Possible benefits of high intakes of vitamin C
There is evidence from a variety of studies that high vitamin C status and a high plasma concentration of the vitamin is associated with reduced all-cause mortality.
At intakes above about 100–120 mg/day the body’s capacity to metabolize vitamin C is saturated, and any further intake is excreted in the urine unchanged. Therefore, it would not seem justifiable to recom-mend higher levels of intake. However, in addition to its antioxidant role and its role in reducing the tocopheroxyl radical, and thus sparing vitamin E, vitamin C is important in the absorption of iron, and in preventing the formation of nitrosa-mines. Both of these actions depend on the presence of the vitamin in the gut together with food, and intakes totaling more than 100 mg/day may be beneficial.
Inorganic dietary iron is absorbed as Fe2+ and not as Fe3+; ascorbic acid in the intestinal lumen will both maintain iron in the reduced state and chelate it, thus increasing the amount absorbed. A dose of 25 mg of vitamin C taken together with a meal increases the absorption of iron by around 65%, while a 1 g dose gives a ninefold increase. This occurs only when ascorbic acid is present together with the test meal; neither intravenous administration of vitamin C nor intake several hours before the test meal has any effect on iron absorption. Optimum iron absorption may therefore require significantly more than 100 mg of vitamin C/day.
The safety of nitrates and nitrites used in curing meat, a traditional method of preservation, has been ques-tioned because of the formation of nitrosamines by reaction between nitrite and amines naturally present in foods under the acid conditions in the stomach. In experimental animals nitrosamines are potent car-cinogens, and some authorities have limited the amounts of these salts that are permitted, although there is no evidence of any hazard to humans from endogenous nitrosamine formation. Ascorbate can prevent the formation of nitrosamines by reacting non-enzymically with nitrite and other nitrosating reagents, forming NO, NO2, and N2. Again, this is an effect of ascorbate present in the stomach at the same time as the dietary nitrites and amines, rather than an effect of vitamin C nutritional status.
Several studies have reported low ascorbate status in patients with advanced cancer, which is perhaps an unsurprising finding in seriously ill patients. With very little experimental evidence, it has been sug-gested that very high intakes of vitamin C (of the order of 10 g/day or more) may be beneficial in enhancing host resistance to cancer and preventing the development of the acquired immunodeficiency syndrome (AIDS) in people who are human immu-nodeficiency virus (HIV) positive. In controlled studies with patients matched for age, gender, site and stage of primary tumors and metastases, and previous chemotherapy, there was no beneficial effect of high-dose ascorbic acid in the treatment of advanced cancer.
High doses of vitamin C have been recommended for the prevention and treatment of the common cold, with some evidence that the vitamin reduces the duration of symptoms. However, the evidence from controlled trials is unconvincing.
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