Megadoses of vitamin C are often recommended for good health. However, many argue that such high doses are not required and that most of the vitamin C is simply metabolised and excreted. One study investigated the absorption kinetics of vitamin C and found that there was a rapid rise in blood vitamin C levels with a dose of up to 400 mg, but above this dose further increases in vitamin C levels were only gradual (albeit present up to about a 2000 mg dose). However, what is often forgotten in this discussion is that vitamin C does not have to enter the blood in order to have positive effects. Vitamin C for example is able to have effects in the gut prior to absorption, and here there may be a benefit from very high doses. Vitamin C can for example act as an antioxidant and may prevent the generation of free radicals and lipid peroxides in the gut. Recent research has shown that the function of antioxidants in the gut has been greatly understated. This gut role for vitamin C is little understood, but may play a significant role in promoting health.
Another function for vitamin C in the gut is in the prevention of damaging nitrosamine compounds. These compounds are formed from nitrates and nitrites in foods, and evidence suggests that nitrosamines are involved in the initiation of gut cancer. Vitamin C is able to prevent the formation of nitrosamine because it can instead cause nitrate to be reduced to nitrite and then further reduced to nitric oxide, thus inhibiting the formation of nitrosamines by removing the pool of available nitrite and nitrate. Citrus fruit is an excellent source of vitamin C,
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