This may be partially due to a chronic immune response that continuously produces oxygen free radicals that are mutagenic. There is also an increased amount of gastric cell division that may lead to cancer. Diet plays a major role in cancer development with meat and animal fat increasing the risk and food with fiber decreasing the risk. High fat diets lead to the production of bile acids, promoting colorectal cancer and high meat diets have increased N-nitroso compounds which are carcinogenic. Fruits and vegetables also contain antioxidants.
Exercise is helpful since it decreases transit time of food and aspirin is also protective.
Estrogen in birth control pills and hormone replacement therapy increases cancer risk. Also the earlier the time of birth of the 1st child, the less the risk. Obesity is also a risk factor. In these people, all cells carry the mutation yet cancer only in certain tissues - ex. RB1 gene and retinoblastomas.