Stomach Infection Tied to Bowel Cancer: Key Insights

A bacterium that can live quietly in the stomach for decades may have an unexpected connection with bowel cancer .

A new study estimates that exposure to Helicobacter pylori (H pylori) could be associated with more than one in five cases of bowel cancer worldwide.

That is a striking figure, particularly because H pylori is extremely common . But it comes with an important qualification: the study doesn't prove H pylori causes bowel cancer. Instead, it estimates how much bowel cancer might be linked to the infection.

Bowel cancer, also called colorectal cancer, develops in the colon or rectum. It's one of the most commonly diagnosed cancers worldwide , affecting around 1.9 million people each year.

We already know several factors that can increase the risk of developing bowel cancer. This includes older age, family history, smoking, obesity, alcohol and diets high in processed meat. But researchers are increasingly interested in another possible contributor: the trillions of microorganisms that live in our digestive system (our microbiome), and the infections that can alter this environment.

H pylori is an especially interesting candidate. Not only is the bacterium adapted to survive in the stomach's acidic environment, it is also known to cause gastric cancer .

It's thought around half of the world's population are infected with H pylori- although rates vary greatly between countries. Infection often occurs during childhood.

Most infected people never develop symptoms. But in some, H pylori causes long-term inflammation of the stomach lining . This can lead to ulcers and, in a small proportion, stomach cancer. The evidence that H pylori causes cancer is strong enough that it's classified as a carcinogen .

But the bacterium's relationship with bowel cancer is less clear.

Previous studies have found that people infected with H pylori have a higher risk of developing colorectal cancer.

This latest study attempted to contextualise these findings. It gathered data from 43 studies involving almost 49 million people and combined it with information about the prevalence of both H pylori and bowel cancer globally.

Overall, people exposed to H pylori had about a 1.6 times greater risk of being diagnosed with bowel cancer compared to those without evidence of infection. Using these figures, the researchers calculated that approximately 22% of bowel cancers worldwide could be associated with H pylori.

This doesn't mean H pylori has been shown to cause one in five bowel cancers. The calculation depended heavily on the studies included - and, crucially, assumes that at least some of the observed association is causal.

When the researchers restricted their analysis to 14 population-based and cohort studies - designs generally better suited to studying how risk develops over time - the estimated proportion of bowel cancers associated with H pylori fell to about 12%.

It's also possible that some of the apparent link reflects other differences between people who acquire H pylori and those who don't - including diet , smoking , physical activity levels , socioeconomic conditions conditions and healthcare access.

H pylori and cancer

Nevertheless, there are several plausible ways H pylori might influence bowel cancer.

One is through chronic inflammation. H pylori can provoke an immune response that lasts for decades. Long-lasting inflammation can affect chemical signals involved in cell growth and repair. In some circumstances, this inflammation also creates an environment in which damaged or abnormal cells are able to survive.

Chronic inflammation is already known to contribute to the development of several cancers, so this provides a biologically plausible connection.

Another possibility involves the gut microbiome . By changing inflammation and stomach acid production , H pylori may affect which microorganisms survive in the digestive system. These changes could influence the chemicals produced in the bowel and how its lining responds to damage.

H pylori may also affect hormones and other molecules involved in digestion and cell growth. Although an intriguing possibility, there's no evidence yet of the chain of events that leads from infection in the stomach to cancer in the colon or rectum.

Perhaps the most interesting finding from the study concerns what happens when the infection is treated.

H pylori can usually be eradicated using a combination of antibiotics and medicines that suppress stomach acid. Across four observational studies included in the analysis, people who received treatment for H pylori appeared to have a lower subsequent risk of bowel cancer - with the difference becoming apparent after ten years.

But as these were observational studies, not trials in which people were randomly assigned to treatment or no treatment, so we cannot yet conclude that eliminating H pylori prevents bowel cancer.

So should people now ask to be tested for H pylori to reduce their bowel cancer risk? On the evidence available today, no. We need stronger evidence showing that detecting and treating H pylori actually reduces colorectal cancer incidence.

At present, the most useful ways of reducing your risk of bowel cancer remain the ones supported by strong evidence.

This includes bowel cancer screening , which can find cancers at an earlier, more treatable stage and can sometimes prevent cancer by detecting and removing precancerous growths called polyps. Maintaining a healthy weight, being physically active, not smoking, limiting alcohol intake and eating less processed meat can also reduce risk .

Symptoms such as blood in the stool, a persistent change in bowel habits, unexplained weight loss or ongoing abdominal discomfort should not be ignored, although they are often caused by conditions other than cancer .

The wider significance of this research may be what it tells us about the causes of cancer. We tend to think of bowel cancer mainly in terms of genes, ageing and lifestyle. But some cancers also have an infectious component.

The possibility that an infection acquired in childhood could influence the risk of bowel cancer decades later therefore deserves serious investigation.

Justin Stebbing does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.