Sources of Vitamin C Dietary and High Intakes

It is apparent from the list of rich sources of vitamin C in Table 3 that the major determinant of vitamin C intake is the consumption of fruit and vegetables; deficiency is likely in people whose habitual intake of fruit and vegetables is very low. However, clinical signs of deficiency are rarely seen in developed countries. The range of intakes by healthy adults in Britain reflects fruit and vegetable consumption: the 2.5 percentile intake is 19mg per day (men) and 14 mg per day (women), while the 97.5 percentile intake from foods (excluding supplements) is 170 mg per day (men) and 160 mg per day (women). Smokers may be at increased risk of deficiency; there is some evidence that the rate of ascorbate catabolism is 2- fold higher in smokers than in nonsmokers.
There is a school of thought that human requirements for vitamin C are considerably higher than those  discussed above. The evidence is largely based on observation of the vitamin C intake of gorillas in captivity, assuming that this is the same as their intake in the wild (where they eat considerably less fruit than under zoo conditions), and then assuming that because they have this intake, it is their requirement— an unjustified assumption. Scaling this to human beings suggests a requirement of 1–2 g per day.

Intakes in excess of about 80–100mg per day lead to a quantitative increase in urinary excretion of unmetabolized ascorbate, suggesting saturation of tissue reserves. It is difficult to justify a requirement in excess of tissue storage capacity. A number of studies have reported low ascorbate status in patients with advanced cancer—perhaps an unsurprising finding in seriously ill patients. One study has suggested, on the basis of an uncontrolled open trial, that 10 g daily doses of vitamin C resulted in increased survival. Controlled studies have not demonstrated any beneficial effects of high-dose ascorbic acid in the treatment of advanced cancer.

High doses of ascorbate are popularly recommended for the prevention and treatment of the common cold. The evidence from controlled trials is unconvincing, and meta-analysis shows no evidence of a protective effect against the incidence of colds. There is, however, consistent evidence of a beneficial effect in reducing the severity and duration of symptoms. This may be due to the antioxidant actions of ascorbate against the oxidizing agents produced by, and released from, activated phagocytes, and hence a decreased inflammatory

Scorbutic guinea pigs develop hypercholesterolemia. While there is no evidence that high intakes of vitamin C result in increased cholesterol catabolism, there is evidence that monodehydroascorbate inhibits hydroxymethylglutaryl CoA reductase, resulting in reduced synthesis of cholesterol, and high intakes of ascorbate may have some hypocholesterolaemic action. There is limited evidence of benefits of high intakes of vitamin C in reducing the incidence of stroke, but inconsistent evidence with respect to coronary heart disease.

Regardless of whether or not high intakes of ascorbate have any beneficial effects, large numbers of people habitually take between 1 and 5 g per day of vitamin C supplements. There is little evidence of any significant toxicity from these high intakes. Once the plasma concentration of ascorbate reaches the renal threshold, it is excreted more or less quantitatively with increasing intake. Because the rate of ascorbate catabolism increases with increasing intake, it has been suggested that abrupt cessation of high intakes of ascorbate may result in rebound scurvy because of ‘metabolic conditioning’ and a greatly increased rate of catabolism. While there have been a number of anecdotal reports, there is no evidence that this occurs.


Source: Guide to Nutrition Supplements
Share on :

Recommended Posts :


Post a Comment

Umaee FarmMed Copyright © 2011 -- Template created by O Pregador -- Powered by Blogger