Evidence published in the Lancet in 20011 suggested an inverse relationship between plasma vitamin C and cancer mortality. Vitamin C is thought to be able to prevent disease, possibly including some cancers, because it is an important water-soluble antioxidant in vivo. Evidence suggest that its role in cancer prevention may involve the protection of DNA from oxidative damage, that could otherwise initiate cancer. The current RDA for vitamin C is 60 mg/d, but a diet rich in fruit and vegetables should provide larger intakes than this, perhaps as high as 200 to 300 mg/d. Supplementation with higher intakes is controversial, but plasma levels do rise with increasing intakes, and higher plasma levels of vitamin C are known to increase cellular glutathione levels. Vitamin C is also able to recycle vitamin E by reducing the vitamin E radical, and therefore plays a key part in antioxidant defences against potential carcinogenic compound.
The prevention of oxidation may be beneficial because free radicals and their associated oxidative damage may trigger an inflammatory response. Inflammation is linked to the development of some cancers, particularly gastric cancers, and vitamin c may play a multi-functional role in their prevention. One possible mechanism for this is the scavenging of oxygen radicals in the mucosal lining. Another mechanism that is known is the prevention of the formation of carcinogenic nitrosamines. Nuclear factor-κB is a protein complex that is involved to the cellular response to stress and infection and dysfunction of NF-κB is associated with the development of inflammation and cancer. Vitamin C accumulates in endothelial cells where it inactivates NF-κB. In the initial stages of gastric cancer, intestinal levels of vitamin C are reduced, which suggests that vitamin C and gastric cancer are associated.
Gap junction communication between cells is important to maintain regulation of cell growth, proliferation and differentiation. Break down in gap cell communication may initiate tumours because the cellular differentiation and growth process becomes dysfunctional and unregulated cell proliferation occurs. The carotenoids (such as β-carotene and α-carotene) are able to improve gap cell communication and this may explain some of their ability to inhibit carcinogenesis. Like the carotenoids, vitamin C is an important promoter of gap cell communications, although the exact mechanism by which it does this is not known. However evidence suggests that it is not related to its antioxidant function Instead, vitamin C may regulate gap cell communication by the inhibition of a signal transduction mechanism. Vitamin C therefore appears to have multiple roles in the prevention of cancer including anti-oxidant, anti-inflammatory and signal transduction mechanisms.
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