Fermented foods are those such as kimchi and sauerkraut. They are believed to be a risk factor in the development of gastric cancer.
One article (1) discussed gastric cancer and noted that it is one of the most common cancers in Korea and Japan—both countries in East Asia. The incidence and mortality of this type of cancer has gone down; however, it remains one of the more common types of cancer in that area.
The article noted that gastric cancer is more likely to occur in patients who smoke, those who have a H. pylori infection, and those that consumed fermented foods. Fermented foods are more commonly used in these areas of the world and these are dietary factors that seem to contribute to getting gastric cancer. The article was a review article, meaning that it looked at the results of basic scientific research studies on fermented food and gastric cancer.
Source: 1.Shin A. Gastric Cancer Epidemiology in Korea. J Gastric Cancer. 2011 Sep; 11(3): 135–140.