The association between blood group A and gastric cancer has appeared in numerous studies. A systematic review and meta-analysis published in 2019, for example, reported a modest association between blood group A and increased gastric cancer risk compared with blood group O. An umbrella review published in 2024 also found evidence supporting this association. <Cite refs={["turn985069search4","turn985069search12"]}/>
However, an important distinction must be made: an association does not automatically establish a direct cause.
Blood group A is not a diagnosis, and it does not mean that cancer is inevitable. Researchers are still investigating the biological mechanisms that may explain the observed relationship.
2. How Large Is the Difference in Risk?
One reason the blood type finding has attracted attention is that it has been observed across multiple populations and research studies.
In the 2019 meta-analysis, people with blood group A had approximately 19% higher relative odds of gastric cancer than people with blood group O. This is a statistical comparison between groups, not a statement that 19 out of every 100 people with blood type A will develop stomach cancer. <Cite refs={["turn985069search4","turn985069search10"]}/>
Relative risk measures can easily be misunderstood when they are presented without context.
Imagine, purely as a simplified illustration, that a hypothetical group has a baseline risk of 1%. If another group has a relative risk 20% higher, its corresponding risk would be approximately 1.2%, assuming the comparison applies directly to those populations and time periods.
The difference would be 0.2 percentage points, not 20 percentage points.
This example is not an estimate of any particular person's stomach cancer risk. Actual risk varies substantially according to age, country, family history, infection status, smoking, diet, and other factors.
The key message is that a modest statistical association does not make blood type a powerful standalone predictor.
Doctors do not diagnose stomach cancer by looking at blood group alone. Nor should people undergo invasive testing simply because they have type A blood without considering their symptoms, medical history, and applicable clinical guidance.
3. Could Blood Group A Affect the Stomach's Response to Bacteria?
One possible explanation involves the interaction between ABO-related molecules and microorganisms that live in or interact with the stomach.
The stomach is not merely a container for food. Its lining contains specialized cells that produce mucus, acid, digestive substances, and protective molecules. This lining must tolerate mechanical stress, food components, and exposure to microorganisms while maintaining a barrier against injury.
The bacterium Helicobacter pylori, commonly called H. pylori, can colonize the stomach and persist there for years if it is not eliminated.
Scientists have investigated whether differences in cell-surface carbohydrates associated with blood groups might influence how H. pylori attaches to the stomach lining or interacts with the body's defenses.
Certain bacterial strains possess proteins that help them bind to specific structures on host cells. Differences in these interactions may influence colonization and inflammation.
Nevertheless, the relationship is complex. Studies have not established a simple rule that people with blood group A are always more likely to become infected with H. pylori. A meta-analysis of studies examining blood group and infection found different associations for different blood groups, illustrating that infection risk and cancer risk are not interchangeable. <Cite refs={["turn985069search8","turn985069search13"]}/>
The precise mechanisms linking ABO biology to gastric cancer remain under investigation.
4. Why H. pylori Is a More Important Risk Factor to Understand
Although blood group associations are scientifically interesting, H. pylori infection is a much more established risk factor for stomach cancer.
The bacterium can cause chronic inflammation of the stomach lining. In some people, long-term inflammation leads to progressive changes in the stomach's tissue.
These changes may include atrophic gastritis, in which the normal stomach glands become damaged or reduced, and intestinal metaplasia, in which cells begin to resemble those normally found in the intestine. In some cases, further abnormal changes can develop and eventually contribute to cancer.
This progression does not occur in everyone who has the infection. Many people with H. pylori never develop stomach cancer, and the course of infection varies among individuals.
Factors such as bacterial strain, host genetics, smoking, dietary patterns, and the condition of the stomach lining may influence the outcome. <Cite refs={["turn985069search7","turn985069search11"]}/>
The important distinction is that H. pylori is an established infectious cause of gastric cancer, whereas blood group A is an associated characteristic whose biological role is not fully understood.
This difference matters when deciding which health actions are most useful. People cannot change their inherited blood group, but appropriate testing and treatment for H. pylori can be possible when clinically indicated.
5. How Is H. pylori Infection Diagnosed?
Several tests can help identify H. pylori infection.
Urea breath test: The person drinks a specially prepared solution, and the breath is analyzed for evidence that the bacterium is breaking down urea. This is commonly used to identify active infection and can also be used to check whether treatment has worked.