Vitamin C intake is negatively correlated with cancers of the digestive system including the esophagus, stomach, oral cavity and pancreas. Vitamin C protects against human cancer in the gut because it inhibits the formation of carcinogenic nitrosamines. Nitrosamines (figure 1) form when nitrites (NO2−) (a preservative in cured meats and fish), react with stomach acid to form nitrous acid (HNO2) which in turn reacts with amines (e.g. amino acids). Nitrosamines are known to be carcinogenic and because they are in contact with enterocytes, can induce the formation of digestive system cancers. Inhibition of this reaction depends on the reduction of nitrite to nitric oxide (NO) by vitamin C, which prevents the reaction of nitrous acid with amines. For this inhibition to occur it is important for vitamin C to be present in the stomach at the same time as the nitrite compounds.
Figure 1. Nitrosamine structure. R1 and R2 can be various different groups.
Research indicates that the gastric juice secreted by the stomach during digestion may contain high levels of vitamin C. A study published in the American Journal of Nutrition in 19911 measured the levels of ascorbic acid secreted in the gastric juice and found that in some cases they actually exceeded plasma levels. This suggests that high intakes of vitamin C are able to protect from the formation of carcinogenic nitrosamines even if vitamin C is not consumed at the same time as the food containing the nitrite compounds. High intakes of vitamin C are important to protect from cancers of the gut, but of equal importance is the avoidance of cured meats and fish that contain high levels of nitrites. Nitrites are added to the foods as a preservative. These sorts of foods have no place in a healthy diet.
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