates iron distribution and storage bymaintaining a normal ratio of ferritin to hemosiderin. As a specific electron donor, vitamin Calso appears to participate in the synthesis of brain neurotransmitters and pituitary peptidehormones as mentioned above.Health BenefitsIn 1970, Linus Pauling was the first to realize vitamin C's crucial importance in themaintenance of a healthy immune system, and proposed that regular intake of vitamin Ccould help prevent and shorten the duration of the common cold. However, the use of vitaminC in the prevention and treatment of the common cold and respiratory infections remainscontroversial. Block et al (2004) investigated if antioxidants reduce plasma C-reactive protein(CRP) in active and passive smokers since CRP may have effect on the progression ofatherosclerosis. The results showed that vitamin C supplementation (515 mg/d) caused a 24%reduction (p < 0.036) in plasma CRP, whereas the antioxidant mixture and placebo yielded a4.7% reduction. Vitamin C may have a role inhibiting series-2 prostaglandins in carcinomacells (Beetens and Hermen, 1983). It also has been shown to stabilize protein, p53, which isinvolved in the control of cell proliferation (Reddy et al., 2001), and approximately 50% ofall cancers have a deficiency in protein p53. Ascorbic acid is involved in the synthesis ofcorticosteroids, modulation of components in microsomal drug-metabolizing system, thenervous system, and conversion of cholesterol to bile acids (Katsuki 1996). Ascorbate and itsradical potentiate the activation of transcription factor NF- B, which has been associatedwith inhibition of cell proliferation (Sakagami and Satoh, 1997). Recent research has shownthat vitamin C has a complex protective role against toxic compounds formed from oxidizedlipids, preventing genetic damage and inflammation. This appears to be a major pathway by