Saliva Could Help Detect Gastrointestinal Cancer

Gastrointestinal cancers rank among the most prevalent cancers globally and are a major cause of death. Being diagnosed is not especially pleasant, as it commonly means putting a camera where the Sun don't shine.

Yet doctors may soon be able to identify this form of cancer by examining the opposite end.

New findings, published in Cell Host & Microbe, indicate that a straightforward and much less invasive oral sample may be sufficient to determine whether a person has a gastrointestinal cancer.

"Many species of microbes associated with the mouth are also present in the gut and may flourish there," says Jihyun Kim, a systems biologist at Yonsei University in South Korea and the new study's lead author.

"These observations made us wonder whether oral microbes reach and persist in the gastrointestinal tract differently in people with cancer."

Saliva samples and gastrointestinal cancer

The researchers compared microbes present in oral and faecal samples from healthy controls (HC), people with metabolic syndromes such as type 2 diabetes (T2D), and gastrointestinal cancer patients (GC/CRC). (Jang et al., Cell Host Microbe, 2026)

To investigate, the scientists gathered saliva and faecal specimens from 507 participants. This included 77 people with stomach cancer, 86 with colorectal cancer, and 215 with metabolic conditions including hypertension, type 2 diabetes, and metabolic syndrome. They also tested a control group of 129 healthy people.

The team then sequenced microbial genes in the samples to generate a measure with a rather unappealing name: the mouth-to-faeces (MF) index. Put simply, it assessed how extensively the same microbes appeared in both an individual's saliva and faeces.

This allowed the researchers to assess whether the MF index differed among healthy people, those with cancer, and those with metabolic conditions. Their ultimate aim was to establish whether individuals with gastrointestinal cancers show distinct microbial signatures detectable from oral samples alone.

As expected, the MF index was significantly higher in people with gastrointestinal cancers than in healthy controls. In other words, cancer patients had more oral microbes travelling deeper into the gut than would usually be expected.

However, the researchers found no comparable rise among people with metabolic disorders.

"The presence of oral-associated bacteria in the gut of patients with cancer was not entirely unexpected," says Sun Ha Jee, an epidemiologist at Yonsei University.

"What we had not fully anticipated was how the signal varied across disease groups, how much of it could be detected in oral samples alone, and how strongly it depended on the way microbial features were represented."

Colon cancer illustration. (Sebastian Kaulitzki/Science Photo Library/Getty Images)

How oral testing compares with colorectal cancer screening

Intriguingly, the researchers reported that oral-sample tests detected signs of colorectal cancer more accurately than certain standard screening methods currently in use.

For patients without symptoms, one of the earliest tests commonly used is the faecal occult blood test. As its name implies, this looks for traces of blood in stool samples, which may indicate colorectal cancer. A positive result may then lead doctors to request additional tests to confirm a diagnosis.

The issue is that these tests produce a high number of false positives and negatives. Because not every cancerous or precancerous polyp bleeds, many are missed. Conversely, blood may result from other conditions, such as haemorrhoids.

Although it is less invasive than a complete colonoscopy, collecting your own poo, dipping a stick into it, and posting it to a laboratory is still unlikely to be many people's idea of a good time.

If given the option, most patients would likely rather provide a mouth-rinse sample.

"It was notable that cancer-related information could be recovered from oral samples alone," says Yong Chan Lee, a gastroenterologist at Yonsei University.

"Although only a small fraction of oral bacterial variants were also detected in the gut, models built using only these oral features could distinguish patients with cancer from healthy individuals across several independent cohorts."

Related: Sugar-Sweetened Beverages Linked to Stomach Cancer For The First Time

What remains before clinical use

The findings are an intriguing advance, but considerable work remains before oral tests can be used clinically to diagnose gastrointestinal cancers.

For a start, the researchers do not yet know why this association exists: might bacteria from the mouth that move into the gut contribute to cancer, or does cancer make it easier for these bacteria to migrate to and persist in the intestines? It may be some combination of both.

Future studies will explore that causal relationship, develop more sensitive versions of the test, and evaluate its accuracy in patients who do not already have a cancer diagnosis.

The research was published in Cell Host & Microbe.

This article was fact-checked and edited by Clare Watson. Although we take pride in our process, we are only human. If you notice an error, please let us know.