Handbook of Vitamin C Research

130Chin-San LiuRecommendations for consumption of high doses of vitamin C are not supported by allresearchers. A recent meta-analysis on a potential effect of vitamin C on the common coldconducted by Douglas et al. showed that there seems to be no justification for routinemegadose (1–3 g/day) vitamin C supplementation in the normal population [49], althoughprophylaxis may be justified in those exposed to severe physical exercise or cold stress orboth. Moreover, numerous reviews suggest that intake of vitamin C much higher than theRDA, although without reaching the megadoses of vitamin C consumption proposed by someresearchers, may reduce the risk or risk factors for chronic diseases such as heart disease andcertain types of cancer [2, 50-53].SupplementsL-ascorbic acid is available in many forms, but there is little scientific evidence that anyone form is better absorbed or more effective than another. Natural and synthetic L-ascorbicacid are chemically identical and there are no known differences in their biological activitiesor bioavailability [54]. Mineral salts of ascorbic acid are buffered and, therefore, less acidicthan ascorbic acid. Sodium ascorbate and calcium ascorbate are the most common forms.Sodium ascorbate generally provides 131 mg of sodium per 1,000 mg of ascorbic acid, andpure calcium ascorbate provides 114 mg of calcium per 1,000 mg of ascorbic acid. One suchsupplement, Ester-C ® , contains mainly calcium ascorbate, but also contains small amounts ofthe vitamin C metabolites dehydroascorbate (oxidized ascorbic acid), calcium threonate, andtrace levels of xylonate and lyxonate. Although these metabolites are supposed to increasethe bioavailability of vitamin C, the only published study in humans found no difference inabsorption or urinary excretion of vitamin C between Ester-C ® and commercially availableascorbic acid tablets [55]. Ester-C ® should not be confused with ascorbyl palmitate, which isalso marketed as "vitamin C ester". Ascorbyl palmitate is actually ascorbic acid that has beenesterified to a fatty acid, resulting in a fat-soluble form of vitamin C. Ascorbyl palmitate hasbeen added to a number of skin creams due to interest in its antioxidant properties as well asthe important role of vitamin C in collagen synthesis [56]. Although ascorbyl palmitate isalso available as an oral supplement, it is likely that most of it is hydrolyzed to ascorbic acidand palmitic acid in the digestive tract before it is absorbed.Vitamin C induced peak values of plasma close to 220 mol/L when 3 g wasadministered orally 6 times per day [57]. Padayatty et al., however, showed that singlesupplement gram doses produced transient peak plasma concentrations that were at most twoorthree-fold higher than those from vitamin C contained in five to nine daily servings offruits and vegetables, which only produced a plasma concentration of 80 mol/L. The U.S.department of Agriculture and the National Cancer Institute recommend that five servings offruits and vegetables be eaten daily, even if recent analysis has suggested that thisconsumption should be higher [58, 59]. If these recommendations are followed, daily vitaminC intake will be 210 to 280 mg, depending on food cofactors [2]. Amounts >500 mg/daywould be difficult to obtain from dietary sources alone and would require supplements [60].