People undergoing immune-boosting therapy for advanced melanoma may respond better if they eat a high-fiber diet, according to a study published in the journal Science that looked at how diet and the gut microbiome might affect cancer patients’ response to immunotherapy (treatments that enlist the immune system to help kill tumors).

The microbiome refers to the trillions of bacteria and other microbes that naturally dwell in the human body, largely in the gut. Those microbes are integral to the normal body processes, from metabolism and nutrient synthesis to brain function and immune defenses. Gut bacteria play a key role in “educating and training” the immune system.

Fiber is one factor in the composition of the gut microbiome. It “feeds” certain types of bacteria, including those that produce short-chain fatty acids that have anti-inflammatory and antitumor benefits.

On average, the study found that patients who ate enough fiber fared better: 76 percent responded to immunotherapy, versus 60 percent of those with low-fiber diets. That meant their tumors had at least partly regressed, or their cancer remained stable for at least six months.

However, no such benefit was found among the 49 patients who said they used probiotic supplements.

And the best response rate (82 percent) was seen among patients who ate plenty of fiber but took no probiotics.