It might seem counterintuitive, but there are a growing number of studies that tell us that consuming more probiotics — live microorganisms often touted for their health benefits — can lead to reduced diversity in the gut microbiome. In some contexts, probiotics might be doing us more harm than good.

Case in point, a new study recently presented at a news conference held by the American Association for Cancer Research has found that cancer patients being treated with immune therapy did worse when their diets were supplemented with probiotics, reports Science News.

Alternatively, the study also found that patients who had diets rich in high fiber foods, such as whole grains, fruits, and vegetables, showed marked improvements in the effectiveness of their treatment, as well as higher gut flora diversity.

The study looked at people with melanoma skin cancer who were getting a kind of immune therapy called PD-1 blockade or checkpoint inhibition. Patients who ate a high fiber diet were five times as likely to have the therapy halt the growth of or shrink tumors as those on diets low in fiber. Meanwhile, the 40 percent of patients in the study who said they were taking probiotics saw a decrease in their their gut microbe diversity.

“A lot of people have perceptions that probiotics will have health benefits, but that might not be the case for cancer patients,” said Christine Spencer, a research scientist at the Parker Institute for Cancer Immunotherapy in San Francisco.

Microbiome diversity is key

Gut flora diversity is particularly important for patients on immunotherapy, because our microbiome plays a major role in activating our body's immune response. In particular, bacteria in the Ruminococcaceae family have been shown to improve responses to PD-1 blockade immunotherapy, but researchers don't know why some people naturally have more of this bacteria than others. The best way we know of, so far, to improve our chances of having these helpful bugs in our system is to increase microbiome diversity on the whole.

To be fair, the study didn't distinguish between sources for probiotics. For instance, it's unclear whether some sources might be more helpful — or at least, less harmful — than others. Nevertheless, the only sure-fire way to increase microbiome diversity appears to be with a high fiber diet. Even in cases where eating more fruit and vegetables didn't boost the immune therapy's effectiveness, researchers noted that no one got worse. So you can't go wrong with fiber like you can with probiotics.

More data from the study, including more specifics on probiotics’ effect on immune therapy, is scheduled to be presented on April 2 in Atlanta during the cancer research association’s annual meeting.

High fiber diets improve cancer treatments; probiotics make things worse
If you want to get the most out of immune therapy treatment for cancer, lay off the kombucha and eat whole foods.