Helicobacter pylori (H pylori), a common bacteria that can infect the stomach, has been linked to more than one in five bowel cancer cases worldwide, according to a new analysis.

The bacteria has already been associated with stomach cancer in some individuals. Now, research suggests exposure might also be linked with up to a fifth of bowel cancer cases, too.

Meanwhile, those born in more recent decades may be more at risk of developing bowel cancer after exposure, the analysis suggested.

Bowel cancer is the second leading cause of cancer-related death globally. In recent years, cases have been on the rise, including in younger people.

Cancer Research UK said one in every 20 bowel cancer cancer cases in the UK occur in the under-50s. It’s not clear why – scientists and doctors have theorised it could be due to diet (ie. more reliance on processed foods), reduced activity levels or even environmental exposures.

H pylori are a group of bacteria which can be spread from person-to-person through contact with infected bodily fluids or contaminated food or water.

The germ is typically picked up in childhood and it’s thought up to two in five people in the UK are infected with it.

The bacteria can cause long-lasting inflammation of the gastric lining which can, in some cases, lead to issues with the stomach over time, including ulcers and cancer. As a result, it is categorised as a group 1 (definite) human carcinogen.

According to Cancer Research UK (CRUK), for most people, infection with H pylori will not cause them any problems and therefore doesn’t require treatment.

However, it can be “a cause of ongoing indigestion, bloating or nausea”, the cancer charity added.

CRUK noted that in addition to stomach cancer, the bacteria has also been linked to an increased risk of non-Hodgkin lymphoma.

Infection with the bacteria can typically be diagnosed via a blood test, breath test or stool sample.

Previous research had already established a link between H pylori and an increased risk of bowel cancer.

For the latest study, researchers set out to estimate the extent to which exposure to H pylori might be related to the global toll of cases.

They drew on data from systematic reviews and pooled data analyses and determined that exposure to H pylori was associated with a 1.6-fold greater risk of bowel cancer than no exposure.

Eradication therapy – which involves taking a combination of antibiotics and proton pump inhibitors (PPIs) to get rid of the bacteria – was associated with a reduced risk of the disease, but only after 10 years.

Estimates from 43 studies indicated that H pylori exposure might be linked to around 22% of bowel cancer cases worldwide.

Researchers also noted the proportion of bowel cancers linked to exposure was greater among those born in more recent decades, rising from 9.5% in those born before 1947, to around 10% among those born in 1948-62, and 15% among those born in 1963-77.

It’s worth noting that there are other influential risk factors in the development of bowel cancer, such as genetics, age, lifestyle and diet. The researchers pointed out that these factors account for a larger estimated proportion of bowel cancer cases than H pylori.

That said, they called for long term studies to determine if eradication therapy with antibiotics significantly lowers bowel cancer risk in people who have the bacteria.

The National Cancer Institute in the United States previously warned that unnecessary or inappropriate H pylori eradication treatment may contribute to an increase in antibiotic resistance.