Gut bacteria and cancer: what has been proven and what has not

The short answer

Gut bacteria are linked to cancer, but not in the way headlines make it sound. Only one bacterium is proven to lead to cancer, and it can be treated. The rest is serious, unfinished research.

Billions of bacteria, fungi, and viruses live in the gut. Together, they are called the microbiome.

They are not parasites. They break down our food, produce a few vitamins, and take up space that more dangerous microbes would otherwise occupy. They also help the body's defenses learn what is foreign and what is not.

That is why the idea that the microbiome might be linked to cancer is not a fantasy. The real question is how large that link actually is.

A bacterium that lives in the stomach, Helicobacter pylori. The World Health Organization classified it among the causes of cancer back in 1994.

Nearly two-thirds of the world's population carries it. Most stomach cancers are attributed to it.

Here is the part that matters. In a study in China, two weeks of antibiotics that eliminated the bacterium reduced the number of gastric cancer cases by nearly 50%, over 22 years of follow-up.

This shows the direction makes sense: a microbe found, treated, fewer cancers afterward.

Cancer Research UK recently gathered, in one piece, what the major research teams are tracking.

Some researchers are checking whether a gut with too little variety in the first years of life is linked to childhood leukemia. Others are studying a substance produced by certain strains of Escherichia coli, which leaves marks in DNA and shows up in colon cancers in young people.

The shared idea is that signs of risk might be visible long before the disease itself. If confirmed, it could allow earlier intervention.

The researchers quoted in the piece all say some version of the same sentence: for diseases that build up over years, prevention can do more than treatment given late.

Three things are missing, and they are big gaps.

There is no definition of a healthy microbiome. Researchers say openly that it is hard to pin down what makes one healthy.

Then, in most cases, it is not known which way the link runs: do changed bacteria cause the disease, or does the disease change the bacteria? This is one of the field's biggest difficulties.

And there is still no treatment that uses the microbiome to prevent cancer in humans.

Fiber in food.

When fiber reaches the bacteria in the gut, they produce butyrate — a substance that keeps the cells of the gut wall healthy. Cancer Research UK writes that this makes it harder for tumors to form.

It is the one part of the microbiome story that has made it out of the laboratory and onto the plate.

• Have I ever been tested for Helicobacter pylori?
• In my situation, would such a test be worthwhile?
• What can I eat now, given the treatment I am on?
• Is it worth paying for a microbiome test?
• Do bacterial supplements conflict with my treatment?

If you have read somewhere that a product "repairs" your microbiome, write down its name and ask about it at your next check-up. For now, the one thing in this area with solid evidence goes on your plate, not in a box you buy.