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Diet and probiotic supplements could make cancer immunotherapy less effective for some people. Getty Images

Probiotic supplements and certain foods could affect how well people respond to cancer immunotherapy, due to the way they influence the gut bacteria, suggests a new study.

This type of cancer treatment uses the body’s own immune system to prevent, target, and eliminate cancer. While this treatment is still relatively new, have been approved to treat several types of cancer.

The new study included people with metastatic melanoma, a later-stage type of skin cancer.

Patients underwent immunotherapy with anti-PD-1 checkpoint inhibitors.

This treatment turns off the “brakes” that cancer cells use to keep T cells — part of the immune system — from killing them.

Researchers found that patients using probiotic supplements were 70 percent less likely to respond to this type of immunotherapy.

Probiotic supplements contain live bacteria that “seed” your gut with those microorganisms.

They’re classified by the Food and Drug Administration as food supplements, so they don’t undergo rigorous testing.

People taking probiotics also had a lower diversity in their gut microbiome, the trillions of bacteria and other microorganisms that inhabit your intestines.

Study author Christine Spencer, PhD, a research scientist at the Parker Institute for Cancer Immunotherapy in San Francisco, said that in their previous study in Science, they found “a strong link between having a lot of bacterial diversity in your gut and response to cancer immunotherapy,” specifically to anti-PD-1 checkpoint inhibitors.

Certain types of bacteria, including those that digest and ferment fiber, were also associated with a better treatment response.

In the current study, diet also had an effect on treatment.

Patients who reported eating a high-fiber diet were five times more likely to respond to immunotherapy treatment with anti-PD-1 checkpoint inhibitors.

In addition, people who ate lots of whole-grain foods had more bacteria that are linked to better outcomes from this type of immunotherapy, based on earlier research.

On the other hand, people in the study with higher amounts of processed meat or added sugars in their diet had fewer bacteria associated with a good immunotherapy response.

Researchers collected fecal samples from 113 patients with metastatic melanoma before they underwent treatment. They used genetic sequencing to determine which bacteria were present in the gut and in what amounts.

They also asked patients about their diet and use of probiotics and antibiotics.

Researchers presented their results today at the American Association for Cancer Research (AACR) 2019 Annual Meeting in Atlanta.

However, the study hasn’t been published in a peer-reviewed journal, so the results should be viewed as preliminary.

The current study focused on one type of immunotherapy and cancer.

But other researchers are looking at the role of the microbiome in treating other cancers, such as cervical cancer, with some using other types of immunotherapies.

Dr. Cynthia Sears, a professor of medicine at the Johns Hopkins Kimmel Cancer Center, said this field is still at an early stage, with a small number of papers and little data on how patients do over the long term.

But she said the good news is that some of the early outcomes are now being tested in clinical trials to see if they make a difference for a person’s immunotherapy treatment.

One of these clinical trials is being carried out by the Parker Institute in adults with metastatic melanoma. Some patients will be given an oral microbiome pill with specific kinds of bacteria to see if it improves their response to checkpoint inhibitors.

However, this isn’t just any over-the-counter probiotic.

Scientists from Seres Therapeutics developed the microbiome pill based on gut bacteria found in people who responded well to immunotherapy.

Researchers at MD Anderson Cancer Center are also planning a study to see if changing a patient’s diet can shift their gut microbiome and response to immunotherapy.

Sears said other studies use fecal microbiota transplants (FMT) to alter a person’s gut microbiome. This involves transferring a stool sample from a healthy donor to a patient’s intestines.

FMT has had some success in treating acute diseases like Clostridium difficile infection of the intestines. But Sears said there’s “much less security” about the chances of success in chronic diseases like cancer.

These techniques are also not ready to be implemented in the clinic. But they’re promising enough that more researchers are keeping the microbiome in mind.

“In the Parker Institute trials,” said Spencer, “we are now adding on gut microbiome collection and analysis to all of our clinical studies.”

Should people undergoing cancer treatment stop taking probiotics?

“Based on our early results, cancer patients and doctors should carefully consider the use of over-the-counter probiotic supplements, especially before beginning immunotherapy treatment,” said study author Dr. Jennifer Wargo, a researcher at the MD Anderson Cancer Center, in a press release.

This runs counter to popular beliefs about the health benefits of probiotics, which include a healthier gut, heart, and mind.

The new study isn’t alone, though.

Sears pointed to several recent studies that have raised significant questions about the value of probiotics, including for treating diarrhea in children and restoring the gut microbiome after antibiotics.

“People love to say that we should all take probiotics, but it’s not that simple,” said Sears. “It’s quite possible that a probiotic would be beneficial, but it has to be the right bacteria.”

And that’s something researchers are still trying to figure out.

As for diet, Sears said you can easily alter your gut microbiome with food. If a meat eater became a vegetarian today, their microbiome would be different in just a few days.

“Within each of us, we have these bacteria and we can modulate them,” said Sears, “in part by diet.”

The effect of diet on immunotherapy, though, needs to be clarified. For example, it may depend not just on how much fiber you’re eating, but which kinds.

“Nonetheless,” said Sears, “if you’re trying to prevent colon cancer, a high-fiber diet is thought to be good.”

Many nutrition studies also point to the health benefits of high-fiber diets, along with limiting the intake of processed meat and added sugars, for preventing chronic diseases.

The mechanisms behind these benefits may be connected with how diet affects immunotherapy, although even that remains to be seen.

“The gut microbiome is intimately linked to a healthy immune system,” said Spencer, “so diet is likely important to maintaining both a healthy gut microbiome and a healthy immune system.”