Stomach cancer develops through a combination of chronic infection, dietary habits, and genetic damage that unfolds over years or even decades. The single biggest driver is a bacterial infection most people have never heard of, but lifestyle factors like diet, smoking, and obesity also play significant roles. Globally, stomach cancer accounts for about 4.9% of all new cancer diagnoses and 6.8% of all cancer deaths, making it the fifth most commonly diagnosed cancer worldwide.
The Bacterial Infection Behind Most Cases
A bacterium called Helicobacter pylori (H. pylori) is the best-established cause of stomach cancer, particularly in the lower and middle portions of the stomach. Roughly half the world’s population carries this infection, though only a small fraction develop cancer from it. The process works through two pathways operating simultaneously: chronic inflammation and direct DNA damage to the cells lining the stomach.
When H. pylori colonizes the stomach lining, the immune system sends waves of defensive cells to the site, including neutrophils and other immune cells. These defenders release highly reactive molecules meant to kill the bacteria, but those same molecules also damage the DNA of nearby stomach cells. Over time, the bacterium also interferes with the stomach’s built-in DNA repair systems. Studies on human stomach tissue have shown that people infected with H. pylori have fewer functioning repair proteins in their stomach lining compared to uninfected individuals. The result is a double hit: more DNA damage occurring and less ability to fix it.
This matters because a meta-analysis of available evidence found that successfully treating and eliminating H. pylori with antibiotics reduces stomach cancer risk by 39%. That said, the risk doesn’t drop to zero. Even after the infection is cleared, heavy smokers still face a 34% higher risk of stomach cancer compared to people who never smoked, and heavy drinkers face a 23% higher risk.
How Normal Tissue Becomes Cancer
Stomach cancer doesn’t appear overnight. The transformation from healthy tissue to cancer follows a well-documented sequence that typically takes many years. It starts with chronic, non-damaging inflammation of the stomach lining. If that inflammation persists, it progresses to atrophic gastritis, where the stomach’s acid-producing glands begin to disappear. Next, the stomach lining starts replacing its normal cells with cells that resemble intestinal tissue, a change called intestinal metaplasia. These abnormal cells can then develop increasingly disordered growth patterns (dysplasia) before eventually becoming invasive cancer.
Not everyone who starts down this path reaches the end. At each stage, the process can stall or even partially reverse, especially if the underlying cause (like H. pylori infection) is removed early. This is why catching and treating the infection before significant tissue changes occur offers the greatest protective benefit.
Two Different Types With Different Causes
The stomach has distinct zones, and cancer in different locations often has different origins. Cancer in the lower and middle stomach (non-cardia cancer) is most strongly linked to H. pylori infection. Cancer at the top of the stomach, where it meets the esophagus (cardia cancer), has two separate pathways.
One type of cardia cancer develops from chronic acid reflux, where stomach acid repeatedly washes up into the junction between the esophagus and stomach. This version tends to occur in people who do not have H. pylori infection, because the infection actually reduces stomach acid production. In an ironic twist, H. pylori appears to be somewhat protective against acid reflux-related cancers, since the lower acid levels in infected stomachs mean reflux episodes cause less damage to the esophageal lining.
The other type of cardia cancer develops when H. pylori infection gradually migrates from the lower stomach upward as atrophic gastritis spreads. This version behaves much more like non-cardia stomach cancer.
Diet and Salt-Preserved Foods
What you eat plays a meaningful role, particularly if your diet is heavy in salt-preserved or pickled foods. Research tracking dietary patterns has found that people eating the most salted fish, salted fish roe, pickled vegetables, and dried fish had roughly 2.4 to 2.9 times the stomach cancer risk compared to those eating the least, with the association holding for both men and women.
High salt concentrations damage the protective mucus barrier that coats the stomach lining, leading to inflammation, erosion, and increased cell turnover. When stomach cells are dividing more rapidly to repair this damage, they become more vulnerable to cancer-causing compounds. Salt-preserved foods carry an additional threat: during the preservation process and during digestion, nitrates and nitrites in these foods can convert into chemical carcinogens inside the stomach.
There’s also an interaction between diet and infection. Frequent consumption of pickled vegetables has been associated with higher rates of H. pylori infection itself. People eating pickled vegetables five to seven days per week had nearly double the odds of carrying the infection compared to those eating them less than once a week. This creates a compounding effect: the diet both promotes the infection and amplifies the damage it causes.
Smoking, Alcohol, and Body Weight
Tobacco use increases stomach cancer risk in a dose-dependent pattern. Moderate smokers face about a 12% higher risk, while heavy smokers face a 34% higher risk compared to people who have never smoked. These elevated risks persist even after H. pylori infection has been treated, meaning smoking is an independent contributor to stomach cancer rather than just something that worsens the infection’s effects.
Heavy alcohol consumption raises risk by about 23%. Abdominal obesity, specifically carrying excess fat around the midsection rather than overall body weight alone, is associated with an 11% increase in risk. None of these factors in isolation carries the same weight as H. pylori infection, but they stack. A person with a history of H. pylori who also smokes, drinks heavily, and carries abdominal fat faces a meaningfully compounded risk.
Previous Stomach Surgery
People who have had stomach surgery for non-cancerous reasons, including weight-loss procedures, carry an elevated long-term risk that is easy to overlook. After gastric bypass surgery, the rearranged anatomy makes it difficult to examine the entire stomach with a standard endoscope. In a review of cancer cases found after Roux-en-Y gastric bypass, 75% of tumors were discovered in the bypassed portion of the stomach that is no longer accessible through normal routes. Cancers in this excluded segment took an average of 11.5 years to detect, more than double the 4.7 years for cancers found in the accessible portion.
The symptoms of stomach cancer, such as abdominal pain, nausea, vomiting, and bloody stools, overlap heavily with common side effects of bariatric surgery. Both patients and doctors may attribute warning signs to the surgery itself rather than investigating further. This diagnostic blind spot is worth being aware of if you’ve had any procedure that altered your stomach’s anatomy.
Putting the Risk Factors Together
Stomach cancer rarely comes from a single cause. It typically results from a long interaction between infection, diet, lifestyle, and time. H. pylori infection is the dominant factor for most cases worldwide, but it operates within a context shaped by what you eat, whether you smoke, how much you drink, and your body composition. The slow, stepwise progression from inflammation to cancer means there are multiple points where the process can be interrupted, whether through treating the underlying infection, shifting dietary habits, or maintaining surveillance after stomach surgery.