Epidemiology of gastric cancer in Chile: II - Nitrate exposures and stomach cancer frequency.

Nitrate levels in urine and nitrite levels in saliva were determined in school children 11-13 years of age in 2 areas of high stomach cancer mortality located in central Chile and in 2 areas of low stomach cancer mortality located at the extremes of Chile. Levels of both nitrates and nitrites were concurrently determined in selected vegetables obtained in these 4 study areas. Levels of nitrates were significantly higher in the urine of schoolchildren and in vegetables obtained from the northern low-risk area. Levels of nitrites were similar in saliva specimens from all 4 areas, but levels in vegetables were inconsistent. These results are apparently incompatible with the hypothesis of a high nitrate intake in areas of high stomach cancer incidence compared to areas of low incidence. This paradoxical finding suggests that cofactors may be involved in the pathopoiesis of stomach cancer that promote or inhibit the transformation of dietary nitrates to nitrosamines. The soil in the high-risk areas is primarily of volcanic origin, which is reported to be low in levels of selenium. On the other hand, residents of Antofagasta, the northern low-risk area, are known to be exposed to high levels of arsenic. It is possible that the diet of residents of the high incidence area which may be deficient in selenium may enhance promotion of nitrates to nitrosamines as has been reported in the literature, or conversely, that high levels of arsenic or other factors in the diet of individuals in Antofagasta may play a protective role. Other factors may be responsible for the low rate observed in the southern low-risk area.