Immunotherapy is a cancer treatment with several benefits. For this reason, improving its effectiveness is vital. In studying the gut microbiome, scientists have found some rather unusual results.

Cancer immunotherapy is a relatively young field. However, it has the potential for long-term remission and less likely side effects. According to the Cancer Research Institute, scientists have shown that it is effective at treating cancers that are resistant to both chemotherapy and radiation therapy. Immunotherapy works by helping the immune system fight off the disease. Cancer cells normally go undetected by the immune system, but the treatment uses drugs and other substances to produce a stronger response.

Checkpoint inhibitors are one type of immunotherapy. They affect cancer cells' ability to dodge immune system attacks. However, they only work for 20%–30% of people with cancer. Scientists have recently found that the gut microbiome, which comprises trillions of intestinal microorganisms, has the ability to control the immune system.

A group of researchers from the Parker Institute for Cancer Immunotherapy in San Francisco, CA, and the University of Texas MD Anderson Cancer Center in Houston has examined whether this could be impacting immunotherapy success rates.

A surprising result

The preliminary study is the first to look at the link between immunotherapy, the gut microbiome, and diet in people with cancer. In all, 113 individuals with metastatic melanoma who had started treatment at MD Anderson took part. The scientists presented their findings at the American Association for Cancer Research's recent annual meetings, which took place in Atlanta, GA.

The participants filled out a lifestyle survey on their diet, medication, and use of supplements. The researchers also analyzed their fecal samples to build up a picture of each individual gut microbiome. They also tracked the participants' treatment progress.

One surprising finding came to light. Taking over-the-counter probiotic supplements correlated with a 70% lower chance of responding to checkpoint inhibitor immunotherapy. Almost half (42%) of the participants reported taking such supplements.

The researchers also noticed a relationship between probiotics and lower gut microbiome diversity. Scientists had already seen this in people with cancers that respond poorly to immunotherapy.

"The general perception is [that probiotics] make your gut microbiome healthier," says first study author Christine Spencer, a research scientist at the Parker Institute. "While more research is needed, our data suggest that may not be the case for cancer patients."

Manipulating the gut microbiome

Dietary choices also appeared to have an impact. People who ate a high-fiber diet were five times as likely to respond to immunotherapy and had more bacteria linked to a positive response. People with diets high in added sugar and processed meat, on the other hand, had fewer of these bacteria.

Spencer and team were less shocked by this result. "Eating a high-fiber diet has long been shown to have health benefits," she explains. "In this case, we see signs that it is also linked to a better response to cancer immunotherapy. Definitely another good reason to load up on whole grains, vegetables, and fruits."

Overall, the study may partly explain why some cancers do not respond well to immunotherapy treatment. It also suggests that certain dietary factors—especially careful consideration of probiotic supplements—may have an impact on success rates.

Spencer admits that improving the effectiveness of immunotherapy might not be as simple as that. "But this study," she says, "does point to diet playing a role in immunotherapy response via the gut microbiome, and we hope these findings will spur more studies on this topic in the cancer research community."

More trials are beginning. One is currently using an oral pill in an attempt to positively influence the gut microbiome and immunotherapy response. MD Anderson staff are planning another that will examine the effects of different diets on people with cancer.