The causes of stomach cancer are unknown. However, certain forms of Helicobacter pylori or H. pylori bacteria are strongly associated with this cancer. In fact, it is believed that the large proportion of stomach cancer cases in developing countries are due to this bacteria and a reduced access to antibiotics. Recent studies have suggested, however, that the bacteria may be a greater risk factor for cancer of the lower stomach versus types of upper stomach cancer. The studies may explain why there has been a decline in lower stomach cancer in Western countries that utilize more antibiotics for intestinal conditions.

Foods preserved by drying, smoking, salting or pickling also are strongly associated with the disease. Countries where the diets are heavy in these types of foods have a high incidence of stomach cancer.

In the United States, where stomach cancers are rare, a person’s risk is determined by the presence of one or more risk factors. These factors may increase the likelihood that a person will develop stomach cancer but do not indicate that a person will definitely develop the disease. Some people may not have any risk factors and still develop stomach cancer. Some risk factors are out of a person’s control, and others involve elements that can be altered.

Uncontrollable or inherited risk factors for stomach cancer include:

• Age. The majority of the cases develop in individuals aged 65 and older.
  
  
• Gender. Men have about twice the risk of this disease than women.
  
  
• Race and ethnicity. Stomach cancer is most common in Asians and Pacific Islanders. African Americans and Hispanics are at higher risk than whites. The cause of this increase is believed to lie primarily in the traditional diets of these groups.
  
  
• Family history of close relative with stomach cancer.
  
  
• Type A blood. Including both positive and negative, type A blood contains antigens (immune system substances) in the red blood cells that may contribute to higher risk. The reason why people with type A blood are more likely to get stomach cancer is unknown.
  
  
• Familial adenomatous polyposis (FAP) or other cancer syndromes.

Medical or infectious conditions that are risk factors include:

• Helicobacter pylori (H. pylori) bacteria. This bacterial infection of the stomach is strongly associated with stomach cancer, particularly in the lower stomach. Long-term infection with this type of bacteria may lead to inflammation of the stomach lining resulting in pre-cancerous changes.
  
  
• Chronic gastritis (stomach inflammation)
  
  
• Gastric polyps
  
  
• Previous stomach surgery
  
  
• Epstein-Barr virus (virus that causes infectious mononucleosis)
  
  
• Menetrier’s disease (rare disease that alters the lining of the stomach)
  
  
• Pernicious anemia.  This disease causes the stomach to atrophy (become thinner than normal) and produce an inadequate amount of protein, which results in a shortage of red blood cells.

Dietary risk factors include:

• Nitrates and nitrites used as preservatives in food
  
  
• Salty, smoked or pickled foods or red meats. Research has indicated that eating red meat more than 13 times per week doubles the risk of stomach cancer. The risk increases if the meet is barbecued and cooked well done.
  
  
• Poor refrigeration of foods
  
  
• Obesity

Other lifestyle risk factors include:

• Smoking
  
  
• Abuse of alcohol
  
  
• Environmental exposure to toxins such as coal products, asbestos, rubber and nickel
  
  
• Low socioeconomic levels
  
  
• Geographic locations where diets of high-salt and pickled foods are common such as Asian, Eastern European and Latin American countries
  
  
• Areas and populations with reduced access to antibiotics to treat medical conditions